qemu

qcow2-cluster.c (87667B)

---

 /*
  * Block driver for the QCOW version 2 format
  *
  * Copyright (c) 2004-2006 Fabrice Bellard
  *
  * Permission is hereby granted, free of charge, to any person obtaining a copy
  * of this software and associated documentation files (the "Software"), to deal
  * in the Software without restriction, including without limitation the rights
  * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
  * copies of the Software, and to permit persons to whom the Software is
  * furnished to do so, subject to the following conditions:
  *
  * The above copyright notice and this permission notice shall be included in
  * all copies or substantial portions of the Software.
  *
  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
  * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
  * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
  * THE SOFTWARE.
  */
 
 #include "qemu/osdep.h"
 #include <zlib.h>
 
 #include "qapi/error.h"
 #include "qcow2.h"
 #include "qemu/bswap.h"
 #include "qemu/memalign.h"
 #include "trace.h"
 
 int coroutine_fn qcow2_shrink_l1_table(BlockDriverState *bs,
                                        uint64_t exact_size)
 {
     BDRVQcow2State *s = bs->opaque;
     int new_l1_size, i, ret;
 
     if (exact_size >= s->l1_size) {
         return 0;
     }
 
     new_l1_size = exact_size;
 
 #ifdef DEBUG_ALLOC2
     fprintf(stderr, "shrink l1_table from %d to %d\n", s->l1_size, new_l1_size);
 #endif
 
     BLKDBG_EVENT(bs->file, BLKDBG_L1_SHRINK_WRITE_TABLE);
     ret = bdrv_co_pwrite_zeroes(bs->file,
                                 s->l1_table_offset + new_l1_size * L1E_SIZE,
                                 (s->l1_size - new_l1_size) * L1E_SIZE, 0);
     if (ret < 0) {
         goto fail;
     }
 
     ret = bdrv_co_flush(bs->file->bs);
     if (ret < 0) {
         goto fail;
     }
 
     BLKDBG_EVENT(bs->file, BLKDBG_L1_SHRINK_FREE_L2_CLUSTERS);
     for (i = s->l1_size - 1; i > new_l1_size - 1; i--) {
         if ((s->l1_table[i] & L1E_OFFSET_MASK) == 0) {
             continue;
         }
         qcow2_free_clusters(bs, s->l1_table[i] & L1E_OFFSET_MASK,
                             s->cluster_size, QCOW2_DISCARD_ALWAYS);
         s->l1_table[i] = 0;
     }
     return 0;
 
 fail:
     /*
      * If the write in the l1_table failed the image may contain a partially
      * overwritten l1_table. In this case it would be better to clear the
      * l1_table in memory to avoid possible image corruption.
      */
     memset(s->l1_table + new_l1_size, 0,
            (s->l1_size - new_l1_size) * L1E_SIZE);
     return ret;
 }
 
 int qcow2_grow_l1_table(BlockDriverState *bs, uint64_t min_size,
                         bool exact_size)
 {
     BDRVQcow2State *s = bs->opaque;
     int new_l1_size2, ret, i;
     uint64_t *new_l1_table;
     int64_t old_l1_table_offset, old_l1_size;
     int64_t new_l1_table_offset, new_l1_size;
     uint8_t data[12];
 
     if (min_size <= s->l1_size)
         return 0;
 
     /* Do a sanity check on min_size before trying to calculate new_l1_size
      * (this prevents overflows during the while loop for the calculation of
      * new_l1_size) */
     if (min_size > INT_MAX / L1E_SIZE) {
         return -EFBIG;
     }
 
     if (exact_size) {
         new_l1_size = min_size;
     } else {
         /* Bump size up to reduce the number of times we have to grow */
         new_l1_size = s->l1_size;
         if (new_l1_size == 0) {
             new_l1_size = 1;
         }
         while (min_size > new_l1_size) {
             new_l1_size = DIV_ROUND_UP(new_l1_size * 3, 2);
         }
     }
 
     QEMU_BUILD_BUG_ON(QCOW_MAX_L1_SIZE > INT_MAX);
     if (new_l1_size > QCOW_MAX_L1_SIZE / L1E_SIZE) {
         return -EFBIG;
     }
 
 #ifdef DEBUG_ALLOC2
     fprintf(stderr, "grow l1_table from %d to %" PRId64 "\n",
             s->l1_size, new_l1_size);
 #endif
 
     new_l1_size2 = L1E_SIZE * new_l1_size;
     new_l1_table = qemu_try_blockalign(bs->file->bs, new_l1_size2);
     if (new_l1_table == NULL) {
         return -ENOMEM;
     }
     memset(new_l1_table, 0, new_l1_size2);
 
     if (s->l1_size) {
         memcpy(new_l1_table, s->l1_table, s->l1_size * L1E_SIZE);
     }
 
     /* write new table (align to cluster) */
     BLKDBG_EVENT(bs->file, BLKDBG_L1_GROW_ALLOC_TABLE);
     new_l1_table_offset = qcow2_alloc_clusters(bs, new_l1_size2);
     if (new_l1_table_offset < 0) {
         qemu_vfree(new_l1_table);
         return new_l1_table_offset;
     }
 
     ret = qcow2_cache_flush(bs, s->refcount_block_cache);
     if (ret < 0) {
         goto fail;
     }
 
     /* the L1 position has not yet been updated, so these clusters must
      * indeed be completely free */
     ret = qcow2_pre_write_overlap_check(bs, 0, new_l1_table_offset,
                                         new_l1_size2, false);
     if (ret < 0) {
         goto fail;
     }
 
     BLKDBG_EVENT(bs->file, BLKDBG_L1_GROW_WRITE_TABLE);
     for(i = 0; i < s->l1_size; i++)
         new_l1_table[i] = cpu_to_be64(new_l1_table[i]);
     ret = bdrv_pwrite_sync(bs->file, new_l1_table_offset, new_l1_size2,
                            new_l1_table, 0);
     if (ret < 0)
         goto fail;
     for(i = 0; i < s->l1_size; i++)
         new_l1_table[i] = be64_to_cpu(new_l1_table[i]);
 
     /* set new table */
     BLKDBG_EVENT(bs->file, BLKDBG_L1_GROW_ACTIVATE_TABLE);
     stl_be_p(data, new_l1_size);
     stq_be_p(data + 4, new_l1_table_offset);
     ret = bdrv_pwrite_sync(bs->file, offsetof(QCowHeader, l1_size),
                            sizeof(data), data, 0);
     if (ret < 0) {
         goto fail;
     }
     qemu_vfree(s->l1_table);
     old_l1_table_offset = s->l1_table_offset;
     s->l1_table_offset = new_l1_table_offset;
     s->l1_table = new_l1_table;
     old_l1_size = s->l1_size;
     s->l1_size = new_l1_size;
     qcow2_free_clusters(bs, old_l1_table_offset, old_l1_size * L1E_SIZE,
                         QCOW2_DISCARD_OTHER);
     return 0;
  fail:
     qemu_vfree(new_l1_table);
     qcow2_free_clusters(bs, new_l1_table_offset, new_l1_size2,
                         QCOW2_DISCARD_OTHER);
     return ret;
 }
 
 /*
  * l2_load
  *
  * @bs: The BlockDriverState
  * @offset: A guest offset, used to calculate what slice of the L2
  *          table to load.
  * @l2_offset: Offset to the L2 table in the image file.
  * @l2_slice: Location to store the pointer to the L2 slice.
  *
  * Loads a L2 slice into memory (L2 slices are the parts of L2 tables
  * that are loaded by the qcow2 cache). If the slice is in the cache,
  * the cache is used; otherwise the L2 slice is loaded from the image
  * file.
  */
 static int l2_load(BlockDriverState *bs, uint64_t offset,
                    uint64_t l2_offset, uint64_t **l2_slice)
 {
     BDRVQcow2State *s = bs->opaque;
     int start_of_slice = l2_entry_size(s) *
         (offset_to_l2_index(s, offset) - offset_to_l2_slice_index(s, offset));
 
     return qcow2_cache_get(bs, s->l2_table_cache, l2_offset + start_of_slice,
                            (void **)l2_slice);
 }
 
 /*
  * Writes an L1 entry to disk (note that depending on the alignment
  * requirements this function may write more that just one entry in
  * order to prevent bdrv_pwrite from performing a read-modify-write)
  */
 int qcow2_write_l1_entry(BlockDriverState *bs, int l1_index)
 {
     BDRVQcow2State *s = bs->opaque;
     int l1_start_index;
     int i, ret;
     int bufsize = MAX(L1E_SIZE,
                       MIN(bs->file->bs->bl.request_alignment, s->cluster_size));
     int nentries = bufsize / L1E_SIZE;
     g_autofree uint64_t *buf = g_try_new0(uint64_t, nentries);
 
     if (buf == NULL) {
         return -ENOMEM;
     }
 
     l1_start_index = QEMU_ALIGN_DOWN(l1_index, nentries);
     for (i = 0; i < MIN(nentries, s->l1_size - l1_start_index); i++) {
         buf[i] = cpu_to_be64(s->l1_table[l1_start_index + i]);
     }
 
     ret = qcow2_pre_write_overlap_check(bs, QCOW2_OL_ACTIVE_L1,
             s->l1_table_offset + L1E_SIZE * l1_start_index, bufsize, false);
     if (ret < 0) {
         return ret;
     }
 
     BLKDBG_EVENT(bs->file, BLKDBG_L1_UPDATE);
     ret = bdrv_pwrite_sync(bs->file,
                            s->l1_table_offset + L1E_SIZE * l1_start_index,
                            bufsize, buf, 0);
     if (ret < 0) {
         return ret;
     }
 
     return 0;
 }
 
 /*
  * l2_allocate
  *
  * Allocate a new l2 entry in the file. If l1_index points to an already
  * used entry in the L2 table (i.e. we are doing a copy on write for the L2
  * table) copy the contents of the old L2 table into the newly allocated one.
  * Otherwise the new table is initialized with zeros.
  *
  */
 
 static int l2_allocate(BlockDriverState *bs, int l1_index)
 {
     BDRVQcow2State *s = bs->opaque;
     uint64_t old_l2_offset;
     uint64_t *l2_slice = NULL;
     unsigned slice, slice_size2, n_slices;
     int64_t l2_offset;
     int ret;
 
     old_l2_offset = s->l1_table[l1_index];
 
     trace_qcow2_l2_allocate(bs, l1_index);
 
     /* allocate a new l2 entry */
 
     l2_offset = qcow2_alloc_clusters(bs, s->l2_size * l2_entry_size(s));
     if (l2_offset < 0) {
         ret = l2_offset;
         goto fail;
     }
 
     /* The offset must fit in the offset field of the L1 table entry */
     assert((l2_offset & L1E_OFFSET_MASK) == l2_offset);
 
     /* If we're allocating the table at offset 0 then something is wrong */
     if (l2_offset == 0) {
         qcow2_signal_corruption(bs, true, -1, -1, "Preventing invalid "
                                 "allocation of L2 table at offset 0");
         ret = -EIO;
         goto fail;
     }
 
     ret = qcow2_cache_flush(bs, s->refcount_block_cache);
     if (ret < 0) {
         goto fail;
     }
 
     /* allocate a new entry in the l2 cache */
 
     slice_size2 = s->l2_slice_size * l2_entry_size(s);
     n_slices = s->cluster_size / slice_size2;
 
     trace_qcow2_l2_allocate_get_empty(bs, l1_index);
     for (slice = 0; slice < n_slices; slice++) {
         ret = qcow2_cache_get_empty(bs, s->l2_table_cache,
                                     l2_offset + slice * slice_size2,
                                     (void **) &l2_slice);
         if (ret < 0) {
             goto fail;
         }
 
         if ((old_l2_offset & L1E_OFFSET_MASK) == 0) {
             /* if there was no old l2 table, clear the new slice */
             memset(l2_slice, 0, slice_size2);
         } else {
             uint64_t *old_slice;
             uint64_t old_l2_slice_offset =
                 (old_l2_offset & L1E_OFFSET_MASK) + slice * slice_size2;
 
             /* if there was an old l2 table, read a slice from the disk */
             BLKDBG_EVENT(bs->file, BLKDBG_L2_ALLOC_COW_READ);
             ret = qcow2_cache_get(bs, s->l2_table_cache, old_l2_slice_offset,
                                   (void **) &old_slice);
             if (ret < 0) {
                 goto fail;
             }
 
             memcpy(l2_slice, old_slice, slice_size2);
 
             qcow2_cache_put(s->l2_table_cache, (void **) &old_slice);
         }
 
         /* write the l2 slice to the file */
         BLKDBG_EVENT(bs->file, BLKDBG_L2_ALLOC_WRITE);
 
         trace_qcow2_l2_allocate_write_l2(bs, l1_index);
         qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_slice);
         qcow2_cache_put(s->l2_table_cache, (void **) &l2_slice);
     }
 
     ret = qcow2_cache_flush(bs, s->l2_table_cache);
     if (ret < 0) {
         goto fail;
     }
 
     /* update the L1 entry */
     trace_qcow2_l2_allocate_write_l1(bs, l1_index);
     s->l1_table[l1_index] = l2_offset | QCOW_OFLAG_COPIED;
     ret = qcow2_write_l1_entry(bs, l1_index);
     if (ret < 0) {
         goto fail;
     }
 
     trace_qcow2_l2_allocate_done(bs, l1_index, 0);
     return 0;
 
 fail:
     trace_qcow2_l2_allocate_done(bs, l1_index, ret);
     if (l2_slice != NULL) {
         qcow2_cache_put(s->l2_table_cache, (void **) &l2_slice);
     }
     s->l1_table[l1_index] = old_l2_offset;
     if (l2_offset > 0) {
         qcow2_free_clusters(bs, l2_offset, s->l2_size * l2_entry_size(s),
                             QCOW2_DISCARD_ALWAYS);
     }
     return ret;
 }
 
 /*
  * For a given L2 entry, count the number of contiguous subclusters of
  * the same type starting from @sc_from. Compressed clusters are
  * treated as if they were divided into subclusters of size
  * s->subcluster_size.
  *
  * Return the number of contiguous subclusters and set @type to the
  * subcluster type.
  *
  * If the L2 entry is invalid return -errno and set @type to
  * QCOW2_SUBCLUSTER_INVALID.
  */
 static int qcow2_get_subcluster_range_type(BlockDriverState *bs,
                                            uint64_t l2_entry,
                                            uint64_t l2_bitmap,
                                            unsigned sc_from,
                                            QCow2SubclusterType *type)
 {
     BDRVQcow2State *s = bs->opaque;
     uint32_t val;
 
     *type = qcow2_get_subcluster_type(bs, l2_entry, l2_bitmap, sc_from);
 
     if (*type == QCOW2_SUBCLUSTER_INVALID) {
         return -EINVAL;
     } else if (!has_subclusters(s) || *type == QCOW2_SUBCLUSTER_COMPRESSED) {
         return s->subclusters_per_cluster - sc_from;
     }
 
     switch (*type) {
     case QCOW2_SUBCLUSTER_NORMAL:
         val = l2_bitmap | QCOW_OFLAG_SUB_ALLOC_RANGE(0, sc_from);
         return cto32(val) - sc_from;
 
     case QCOW2_SUBCLUSTER_ZERO_PLAIN:
     case QCOW2_SUBCLUSTER_ZERO_ALLOC:
         val = (l2_bitmap | QCOW_OFLAG_SUB_ZERO_RANGE(0, sc_from)) >> 32;
         return cto32(val) - sc_from;
 
     case QCOW2_SUBCLUSTER_UNALLOCATED_PLAIN:
     case QCOW2_SUBCLUSTER_UNALLOCATED_ALLOC:
         val = ((l2_bitmap >> 32) | l2_bitmap)
             & ~QCOW_OFLAG_SUB_ALLOC_RANGE(0, sc_from);
         return ctz32(val) - sc_from;
 
     default:
         g_assert_not_reached();
     }
 }
 
 /*
  * Return the number of contiguous subclusters of the exact same type
  * in a given L2 slice, starting from cluster @l2_index, subcluster
  * @sc_index. Allocated subclusters are required to be contiguous in
  * the image file.
  * At most @nb_clusters are checked (note that this means clusters,
  * not subclusters).
  * Compressed clusters are always processed one by one but for the
  * purpose of this count they are treated as if they were divided into
  * subclusters of size s->subcluster_size.
  * On failure return -errno and update @l2_index to point to the
  * invalid entry.
  */
 static int count_contiguous_subclusters(BlockDriverState *bs, int nb_clusters,
                                         unsigned sc_index, uint64_t *l2_slice,
                                         unsigned *l2_index)
 {
     BDRVQcow2State *s = bs->opaque;
     int i, count = 0;
     bool check_offset = false;
     uint64_t expected_offset = 0;
     QCow2SubclusterType expected_type = QCOW2_SUBCLUSTER_NORMAL, type;
 
     assert(*l2_index + nb_clusters <= s->l2_slice_size);
 
     for (i = 0; i < nb_clusters; i++) {
         unsigned first_sc = (i == 0) ? sc_index : 0;
         uint64_t l2_entry = get_l2_entry(s, l2_slice, *l2_index + i);
         uint64_t l2_bitmap = get_l2_bitmap(s, l2_slice, *l2_index + i);
         int ret = qcow2_get_subcluster_range_type(bs, l2_entry, l2_bitmap,
                                                   first_sc, &type);
         if (ret < 0) {
             *l2_index += i; /* Point to the invalid entry */
             return -EIO;
         }
         if (i == 0) {
             if (type == QCOW2_SUBCLUSTER_COMPRESSED) {
                 /* Compressed clusters are always processed one by one */
                 return ret;
             }
             expected_type = type;
             expected_offset = l2_entry & L2E_OFFSET_MASK;
             check_offset = (type == QCOW2_SUBCLUSTER_NORMAL ||
                             type == QCOW2_SUBCLUSTER_ZERO_ALLOC ||
                             type == QCOW2_SUBCLUSTER_UNALLOCATED_ALLOC);
         } else if (type != expected_type) {
             break;
         } else if (check_offset) {
             expected_offset += s->cluster_size;
             if (expected_offset != (l2_entry & L2E_OFFSET_MASK)) {
                 break;
             }
         }
         count += ret;
         /* Stop if there are type changes before the end of the cluster */
         if (first_sc + ret < s->subclusters_per_cluster) {
             break;
         }
     }
 
     return count;
 }
 
 static int coroutine_fn do_perform_cow_read(BlockDriverState *bs,
                                             uint64_t src_cluster_offset,
                                             unsigned offset_in_cluster,
                                             QEMUIOVector *qiov)
 {
     int ret;
 
     if (qiov->size == 0) {
         return 0;
     }
 
     BLKDBG_EVENT(bs->file, BLKDBG_COW_READ);
 
     if (!bs->drv) {
         return -ENOMEDIUM;
     }
 
     /*
      * We never deal with requests that don't satisfy
      * bdrv_check_qiov_request(), and aligning requests to clusters never
      * breaks this condition. So, do some assertions before calling
      * bs->drv->bdrv_co_preadv_part() which has int64_t arguments.
      */
     assert(src_cluster_offset <= INT64_MAX);
     assert(src_cluster_offset + offset_in_cluster <= INT64_MAX);
     /* Cast qiov->size to uint64_t to silence a compiler warning on -m32 */
     assert((uint64_t)qiov->size <= INT64_MAX);
     bdrv_check_qiov_request(src_cluster_offset + offset_in_cluster, qiov->size,
                             qiov, 0, &error_abort);
     /*
      * Call .bdrv_co_readv() directly instead of using the public block-layer
      * interface.  This avoids double I/O throttling and request tracking,
      * which can lead to deadlock when block layer copy-on-read is enabled.
      */
     ret = bs->drv->bdrv_co_preadv_part(bs,
                                        src_cluster_offset + offset_in_cluster,
                                        qiov->size, qiov, 0, 0);
     if (ret < 0) {
         return ret;
     }
 
     return 0;
 }
 
 static int coroutine_fn do_perform_cow_write(BlockDriverState *bs,
                                              uint64_t cluster_offset,
                                              unsigned offset_in_cluster,
                                              QEMUIOVector *qiov)
 {
     BDRVQcow2State *s = bs->opaque;
     int ret;
 
     if (qiov->size == 0) {
         return 0;
     }
 
     ret = qcow2_pre_write_overlap_check(bs, 0,
             cluster_offset + offset_in_cluster, qiov->size, true);
     if (ret < 0) {
         return ret;
     }
 
     BLKDBG_EVENT(bs->file, BLKDBG_COW_WRITE);
     ret = bdrv_co_pwritev(s->data_file, cluster_offset + offset_in_cluster,
                           qiov->size, qiov, 0);
     if (ret < 0) {
         return ret;
     }
 
     return 0;
 }
 
 
 /*
  * get_host_offset
  *
  * For a given offset of the virtual disk find the equivalent host
  * offset in the qcow2 file and store it in *host_offset. Neither
  * offset needs to be aligned to a cluster boundary.
  *
  * If the cluster is unallocated then *host_offset will be 0.
  * If the cluster is compressed then *host_offset will contain the l2 entry.
  *
  * On entry, *bytes is the maximum number of contiguous bytes starting at
  * offset that we are interested in.
  *
  * On exit, *bytes is the number of bytes starting at offset that have the same
  * subcluster type and (if applicable) are stored contiguously in the image
  * file. The subcluster type is stored in *subcluster_type.
  * Compressed clusters are always processed one by one.
  *
  * Returns 0 on success, -errno in error cases.
  */
 int qcow2_get_host_offset(BlockDriverState *bs, uint64_t offset,
                           unsigned int *bytes, uint64_t *host_offset,
                           QCow2SubclusterType *subcluster_type)
 {
     BDRVQcow2State *s = bs->opaque;
     unsigned int l2_index, sc_index;
     uint64_t l1_index, l2_offset, *l2_slice, l2_entry, l2_bitmap;
     int sc;
     unsigned int offset_in_cluster;
     uint64_t bytes_available, bytes_needed, nb_clusters;
     QCow2SubclusterType type;
     int ret;
 
     offset_in_cluster = offset_into_cluster(s, offset);
     bytes_needed = (uint64_t) *bytes + offset_in_cluster;
 
     /* compute how many bytes there are between the start of the cluster
      * containing offset and the end of the l2 slice that contains
      * the entry pointing to it */
     bytes_available =
         ((uint64_t) (s->l2_slice_size - offset_to_l2_slice_index(s, offset)))
         << s->cluster_bits;
 
     if (bytes_needed > bytes_available) {
         bytes_needed = bytes_available;
     }
 
     *host_offset = 0;
 
     /* seek to the l2 offset in the l1 table */
 
     l1_index = offset_to_l1_index(s, offset);
     if (l1_index >= s->l1_size) {
         type = QCOW2_SUBCLUSTER_UNALLOCATED_PLAIN;
         goto out;
     }
 
     l2_offset = s->l1_table[l1_index] & L1E_OFFSET_MASK;
     if (!l2_offset) {
         type = QCOW2_SUBCLUSTER_UNALLOCATED_PLAIN;
         goto out;
     }
 
     if (offset_into_cluster(s, l2_offset)) {
         qcow2_signal_corruption(bs, true, -1, -1, "L2 table offset %#" PRIx64
                                 " unaligned (L1 index: %#" PRIx64 ")",
                                 l2_offset, l1_index);
         return -EIO;
     }
 
     /* load the l2 slice in memory */
 
     ret = l2_load(bs, offset, l2_offset, &l2_slice);
     if (ret < 0) {
         return ret;
     }
 
     /* find the cluster offset for the given disk offset */
 
     l2_index = offset_to_l2_slice_index(s, offset);
     sc_index = offset_to_sc_index(s, offset);
     l2_entry = get_l2_entry(s, l2_slice, l2_index);
     l2_bitmap = get_l2_bitmap(s, l2_slice, l2_index);
 
     nb_clusters = size_to_clusters(s, bytes_needed);
     /* bytes_needed <= *bytes + offset_in_cluster, both of which are unsigned
      * integers; the minimum cluster size is 512, so this assertion is always
      * true */
     assert(nb_clusters <= INT_MAX);
 
     type = qcow2_get_subcluster_type(bs, l2_entry, l2_bitmap, sc_index);
     if (s->qcow_version < 3 && (type == QCOW2_SUBCLUSTER_ZERO_PLAIN ||
                                 type == QCOW2_SUBCLUSTER_ZERO_ALLOC)) {
         qcow2_signal_corruption(bs, true, -1, -1, "Zero cluster entry found"
                                 " in pre-v3 image (L2 offset: %#" PRIx64
                                 ", L2 index: %#x)", l2_offset, l2_index);
         ret = -EIO;
         goto fail;
     }
     switch (type) {
     case QCOW2_SUBCLUSTER_INVALID:
         break; /* This is handled by count_contiguous_subclusters() below */
     case QCOW2_SUBCLUSTER_COMPRESSED:
         if (has_data_file(bs)) {
             qcow2_signal_corruption(bs, true, -1, -1, "Compressed cluster "
                                     "entry found in image with external data "
                                     "file (L2 offset: %#" PRIx64 ", L2 index: "
                                     "%#x)", l2_offset, l2_index);
             ret = -EIO;
             goto fail;
         }
         *host_offset = l2_entry;
         break;
     case QCOW2_SUBCLUSTER_ZERO_PLAIN:
     case QCOW2_SUBCLUSTER_UNALLOCATED_PLAIN:
         break;
     case QCOW2_SUBCLUSTER_ZERO_ALLOC:
     case QCOW2_SUBCLUSTER_NORMAL:
     case QCOW2_SUBCLUSTER_UNALLOCATED_ALLOC: {
         uint64_t host_cluster_offset = l2_entry & L2E_OFFSET_MASK;
         *host_offset = host_cluster_offset + offset_in_cluster;
         if (offset_into_cluster(s, host_cluster_offset)) {
             qcow2_signal_corruption(bs, true, -1, -1,
                                     "Cluster allocation offset %#"
                                     PRIx64 " unaligned (L2 offset: %#" PRIx64
                                     ", L2 index: %#x)", host_cluster_offset,
                                     l2_offset, l2_index);
             ret = -EIO;
             goto fail;
         }
         if (has_data_file(bs) && *host_offset != offset) {
             qcow2_signal_corruption(bs, true, -1, -1,
                                     "External data file host cluster offset %#"
                                     PRIx64 " does not match guest cluster "
                                     "offset: %#" PRIx64
                                     ", L2 index: %#x)", host_cluster_offset,
                                     offset - offset_in_cluster, l2_index);
             ret = -EIO;
             goto fail;
         }
         break;
     }
     default:
         abort();
     }
 
     sc = count_contiguous_subclusters(bs, nb_clusters, sc_index,
                                       l2_slice, &l2_index);
     if (sc < 0) {
         qcow2_signal_corruption(bs, true, -1, -1, "Invalid cluster entry found "
                                 " (L2 offset: %#" PRIx64 ", L2 index: %#x)",
                                 l2_offset, l2_index);
         ret = -EIO;
         goto fail;
     }
     qcow2_cache_put(s->l2_table_cache, (void **) &l2_slice);
 
     bytes_available = ((int64_t)sc + sc_index) << s->subcluster_bits;
 
 out:
     if (bytes_available > bytes_needed) {
         bytes_available = bytes_needed;
     }
 
     /* bytes_available <= bytes_needed <= *bytes + offset_in_cluster;
      * subtracting offset_in_cluster will therefore definitely yield something
      * not exceeding UINT_MAX */
     assert(bytes_available - offset_in_cluster <= UINT_MAX);
     *bytes = bytes_available - offset_in_cluster;
 
     *subcluster_type = type;
 
     return 0;
 
 fail:
     qcow2_cache_put(s->l2_table_cache, (void **)&l2_slice);
     return ret;
 }
 
 /*
  * get_cluster_table
  *
  * for a given disk offset, load (and allocate if needed)
  * the appropriate slice of its l2 table.
  *
  * the cluster index in the l2 slice is given to the caller.
  *
  * Returns 0 on success, -errno in failure case
  */
 static int get_cluster_table(BlockDriverState *bs, uint64_t offset,
                              uint64_t **new_l2_slice,
                              int *new_l2_index)
 {
     BDRVQcow2State *s = bs->opaque;
     unsigned int l2_index;
     uint64_t l1_index, l2_offset;
     uint64_t *l2_slice = NULL;
     int ret;
 
     /* seek to the l2 offset in the l1 table */
 
     l1_index = offset_to_l1_index(s, offset);
     if (l1_index >= s->l1_size) {
         ret = qcow2_grow_l1_table(bs, l1_index + 1, false);
         if (ret < 0) {
             return ret;
         }
     }
 
     assert(l1_index < s->l1_size);
     l2_offset = s->l1_table[l1_index] & L1E_OFFSET_MASK;
     if (offset_into_cluster(s, l2_offset)) {
         qcow2_signal_corruption(bs, true, -1, -1, "L2 table offset %#" PRIx64
                                 " unaligned (L1 index: %#" PRIx64 ")",
                                 l2_offset, l1_index);
         return -EIO;
     }
 
     if (!(s->l1_table[l1_index] & QCOW_OFLAG_COPIED)) {
         /* First allocate a new L2 table (and do COW if needed) */
         ret = l2_allocate(bs, l1_index);
         if (ret < 0) {
             return ret;
         }
 
         /* Then decrease the refcount of the old table */
         if (l2_offset) {
             qcow2_free_clusters(bs, l2_offset, s->l2_size * l2_entry_size(s),
                                 QCOW2_DISCARD_OTHER);
         }
 
         /* Get the offset of the newly-allocated l2 table */
         l2_offset = s->l1_table[l1_index] & L1E_OFFSET_MASK;
         assert(offset_into_cluster(s, l2_offset) == 0);
     }
 
     /* load the l2 slice in memory */
     ret = l2_load(bs, offset, l2_offset, &l2_slice);
     if (ret < 0) {
         return ret;
     }
 
     /* find the cluster offset for the given disk offset */
 
     l2_index = offset_to_l2_slice_index(s, offset);
 
     *new_l2_slice = l2_slice;
     *new_l2_index = l2_index;
 
     return 0;
 }
 
 /*
  * alloc_compressed_cluster_offset
  *
  * For a given offset on the virtual disk, allocate a new compressed cluster
  * and put the host offset of the cluster into *host_offset. If a cluster is
  * already allocated at the offset, return an error.
  *
  * Return 0 on success and -errno in error cases
  */
 int coroutine_fn qcow2_alloc_compressed_cluster_offset(BlockDriverState *bs,
                                                        uint64_t offset,
                                                        int compressed_size,
                                                        uint64_t *host_offset)
 {
     BDRVQcow2State *s = bs->opaque;
     int l2_index, ret;
     uint64_t *l2_slice;
     int64_t cluster_offset;
     int nb_csectors;
 
     if (has_data_file(bs)) {
         return 0;
     }
 
     ret = get_cluster_table(bs, offset, &l2_slice, &l2_index);
     if (ret < 0) {
         return ret;
     }
 
     /* Compression can't overwrite anything. Fail if the cluster was already
      * allocated. */
     cluster_offset = get_l2_entry(s, l2_slice, l2_index);
     if (cluster_offset & L2E_OFFSET_MASK) {
         qcow2_cache_put(s->l2_table_cache, (void **) &l2_slice);
         return -EIO;
     }
 
     cluster_offset = qcow2_alloc_bytes(bs, compressed_size);
     if (cluster_offset < 0) {
         qcow2_cache_put(s->l2_table_cache, (void **) &l2_slice);
         return cluster_offset;
     }
 
     nb_csectors =
         (cluster_offset + compressed_size - 1) / QCOW2_COMPRESSED_SECTOR_SIZE -
         (cluster_offset / QCOW2_COMPRESSED_SECTOR_SIZE);
 
     /* The offset and size must fit in their fields of the L2 table entry */
     assert((cluster_offset & s->cluster_offset_mask) == cluster_offset);
     assert((nb_csectors & s->csize_mask) == nb_csectors);
 
     cluster_offset |= QCOW_OFLAG_COMPRESSED |
                       ((uint64_t)nb_csectors << s->csize_shift);
 
     /* update L2 table */
 
     /* compressed clusters never have the copied flag */
 
     BLKDBG_EVENT(bs->file, BLKDBG_L2_UPDATE_COMPRESSED);
     qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_slice);
     set_l2_entry(s, l2_slice, l2_index, cluster_offset);
     if (has_subclusters(s)) {
         set_l2_bitmap(s, l2_slice, l2_index, 0);
     }
     qcow2_cache_put(s->l2_table_cache, (void **) &l2_slice);
 
     *host_offset = cluster_offset & s->cluster_offset_mask;
     return 0;
 }
 
 static int coroutine_fn perform_cow(BlockDriverState *bs, QCowL2Meta *m)
 {
     BDRVQcow2State *s = bs->opaque;
     Qcow2COWRegion *start = &m->cow_start;
     Qcow2COWRegion *end = &m->cow_end;
     unsigned buffer_size;
     unsigned data_bytes = end->offset - (start->offset + start->nb_bytes);
     bool merge_reads;
     uint8_t *start_buffer, *end_buffer;
     QEMUIOVector qiov;
     int ret;
 
     assert(start->nb_bytes <= UINT_MAX - end->nb_bytes);
     assert(start->nb_bytes + end->nb_bytes <= UINT_MAX - data_bytes);
     assert(start->offset + start->nb_bytes <= end->offset);
 
     if ((start->nb_bytes == 0 && end->nb_bytes == 0) || m->skip_cow) {
         return 0;
     }
 
     /* If we have to read both the start and end COW regions and the
      * middle region is not too large then perform just one read
      * operation */
     merge_reads = start->nb_bytes && end->nb_bytes && data_bytes <= 16384;
     if (merge_reads) {
         buffer_size = start->nb_bytes + data_bytes + end->nb_bytes;
     } else {
         /* If we have to do two reads, add some padding in the middle
          * if necessary to make sure that the end region is optimally
          * aligned. */
         size_t align = bdrv_opt_mem_align(bs);
         assert(align > 0 && align <= UINT_MAX);
         assert(QEMU_ALIGN_UP(start->nb_bytes, align) <=
                UINT_MAX - end->nb_bytes);
         buffer_size = QEMU_ALIGN_UP(start->nb_bytes, align) + end->nb_bytes;
     }
 
     /* Reserve a buffer large enough to store all the data that we're
      * going to read */
     start_buffer = qemu_try_blockalign(bs, buffer_size);
     if (start_buffer == NULL) {
         return -ENOMEM;
     }
     /* The part of the buffer where the end region is located */
     end_buffer = start_buffer + buffer_size - end->nb_bytes;
 
     qemu_iovec_init(&qiov, 2 + (m->data_qiov ?
                                 qemu_iovec_subvec_niov(m->data_qiov,
                                                        m->data_qiov_offset,
                                                        data_bytes)
                                 : 0));
 
     qemu_co_mutex_unlock(&s->lock);
     /* First we read the existing data from both COW regions. We
      * either read the whole region in one go, or the start and end
      * regions separately. */
     if (merge_reads) {
         qemu_iovec_add(&qiov, start_buffer, buffer_size);
         ret = do_perform_cow_read(bs, m->offset, start->offset, &qiov);
     } else {
         qemu_iovec_add(&qiov, start_buffer, start->nb_bytes);
         ret = do_perform_cow_read(bs, m->offset, start->offset, &qiov);
         if (ret < 0) {
             goto fail;
         }
 
         qemu_iovec_reset(&qiov);
         qemu_iovec_add(&qiov, end_buffer, end->nb_bytes);
         ret = do_perform_cow_read(bs, m->offset, end->offset, &qiov);
     }
     if (ret < 0) {
         goto fail;
     }
 
     /* Encrypt the data if necessary before writing it */
     if (bs->encrypted) {
         ret = qcow2_co_encrypt(bs,
                                m->alloc_offset + start->offset,
                                m->offset + start->offset,
                                start_buffer, start->nb_bytes);
         if (ret < 0) {
             goto fail;
         }
 
         ret = qcow2_co_encrypt(bs,
                                m->alloc_offset + end->offset,
                                m->offset + end->offset,
                                end_buffer, end->nb_bytes);
         if (ret < 0) {
             goto fail;
         }
     }
 
     /* And now we can write everything. If we have the guest data we
      * can write everything in one single operation */
     if (m->data_qiov) {
         qemu_iovec_reset(&qiov);
         if (start->nb_bytes) {
             qemu_iovec_add(&qiov, start_buffer, start->nb_bytes);
         }
         qemu_iovec_concat(&qiov, m->data_qiov, m->data_qiov_offset, data_bytes);
         if (end->nb_bytes) {
             qemu_iovec_add(&qiov, end_buffer, end->nb_bytes);
         }
         /* NOTE: we have a write_aio blkdebug event here followed by
          * a cow_write one in do_perform_cow_write(), but there's only
          * one single I/O operation */
         BLKDBG_EVENT(bs->file, BLKDBG_WRITE_AIO);
         ret = do_perform_cow_write(bs, m->alloc_offset, start->offset, &qiov);
     } else {
         /* If there's no guest data then write both COW regions separately */
         qemu_iovec_reset(&qiov);
         qemu_iovec_add(&qiov, start_buffer, start->nb_bytes);
         ret = do_perform_cow_write(bs, m->alloc_offset, start->offset, &qiov);
         if (ret < 0) {
             goto fail;
         }
 
         qemu_iovec_reset(&qiov);
         qemu_iovec_add(&qiov, end_buffer, end->nb_bytes);
         ret = do_perform_cow_write(bs, m->alloc_offset, end->offset, &qiov);
     }
 
 fail:
     qemu_co_mutex_lock(&s->lock);
 
     /*
      * Before we update the L2 table to actually point to the new cluster, we
      * need to be sure that the refcounts have been increased and COW was
      * handled.
      */
     if (ret == 0) {
         qcow2_cache_depends_on_flush(s->l2_table_cache);
     }
 
     qemu_vfree(start_buffer);
     qemu_iovec_destroy(&qiov);
     return ret;
 }
 
 int coroutine_fn qcow2_alloc_cluster_link_l2(BlockDriverState *bs,
                                              QCowL2Meta *m)
 {
     BDRVQcow2State *s = bs->opaque;
     int i, j = 0, l2_index, ret;
     uint64_t *old_cluster, *l2_slice;
     uint64_t cluster_offset = m->alloc_offset;
 
     trace_qcow2_cluster_link_l2(qemu_coroutine_self(), m->nb_clusters);
     assert(m->nb_clusters > 0);
 
     old_cluster = g_try_new(uint64_t, m->nb_clusters);
     if (old_cluster == NULL) {
         ret = -ENOMEM;
         goto err;
     }
 
     /* copy content of unmodified sectors */
     ret = perform_cow(bs, m);
     if (ret < 0) {
         goto err;
     }
 
     /* Update L2 table. */
     if (s->use_lazy_refcounts) {
         qcow2_mark_dirty(bs);
     }
     if (qcow2_need_accurate_refcounts(s)) {
         qcow2_cache_set_dependency(bs, s->l2_table_cache,
                                    s->refcount_block_cache);
     }
 
     ret = get_cluster_table(bs, m->offset, &l2_slice, &l2_index);
     if (ret < 0) {
         goto err;
     }
     qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_slice);
 
     assert(l2_index + m->nb_clusters <= s->l2_slice_size);
     assert(m->cow_end.offset + m->cow_end.nb_bytes <=
            m->nb_clusters << s->cluster_bits);
     for (i = 0; i < m->nb_clusters; i++) {
         uint64_t offset = cluster_offset + ((uint64_t)i << s->cluster_bits);
         /* if two concurrent writes happen to the same unallocated cluster
          * each write allocates separate cluster and writes data concurrently.
          * The first one to complete updates l2 table with pointer to its
          * cluster the second one has to do RMW (which is done above by
          * perform_cow()), update l2 table with its cluster pointer and free
          * old cluster. This is what this loop does */
         if (get_l2_entry(s, l2_slice, l2_index + i) != 0) {
             old_cluster[j++] = get_l2_entry(s, l2_slice, l2_index + i);
         }
 
         /* The offset must fit in the offset field of the L2 table entry */
         assert((offset & L2E_OFFSET_MASK) == offset);
 
         set_l2_entry(s, l2_slice, l2_index + i, offset | QCOW_OFLAG_COPIED);
 
         /* Update bitmap with the subclusters that were just written */
         if (has_subclusters(s) && !m->prealloc) {
             uint64_t l2_bitmap = get_l2_bitmap(s, l2_slice, l2_index + i);
             unsigned written_from = m->cow_start.offset;
             unsigned written_to = m->cow_end.offset + m->cow_end.nb_bytes;
             int first_sc, last_sc;
             /* Narrow written_from and written_to down to the current cluster */
             written_from = MAX(written_from, i << s->cluster_bits);
             written_to   = MIN(written_to, (i + 1) << s->cluster_bits);
             assert(written_from < written_to);
             first_sc = offset_to_sc_index(s, written_from);
             last_sc  = offset_to_sc_index(s, written_to - 1);
             l2_bitmap |= QCOW_OFLAG_SUB_ALLOC_RANGE(first_sc, last_sc + 1);
             l2_bitmap &= ~QCOW_OFLAG_SUB_ZERO_RANGE(first_sc, last_sc + 1);
             set_l2_bitmap(s, l2_slice, l2_index + i, l2_bitmap);
         }
      }
 
 
     qcow2_cache_put(s->l2_table_cache, (void **) &l2_slice);
 
     /*
      * If this was a COW, we need to decrease the refcount of the old cluster.
      *
      * Don't discard clusters that reach a refcount of 0 (e.g. compressed
      * clusters), the next write will reuse them anyway.
      */
     if (!m->keep_old_clusters && j != 0) {
         for (i = 0; i < j; i++) {
             qcow2_free_any_cluster(bs, old_cluster[i], QCOW2_DISCARD_NEVER);
         }
     }
 
     ret = 0;
 err:
     g_free(old_cluster);
     return ret;
  }
 
 /**
  * Frees the allocated clusters because the request failed and they won't
  * actually be linked.
  */
 void qcow2_alloc_cluster_abort(BlockDriverState *bs, QCowL2Meta *m)
 {
     BDRVQcow2State *s = bs->opaque;
     if (!has_data_file(bs) && !m->keep_old_clusters) {
         qcow2_free_clusters(bs, m->alloc_offset,
                             m->nb_clusters << s->cluster_bits,
                             QCOW2_DISCARD_NEVER);
     }
 }
 
 /*
  * For a given write request, create a new QCowL2Meta structure, add
  * it to @m and the BDRVQcow2State.cluster_allocs list. If the write
  * request does not need copy-on-write or changes to the L2 metadata
  * then this function does nothing.
  *
  * @host_cluster_offset points to the beginning of the first cluster.
  *
  * @guest_offset and @bytes indicate the offset and length of the
  * request.
  *
  * @l2_slice contains the L2 entries of all clusters involved in this
  * write request.
  *
  * If @keep_old is true it means that the clusters were already
  * allocated and will be overwritten. If false then the clusters are
  * new and we have to decrease the reference count of the old ones.
  *
  * Returns 0 on success, -errno on failure.
  */
 static int calculate_l2_meta(BlockDriverState *bs, uint64_t host_cluster_offset,
                              uint64_t guest_offset, unsigned bytes,
                              uint64_t *l2_slice, QCowL2Meta **m, bool keep_old)
 {
     BDRVQcow2State *s = bs->opaque;
     int sc_index, l2_index = offset_to_l2_slice_index(s, guest_offset);
     uint64_t l2_entry, l2_bitmap;
     unsigned cow_start_from, cow_end_to;
     unsigned cow_start_to = offset_into_cluster(s, guest_offset);
     unsigned cow_end_from = cow_start_to + bytes;
     unsigned nb_clusters = size_to_clusters(s, cow_end_from);
     QCowL2Meta *old_m = *m;
     QCow2SubclusterType type;
     int i;
     bool skip_cow = keep_old;
 
     assert(nb_clusters <= s->l2_slice_size - l2_index);
 
     /* Check the type of all affected subclusters */
     for (i = 0; i < nb_clusters; i++) {
         l2_entry = get_l2_entry(s, l2_slice, l2_index + i);
         l2_bitmap = get_l2_bitmap(s, l2_slice, l2_index + i);
         if (skip_cow) {
             unsigned write_from = MAX(cow_start_to, i << s->cluster_bits);
             unsigned write_to = MIN(cow_end_from, (i + 1) << s->cluster_bits);
             int first_sc = offset_to_sc_index(s, write_from);
             int last_sc = offset_to_sc_index(s, write_to - 1);
             int cnt = qcow2_get_subcluster_range_type(bs, l2_entry, l2_bitmap,
                                                       first_sc, &type);
             /* Is any of the subclusters of type != QCOW2_SUBCLUSTER_NORMAL ? */
             if (type != QCOW2_SUBCLUSTER_NORMAL || first_sc + cnt <= last_sc) {
                 skip_cow = false;
             }
         } else {
             /* If we can't skip the cow we can still look for invalid entries */
             type = qcow2_get_subcluster_type(bs, l2_entry, l2_bitmap, 0);
         }
         if (type == QCOW2_SUBCLUSTER_INVALID) {
             int l1_index = offset_to_l1_index(s, guest_offset);
             uint64_t l2_offset = s->l1_table[l1_index] & L1E_OFFSET_MASK;
             qcow2_signal_corruption(bs, true, -1, -1, "Invalid cluster "
                                     "entry found (L2 offset: %#" PRIx64
                                     ", L2 index: %#x)",
                                     l2_offset, l2_index + i);
             return -EIO;
         }
     }
 
     if (skip_cow) {
         return 0;
     }
 
     /* Get the L2 entry of the first cluster */
     l2_entry = get_l2_entry(s, l2_slice, l2_index);
     l2_bitmap = get_l2_bitmap(s, l2_slice, l2_index);
     sc_index = offset_to_sc_index(s, guest_offset);
     type = qcow2_get_subcluster_type(bs, l2_entry, l2_bitmap, sc_index);
 
     if (!keep_old) {
         switch (type) {
         case QCOW2_SUBCLUSTER_COMPRESSED:
             cow_start_from = 0;
             break;
         case QCOW2_SUBCLUSTER_NORMAL:
         case QCOW2_SUBCLUSTER_ZERO_ALLOC:
         case QCOW2_SUBCLUSTER_UNALLOCATED_ALLOC:
             if (has_subclusters(s)) {
                 /* Skip all leading zero and unallocated subclusters */
                 uint32_t alloc_bitmap = l2_bitmap & QCOW_L2_BITMAP_ALL_ALLOC;
                 cow_start_from =
                     MIN(sc_index, ctz32(alloc_bitmap)) << s->subcluster_bits;
             } else {
                 cow_start_from = 0;
             }
             break;
         case QCOW2_SUBCLUSTER_ZERO_PLAIN:
         case QCOW2_SUBCLUSTER_UNALLOCATED_PLAIN:
             cow_start_from = sc_index << s->subcluster_bits;
             break;
         default:
             g_assert_not_reached();
         }
     } else {
         switch (type) {
         case QCOW2_SUBCLUSTER_NORMAL:
             cow_start_from = cow_start_to;
             break;
         case QCOW2_SUBCLUSTER_ZERO_ALLOC:
         case QCOW2_SUBCLUSTER_UNALLOCATED_ALLOC:
             cow_start_from = sc_index << s->subcluster_bits;
             break;
         default:
             g_assert_not_reached();
         }
     }
 
     /* Get the L2 entry of the last cluster */
     l2_index += nb_clusters - 1;
     l2_entry = get_l2_entry(s, l2_slice, l2_index);
     l2_bitmap = get_l2_bitmap(s, l2_slice, l2_index);
     sc_index = offset_to_sc_index(s, guest_offset + bytes - 1);
     type = qcow2_get_subcluster_type(bs, l2_entry, l2_bitmap, sc_index);
 
     if (!keep_old) {
         switch (type) {
         case QCOW2_SUBCLUSTER_COMPRESSED:
             cow_end_to = ROUND_UP(cow_end_from, s->cluster_size);
             break;
         case QCOW2_SUBCLUSTER_NORMAL:
         case QCOW2_SUBCLUSTER_ZERO_ALLOC:
         case QCOW2_SUBCLUSTER_UNALLOCATED_ALLOC:
             cow_end_to = ROUND_UP(cow_end_from, s->cluster_size);
             if (has_subclusters(s)) {
                 /* Skip all trailing zero and unallocated subclusters */
                 uint32_t alloc_bitmap = l2_bitmap & QCOW_L2_BITMAP_ALL_ALLOC;
                 cow_end_to -=
                     MIN(s->subclusters_per_cluster - sc_index - 1,
                         clz32(alloc_bitmap)) << s->subcluster_bits;
             }
             break;
         case QCOW2_SUBCLUSTER_ZERO_PLAIN:
         case QCOW2_SUBCLUSTER_UNALLOCATED_PLAIN:
             cow_end_to = ROUND_UP(cow_end_from, s->subcluster_size);
             break;
         default:
             g_assert_not_reached();
         }
     } else {
         switch (type) {
         case QCOW2_SUBCLUSTER_NORMAL:
             cow_end_to = cow_end_from;
             break;
         case QCOW2_SUBCLUSTER_ZERO_ALLOC:
         case QCOW2_SUBCLUSTER_UNALLOCATED_ALLOC:
             cow_end_to = ROUND_UP(cow_end_from, s->subcluster_size);
             break;
         default:
             g_assert_not_reached();
         }
     }
 
     *m = g_malloc0(sizeof(**m));
     **m = (QCowL2Meta) {
         .next           = old_m,
 
         .alloc_offset   = host_cluster_offset,
         .offset         = start_of_cluster(s, guest_offset),
         .nb_clusters    = nb_clusters,
 
         .keep_old_clusters = keep_old,
 
         .cow_start = {
             .offset     = cow_start_from,
             .nb_bytes   = cow_start_to - cow_start_from,
         },
         .cow_end = {
             .offset     = cow_end_from,
             .nb_bytes   = cow_end_to - cow_end_from,
         },
     };
 
     qemu_co_queue_init(&(*m)->dependent_requests);
     QLIST_INSERT_HEAD(&s->cluster_allocs, *m, next_in_flight);
 
     return 0;
 }
 
 /*
  * Returns true if writing to the cluster pointed to by @l2_entry
  * requires a new allocation (that is, if the cluster is unallocated
  * or has refcount > 1 and therefore cannot be written in-place).
  */
 static bool cluster_needs_new_alloc(BlockDriverState *bs, uint64_t l2_entry)
 {
     switch (qcow2_get_cluster_type(bs, l2_entry)) {
     case QCOW2_CLUSTER_NORMAL:
     case QCOW2_CLUSTER_ZERO_ALLOC:
         if (l2_entry & QCOW_OFLAG_COPIED) {
             return false;
         }
         /* fallthrough */
     case QCOW2_CLUSTER_UNALLOCATED:
     case QCOW2_CLUSTER_COMPRESSED:
     case QCOW2_CLUSTER_ZERO_PLAIN:
         return true;
     default:
         abort();
     }
 }
 
 /*
  * Returns the number of contiguous clusters that can be written to
  * using one single write request, starting from @l2_index.
  * At most @nb_clusters are checked.
  *
  * If @new_alloc is true this counts clusters that are either
  * unallocated, or allocated but with refcount > 1 (so they need to be
  * newly allocated and COWed).
  *
  * If @new_alloc is false this counts clusters that are already
  * allocated and can be overwritten in-place (this includes clusters
  * of type QCOW2_CLUSTER_ZERO_ALLOC).
  */
 static int count_single_write_clusters(BlockDriverState *bs, int nb_clusters,
                                        uint64_t *l2_slice, int l2_index,
                                        bool new_alloc)
 {
     BDRVQcow2State *s = bs->opaque;
     uint64_t l2_entry = get_l2_entry(s, l2_slice, l2_index);
     uint64_t expected_offset = l2_entry & L2E_OFFSET_MASK;
     int i;
 
     for (i = 0; i < nb_clusters; i++) {
         l2_entry = get_l2_entry(s, l2_slice, l2_index + i);
         if (cluster_needs_new_alloc(bs, l2_entry) != new_alloc) {
             break;
         }
         if (!new_alloc) {
             if (expected_offset != (l2_entry & L2E_OFFSET_MASK)) {
                 break;
             }
             expected_offset += s->cluster_size;
         }
     }
 
     assert(i <= nb_clusters);
     return i;
 }
 
 /*
  * Check if there already is an AIO write request in flight which allocates
  * the same cluster. In this case we need to wait until the previous
  * request has completed and updated the L2 table accordingly.
  *
  * Returns:
  *   0       if there was no dependency. *cur_bytes indicates the number of
  *           bytes from guest_offset that can be read before the next
  *           dependency must be processed (or the request is complete)
  *
  *   -EAGAIN if we had to wait for another request, previously gathered
  *           information on cluster allocation may be invalid now. The caller
  *           must start over anyway, so consider *cur_bytes undefined.
  */
 static int coroutine_fn handle_dependencies(BlockDriverState *bs,
                                             uint64_t guest_offset,
                                             uint64_t *cur_bytes, QCowL2Meta **m)
 {
     BDRVQcow2State *s = bs->opaque;
     QCowL2Meta *old_alloc;
     uint64_t bytes = *cur_bytes;
 
     QLIST_FOREACH(old_alloc, &s->cluster_allocs, next_in_flight) {
 
         uint64_t start = guest_offset;
         uint64_t end = start + bytes;
         uint64_t old_start = start_of_cluster(s, l2meta_cow_start(old_alloc));
         uint64_t old_end = ROUND_UP(l2meta_cow_end(old_alloc), s->cluster_size);
 
         if (end <= old_start || start >= old_end) {
             /* No intersection */
             continue;
         }
 
         if (old_alloc->keep_old_clusters &&
             (end <= l2meta_cow_start(old_alloc) ||
              start >= l2meta_cow_end(old_alloc)))
         {
             /*
              * Clusters intersect but COW areas don't. And cluster itself is
              * already allocated. So, there is no actual conflict.
              */
             continue;
         }
 
         /* Conflict */
 
         if (start < old_start) {
             /* Stop at the start of a running allocation */
             bytes = old_start - start;
         } else {
             bytes = 0;
         }
 
         /*
          * Stop if an l2meta already exists. After yielding, it wouldn't
          * be valid any more, so we'd have to clean up the old L2Metas
          * and deal with requests depending on them before starting to
          * gather new ones. Not worth the trouble.
          */
         if (bytes == 0 && *m) {
             *cur_bytes = 0;
             return 0;
         }
 
         if (bytes == 0) {
             /*
              * Wait for the dependency to complete. We need to recheck
              * the free/allocated clusters when we continue.
              */
             qemu_co_queue_wait(&old_alloc->dependent_requests, &s->lock);
             return -EAGAIN;
         }
     }
 
     /* Make sure that existing clusters and new allocations are only used up to
      * the next dependency if we shortened the request above */
     *cur_bytes = bytes;
 
     return 0;
 }
 
 /*
  * Checks how many already allocated clusters that don't require a new
  * allocation there are at the given guest_offset (up to *bytes).
  * If *host_offset is not INV_OFFSET, only physically contiguous clusters
  * beginning at this host offset are counted.
  *
  * Note that guest_offset may not be cluster aligned. In this case, the
  * returned *host_offset points to exact byte referenced by guest_offset and
  * therefore isn't cluster aligned as well.
  *
  * Returns:
  *   0:     if no allocated clusters are available at the given offset.
  *          *bytes is normally unchanged. It is set to 0 if the cluster
  *          is allocated and can be overwritten in-place but doesn't have
  *          the right physical offset.
  *
  *   1:     if allocated clusters that can be overwritten in place are
  *          available at the requested offset. *bytes may have decreased
  *          and describes the length of the area that can be written to.
  *
  *  -errno: in error cases
  */
 static int coroutine_fn handle_copied(BlockDriverState *bs,
     uint64_t guest_offset, uint64_t *host_offset, uint64_t *bytes,
     QCowL2Meta **m)
 {
     BDRVQcow2State *s = bs->opaque;
     int l2_index;
     uint64_t l2_entry, cluster_offset;
     uint64_t *l2_slice;
     uint64_t nb_clusters;
     unsigned int keep_clusters;
     int ret;
 
     trace_qcow2_handle_copied(qemu_coroutine_self(), guest_offset, *host_offset,
                               *bytes);
 
     assert(*host_offset == INV_OFFSET || offset_into_cluster(s, guest_offset)
                                       == offset_into_cluster(s, *host_offset));
 
     /*
      * Calculate the number of clusters to look for. We stop at L2 slice
      * boundaries to keep things simple.
      */
     nb_clusters =
         size_to_clusters(s, offset_into_cluster(s, guest_offset) + *bytes);
 
     l2_index = offset_to_l2_slice_index(s, guest_offset);
     nb_clusters = MIN(nb_clusters, s->l2_slice_size - l2_index);
     /* Limit total byte count to BDRV_REQUEST_MAX_BYTES */
     nb_clusters = MIN(nb_clusters, BDRV_REQUEST_MAX_BYTES >> s->cluster_bits);
 
     /* Find L2 entry for the first involved cluster */
     ret = get_cluster_table(bs, guest_offset, &l2_slice, &l2_index);
     if (ret < 0) {
         return ret;
     }
 
     l2_entry = get_l2_entry(s, l2_slice, l2_index);
     cluster_offset = l2_entry & L2E_OFFSET_MASK;
 
     if (!cluster_needs_new_alloc(bs, l2_entry)) {
         if (offset_into_cluster(s, cluster_offset)) {
             qcow2_signal_corruption(bs, true, -1, -1, "%s cluster offset "
                                     "%#" PRIx64 " unaligned (guest offset: %#"
                                     PRIx64 ")", l2_entry & QCOW_OFLAG_ZERO ?
                                     "Preallocated zero" : "Data",
                                     cluster_offset, guest_offset);
             ret = -EIO;
             goto out;
         }
 
         /* If a specific host_offset is required, check it */
         if (*host_offset != INV_OFFSET && cluster_offset != *host_offset) {
             *bytes = 0;
             ret = 0;
             goto out;
         }
 
         /* We keep all QCOW_OFLAG_COPIED clusters */
         keep_clusters = count_single_write_clusters(bs, nb_clusters, l2_slice,
                                                     l2_index, false);
         assert(keep_clusters <= nb_clusters);
 
         *bytes = MIN(*bytes,
                  keep_clusters * s->cluster_size
                  - offset_into_cluster(s, guest_offset));
         assert(*bytes != 0);
 
         ret = calculate_l2_meta(bs, cluster_offset, guest_offset,
                                 *bytes, l2_slice, m, true);
         if (ret < 0) {
             goto out;
         }
 
         ret = 1;
     } else {
         ret = 0;
     }
 
     /* Cleanup */
 out:
     qcow2_cache_put(s->l2_table_cache, (void **) &l2_slice);
 
     /* Only return a host offset if we actually made progress. Otherwise we
      * would make requirements for handle_alloc() that it can't fulfill */
     if (ret > 0) {
         *host_offset = cluster_offset + offset_into_cluster(s, guest_offset);
     }
 
     return ret;
 }
 
 /*
  * Allocates new clusters for the given guest_offset.
  *
  * At most *nb_clusters are allocated, and on return *nb_clusters is updated to
  * contain the number of clusters that have been allocated and are contiguous
  * in the image file.
  *
  * If *host_offset is not INV_OFFSET, it specifies the offset in the image file
  * at which the new clusters must start. *nb_clusters can be 0 on return in
  * this case if the cluster at host_offset is already in use. If *host_offset
  * is INV_OFFSET, the clusters can be allocated anywhere in the image file.
  *
  * *host_offset is updated to contain the offset into the image file at which
  * the first allocated cluster starts.
  *
  * Return 0 on success and -errno in error cases. -EAGAIN means that the
  * function has been waiting for another request and the allocation must be
  * restarted, but the whole request should not be failed.
  */
 static int do_alloc_cluster_offset(BlockDriverState *bs, uint64_t guest_offset,
                                    uint64_t *host_offset, uint64_t *nb_clusters)
 {
     BDRVQcow2State *s = bs->opaque;
 
     trace_qcow2_do_alloc_clusters_offset(qemu_coroutine_self(), guest_offset,
                                          *host_offset, *nb_clusters);
 
     if (has_data_file(bs)) {
         assert(*host_offset == INV_OFFSET ||
                *host_offset == start_of_cluster(s, guest_offset));
         *host_offset = start_of_cluster(s, guest_offset);
         return 0;
     }
 
     /* Allocate new clusters */
     trace_qcow2_cluster_alloc_phys(qemu_coroutine_self());
     if (*host_offset == INV_OFFSET) {
         int64_t cluster_offset =
             qcow2_alloc_clusters(bs, *nb_clusters * s->cluster_size);
         if (cluster_offset < 0) {
             return cluster_offset;
         }
         *host_offset = cluster_offset;
         return 0;
     } else {
         int64_t ret = qcow2_alloc_clusters_at(bs, *host_offset, *nb_clusters);
         if (ret < 0) {
             return ret;
         }
         *nb_clusters = ret;
         return 0;
     }
 }
 
 /*
  * Allocates new clusters for an area that is either still unallocated or
  * cannot be overwritten in-place. If *host_offset is not INV_OFFSET,
  * clusters are only allocated if the new allocation can match the specified
  * host offset.
  *
  * Note that guest_offset may not be cluster aligned. In this case, the
  * returned *host_offset points to exact byte referenced by guest_offset and
  * therefore isn't cluster aligned as well.
  *
  * Returns:
  *   0:     if no clusters could be allocated. *bytes is set to 0,
  *          *host_offset is left unchanged.
  *
  *   1:     if new clusters were allocated. *bytes may be decreased if the
  *          new allocation doesn't cover all of the requested area.
  *          *host_offset is updated to contain the host offset of the first
  *          newly allocated cluster.
  *
  *  -errno: in error cases
  */
 static int coroutine_fn handle_alloc(BlockDriverState *bs,
     uint64_t guest_offset, uint64_t *host_offset, uint64_t *bytes,
     QCowL2Meta **m)
 {
     BDRVQcow2State *s = bs->opaque;
     int l2_index;
     uint64_t *l2_slice;
     uint64_t nb_clusters;
     int ret;
 
     uint64_t alloc_cluster_offset;
 
     trace_qcow2_handle_alloc(qemu_coroutine_self(), guest_offset, *host_offset,
                              *bytes);
     assert(*bytes > 0);
 
     /*
      * Calculate the number of clusters to look for. We stop at L2 slice
      * boundaries to keep things simple.
      */
     nb_clusters =
         size_to_clusters(s, offset_into_cluster(s, guest_offset) + *bytes);
 
     l2_index = offset_to_l2_slice_index(s, guest_offset);
     nb_clusters = MIN(nb_clusters, s->l2_slice_size - l2_index);
     /* Limit total allocation byte count to BDRV_REQUEST_MAX_BYTES */
     nb_clusters = MIN(nb_clusters, BDRV_REQUEST_MAX_BYTES >> s->cluster_bits);
 
     /* Find L2 entry for the first involved cluster */
     ret = get_cluster_table(bs, guest_offset, &l2_slice, &l2_index);
     if (ret < 0) {
         return ret;
     }
 
     nb_clusters = count_single_write_clusters(bs, nb_clusters,
                                               l2_slice, l2_index, true);
 
     /* This function is only called when there were no non-COW clusters, so if
      * we can't find any unallocated or COW clusters either, something is
      * wrong with our code. */
     assert(nb_clusters > 0);
 
     /* Allocate at a given offset in the image file */
     alloc_cluster_offset = *host_offset == INV_OFFSET ? INV_OFFSET :
         start_of_cluster(s, *host_offset);
     ret = do_alloc_cluster_offset(bs, guest_offset, &alloc_cluster_offset,
                                   &nb_clusters);
     if (ret < 0) {
         goto out;
     }
 
     /* Can't extend contiguous allocation */
     if (nb_clusters == 0) {
         *bytes = 0;
         ret = 0;
         goto out;
     }
 
     assert(alloc_cluster_offset != INV_OFFSET);
 
     /*
      * Save info needed for meta data update.
      *
      * requested_bytes: Number of bytes from the start of the first
      * newly allocated cluster to the end of the (possibly shortened
      * before) write request.
      *
      * avail_bytes: Number of bytes from the start of the first
      * newly allocated to the end of the last newly allocated cluster.
      *
      * nb_bytes: The number of bytes from the start of the first
      * newly allocated cluster to the end of the area that the write
      * request actually writes to (excluding COW at the end)
      */
     uint64_t requested_bytes = *bytes + offset_into_cluster(s, guest_offset);
     int avail_bytes = nb_clusters << s->cluster_bits;
     int nb_bytes = MIN(requested_bytes, avail_bytes);
 
     *host_offset = alloc_cluster_offset + offset_into_cluster(s, guest_offset);
     *bytes = MIN(*bytes, nb_bytes - offset_into_cluster(s, guest_offset));
     assert(*bytes != 0);
 
     ret = calculate_l2_meta(bs, alloc_cluster_offset, guest_offset, *bytes,
                             l2_slice, m, false);
     if (ret < 0) {
         goto out;
     }
 
     ret = 1;
 
 out:
     qcow2_cache_put(s->l2_table_cache, (void **) &l2_slice);
     return ret;
 }
 
 /*
  * For a given area on the virtual disk defined by @offset and @bytes,
  * find the corresponding area on the qcow2 image, allocating new
  * clusters (or subclusters) if necessary. The result can span a
  * combination of allocated and previously unallocated clusters.
  *
  * Note that offset may not be cluster aligned. In this case, the returned
  * *host_offset points to exact byte referenced by offset and therefore
  * isn't cluster aligned as well.
  *
  * On return, @host_offset is set to the beginning of the requested
  * area. This area is guaranteed to be contiguous on the qcow2 file
  * but it can be smaller than initially requested. In this case @bytes
  * is updated with the actual size.
  *
  * If any clusters or subclusters were allocated then @m contains a
  * list with the information of all the affected regions. Note that
  * this can happen regardless of whether this function succeeds or
  * not. The caller is responsible for updating the L2 metadata of the
  * allocated clusters (on success) or freeing them (on failure), and
  * for clearing the contents of @m afterwards in both cases.
  *
  * If the request conflicts with another write request in flight, the coroutine
  * is queued and will be reentered when the dependency has completed.
  *
  * Return 0 on success and -errno in error cases
  */
 int coroutine_fn qcow2_alloc_host_offset(BlockDriverState *bs, uint64_t offset,
                                          unsigned int *bytes,
                                          uint64_t *host_offset,
                                          QCowL2Meta **m)
 {
     BDRVQcow2State *s = bs->opaque;
     uint64_t start, remaining;
     uint64_t cluster_offset;
     uint64_t cur_bytes;
     int ret;
 
     trace_qcow2_alloc_clusters_offset(qemu_coroutine_self(), offset, *bytes);
 
 again:
     start = offset;
     remaining = *bytes;
     cluster_offset = INV_OFFSET;
     *host_offset = INV_OFFSET;
     cur_bytes = 0;
     *m = NULL;
 
     while (true) {
 
         if (*host_offset == INV_OFFSET && cluster_offset != INV_OFFSET) {
             *host_offset = cluster_offset;
         }
 
         assert(remaining >= cur_bytes);
 
         start           += cur_bytes;
         remaining       -= cur_bytes;
 
         if (cluster_offset != INV_OFFSET) {
             cluster_offset += cur_bytes;
         }
 
         if (remaining == 0) {
             break;
         }
 
         cur_bytes = remaining;
 
         /*
          * Now start gathering as many contiguous clusters as possible:
          *
          * 1. Check for overlaps with in-flight allocations
          *
          *      a) Overlap not in the first cluster -> shorten this request and
          *         let the caller handle the rest in its next loop iteration.
          *
          *      b) Real overlaps of two requests. Yield and restart the search
          *         for contiguous clusters (the situation could have changed
          *         while we were sleeping)
          *
          *      c) TODO: Request starts in the same cluster as the in-flight
          *         allocation ends. Shorten the COW of the in-fight allocation,
          *         set cluster_offset to write to the same cluster and set up
          *         the right synchronisation between the in-flight request and
          *         the new one.
          */
         ret = handle_dependencies(bs, start, &cur_bytes, m);
         if (ret == -EAGAIN) {
             /* Currently handle_dependencies() doesn't yield if we already had
              * an allocation. If it did, we would have to clean up the L2Meta
              * structs before starting over. */
             assert(*m == NULL);
             goto again;
         } else if (ret < 0) {
             return ret;
         } else if (cur_bytes == 0) {
             break;
         } else {
             /* handle_dependencies() may have decreased cur_bytes (shortened
              * the allocations below) so that the next dependency is processed
              * correctly during the next loop iteration. */
         }
 
         /*
          * 2. Count contiguous COPIED clusters.
          */
         ret = handle_copied(bs, start, &cluster_offset, &cur_bytes, m);
         if (ret < 0) {
             return ret;
         } else if (ret) {
             continue;
         } else if (cur_bytes == 0) {
             break;
         }
 
         /*
          * 3. If the request still hasn't completed, allocate new clusters,
          *    considering any cluster_offset of steps 1c or 2.
          */
         ret = handle_alloc(bs, start, &cluster_offset, &cur_bytes, m);
         if (ret < 0) {
             return ret;
         } else if (ret) {
             continue;
         } else {
             assert(cur_bytes == 0);
             break;
         }
     }
 
     *bytes -= remaining;
     assert(*bytes > 0);
     assert(*host_offset != INV_OFFSET);
     assert(offset_into_cluster(s, *host_offset) ==
            offset_into_cluster(s, offset));
 
     return 0;
 }
 
 /*
  * This discards as many clusters of nb_clusters as possible at once (i.e.
  * all clusters in the same L2 slice) and returns the number of discarded
  * clusters.
  */
 static int discard_in_l2_slice(BlockDriverState *bs, uint64_t offset,
                                uint64_t nb_clusters,
                                enum qcow2_discard_type type, bool full_discard)
 {
     BDRVQcow2State *s = bs->opaque;
     uint64_t *l2_slice;
     int l2_index;
     int ret;
     int i;
 
     ret = get_cluster_table(bs, offset, &l2_slice, &l2_index);
     if (ret < 0) {
         return ret;
     }
 
     /* Limit nb_clusters to one L2 slice */
     nb_clusters = MIN(nb_clusters, s->l2_slice_size - l2_index);
     assert(nb_clusters <= INT_MAX);
 
     for (i = 0; i < nb_clusters; i++) {
         uint64_t old_l2_entry = get_l2_entry(s, l2_slice, l2_index + i);
         uint64_t old_l2_bitmap = get_l2_bitmap(s, l2_slice, l2_index + i);
         uint64_t new_l2_entry = old_l2_entry;
         uint64_t new_l2_bitmap = old_l2_bitmap;
         QCow2ClusterType cluster_type =
             qcow2_get_cluster_type(bs, old_l2_entry);
 
         /*
          * If full_discard is true, the cluster should not read back as zeroes,
          * but rather fall through to the backing file.
          *
          * If full_discard is false, make sure that a discarded area reads back
          * as zeroes for v3 images (we cannot do it for v2 without actually
          * writing a zero-filled buffer). We can skip the operation if the
          * cluster is already marked as zero, or if it's unallocated and we
          * don't have a backing file.
          *
          * TODO We might want to use bdrv_block_status(bs) here, but we're
          * holding s->lock, so that doesn't work today.
          */
         if (full_discard) {
             new_l2_entry = new_l2_bitmap = 0;
         } else if (bs->backing || qcow2_cluster_is_allocated(cluster_type)) {
             if (has_subclusters(s)) {
                 new_l2_entry = 0;
                 new_l2_bitmap = QCOW_L2_BITMAP_ALL_ZEROES;
             } else {
                 new_l2_entry = s->qcow_version >= 3 ? QCOW_OFLAG_ZERO : 0;
             }
         }
 
         if (old_l2_entry == new_l2_entry && old_l2_bitmap == new_l2_bitmap) {
             continue;
         }
 
         /* First remove L2 entries */
         qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_slice);
         set_l2_entry(s, l2_slice, l2_index + i, new_l2_entry);
         if (has_subclusters(s)) {
             set_l2_bitmap(s, l2_slice, l2_index + i, new_l2_bitmap);
         }
         /* Then decrease the refcount */
         qcow2_free_any_cluster(bs, old_l2_entry, type);
     }
 
     qcow2_cache_put(s->l2_table_cache, (void **) &l2_slice);
 
     return nb_clusters;
 }
 
 int qcow2_cluster_discard(BlockDriverState *bs, uint64_t offset,
                           uint64_t bytes, enum qcow2_discard_type type,
                           bool full_discard)
 {
     BDRVQcow2State *s = bs->opaque;
     uint64_t end_offset = offset + bytes;
     uint64_t nb_clusters;
     int64_t cleared;
     int ret;
 
     /* Caller must pass aligned values, except at image end */
     assert(QEMU_IS_ALIGNED(offset, s->cluster_size));
     assert(QEMU_IS_ALIGNED(end_offset, s->cluster_size) ||
            end_offset == bs->total_sectors << BDRV_SECTOR_BITS);
 
     nb_clusters = size_to_clusters(s, bytes);
 
     s->cache_discards = true;
 
     /* Each L2 slice is handled by its own loop iteration */
     while (nb_clusters > 0) {
         cleared = discard_in_l2_slice(bs, offset, nb_clusters, type,
                                       full_discard);
         if (cleared < 0) {
             ret = cleared;
             goto fail;
         }
 
         nb_clusters -= cleared;
         offset += (cleared * s->cluster_size);
     }
 
     ret = 0;
 fail:
     s->cache_discards = false;
     qcow2_process_discards(bs, ret);
 
     return ret;
 }
 
 /*
  * This zeroes as many clusters of nb_clusters as possible at once (i.e.
  * all clusters in the same L2 slice) and returns the number of zeroed
  * clusters.
  */
 static int zero_in_l2_slice(BlockDriverState *bs, uint64_t offset,
                             uint64_t nb_clusters, int flags)
 {
     BDRVQcow2State *s = bs->opaque;
     uint64_t *l2_slice;
     int l2_index;
     int ret;
     int i;
 
     ret = get_cluster_table(bs, offset, &l2_slice, &l2_index);
     if (ret < 0) {
         return ret;
     }
 
     /* Limit nb_clusters to one L2 slice */
     nb_clusters = MIN(nb_clusters, s->l2_slice_size - l2_index);
     assert(nb_clusters <= INT_MAX);
 
     for (i = 0; i < nb_clusters; i++) {
         uint64_t old_l2_entry = get_l2_entry(s, l2_slice, l2_index + i);
         uint64_t old_l2_bitmap = get_l2_bitmap(s, l2_slice, l2_index + i);
         QCow2ClusterType type = qcow2_get_cluster_type(bs, old_l2_entry);
         bool unmap = (type == QCOW2_CLUSTER_COMPRESSED) ||
             ((flags & BDRV_REQ_MAY_UNMAP) && qcow2_cluster_is_allocated(type));
         uint64_t new_l2_entry = unmap ? 0 : old_l2_entry;
         uint64_t new_l2_bitmap = old_l2_bitmap;
 
         if (has_subclusters(s)) {
             new_l2_bitmap = QCOW_L2_BITMAP_ALL_ZEROES;
         } else {
             new_l2_entry |= QCOW_OFLAG_ZERO;
         }
 
         if (old_l2_entry == new_l2_entry && old_l2_bitmap == new_l2_bitmap) {
             continue;
         }
 
         /* First update L2 entries */
         qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_slice);
         set_l2_entry(s, l2_slice, l2_index + i, new_l2_entry);
         if (has_subclusters(s)) {
             set_l2_bitmap(s, l2_slice, l2_index + i, new_l2_bitmap);
         }
 
         /* Then decrease the refcount */
         if (unmap) {
             qcow2_free_any_cluster(bs, old_l2_entry, QCOW2_DISCARD_REQUEST);
         }
     }
 
     qcow2_cache_put(s->l2_table_cache, (void **) &l2_slice);
 
     return nb_clusters;
 }
 
 static int zero_l2_subclusters(BlockDriverState *bs, uint64_t offset,
                                unsigned nb_subclusters)
 {
     BDRVQcow2State *s = bs->opaque;
     uint64_t *l2_slice;
     uint64_t old_l2_bitmap, l2_bitmap;
     int l2_index, ret, sc = offset_to_sc_index(s, offset);
 
     /* For full clusters use zero_in_l2_slice() instead */
     assert(nb_subclusters > 0 && nb_subclusters < s->subclusters_per_cluster);
     assert(sc + nb_subclusters <= s->subclusters_per_cluster);
     assert(offset_into_subcluster(s, offset) == 0);
 
     ret = get_cluster_table(bs, offset, &l2_slice, &l2_index);
     if (ret < 0) {
         return ret;
     }
 
     switch (qcow2_get_cluster_type(bs, get_l2_entry(s, l2_slice, l2_index))) {
     case QCOW2_CLUSTER_COMPRESSED:
         ret = -ENOTSUP; /* We cannot partially zeroize compressed clusters */
         goto out;
     case QCOW2_CLUSTER_NORMAL:
     case QCOW2_CLUSTER_UNALLOCATED:
         break;
     default:
         g_assert_not_reached();
     }
 
     old_l2_bitmap = l2_bitmap = get_l2_bitmap(s, l2_slice, l2_index);
 
     l2_bitmap |=  QCOW_OFLAG_SUB_ZERO_RANGE(sc, sc + nb_subclusters);
     l2_bitmap &= ~QCOW_OFLAG_SUB_ALLOC_RANGE(sc, sc + nb_subclusters);
 
     if (old_l2_bitmap != l2_bitmap) {
         set_l2_bitmap(s, l2_slice, l2_index, l2_bitmap);
         qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_slice);
     }
 
     ret = 0;
 out:
     qcow2_cache_put(s->l2_table_cache, (void **) &l2_slice);
 
     return ret;
 }
 
 int coroutine_fn qcow2_subcluster_zeroize(BlockDriverState *bs, uint64_t offset,
                                           uint64_t bytes, int flags)
 {
     BDRVQcow2State *s = bs->opaque;
     uint64_t end_offset = offset + bytes;
     uint64_t nb_clusters;
     unsigned head, tail;
     int64_t cleared;
     int ret;
 
     /* If we have to stay in sync with an external data file, zero out
      * s->data_file first. */
     if (data_file_is_raw(bs)) {
         assert(has_data_file(bs));
         ret = bdrv_co_pwrite_zeroes(s->data_file, offset, bytes, flags);
         if (ret < 0) {
             return ret;
         }
     }
 
     /* Caller must pass aligned values, except at image end */
     assert(offset_into_subcluster(s, offset) == 0);
     assert(offset_into_subcluster(s, end_offset) == 0 ||
            end_offset >= bs->total_sectors << BDRV_SECTOR_BITS);
 
     /*
      * The zero flag is only supported by version 3 and newer. However, if we
      * have no backing file, we can resort to discard in version 2.
      */
     if (s->qcow_version < 3) {
         if (!bs->backing) {
             return qcow2_cluster_discard(bs, offset, bytes,
                                          QCOW2_DISCARD_REQUEST, false);
         }
         return -ENOTSUP;
     }
 
     head = MIN(end_offset, ROUND_UP(offset, s->cluster_size)) - offset;
     offset += head;
 
     tail = (end_offset >= bs->total_sectors << BDRV_SECTOR_BITS) ? 0 :
         end_offset - MAX(offset, start_of_cluster(s, end_offset));
     end_offset -= tail;
 
     s->cache_discards = true;
 
     if (head) {
         ret = zero_l2_subclusters(bs, offset - head,
                                   size_to_subclusters(s, head));
         if (ret < 0) {
             goto fail;
         }
     }
 
     /* Each L2 slice is handled by its own loop iteration */
     nb_clusters = size_to_clusters(s, end_offset - offset);
 
     while (nb_clusters > 0) {
         cleared = zero_in_l2_slice(bs, offset, nb_clusters, flags);
         if (cleared < 0) {
             ret = cleared;
             goto fail;
         }
 
         nb_clusters -= cleared;
         offset += (cleared * s->cluster_size);
     }
 
     if (tail) {
         ret = zero_l2_subclusters(bs, end_offset, size_to_subclusters(s, tail));
         if (ret < 0) {
             goto fail;
         }
     }
 
     ret = 0;
 fail:
     s->cache_discards = false;
     qcow2_process_discards(bs, ret);
 
     return ret;
 }
 
 /*
  * Expands all zero clusters in a specific L1 table (or deallocates them, for
  * non-backed non-pre-allocated zero clusters).
  *
  * l1_entries and *visited_l1_entries are used to keep track of progress for
  * status_cb(). l1_entries contains the total number of L1 entries and
  * *visited_l1_entries counts all visited L1 entries.
  */
 static int expand_zero_clusters_in_l1(BlockDriverState *bs, uint64_t *l1_table,
                                       int l1_size, int64_t *visited_l1_entries,
                                       int64_t l1_entries,
                                       BlockDriverAmendStatusCB *status_cb,
                                       void *cb_opaque)
 {
     BDRVQcow2State *s = bs->opaque;
     bool is_active_l1 = (l1_table == s->l1_table);
     uint64_t *l2_slice = NULL;
     unsigned slice, slice_size2, n_slices;
     int ret;
     int i, j;
 
     /* qcow2_downgrade() is not allowed in images with subclusters */
     assert(!has_subclusters(s));
 
     slice_size2 = s->l2_slice_size * l2_entry_size(s);
     n_slices = s->cluster_size / slice_size2;
 
     if (!is_active_l1) {
         /* inactive L2 tables require a buffer to be stored in when loading
          * them from disk */
         l2_slice = qemu_try_blockalign(bs->file->bs, slice_size2);
         if (l2_slice == NULL) {
             return -ENOMEM;
         }
     }
 
     for (i = 0; i < l1_size; i++) {
         uint64_t l2_offset = l1_table[i] & L1E_OFFSET_MASK;
         uint64_t l2_refcount;
 
         if (!l2_offset) {
             /* unallocated */
             (*visited_l1_entries)++;
             if (status_cb) {
                 status_cb(bs, *visited_l1_entries, l1_entries, cb_opaque);
             }
             continue;
         }
 
         if (offset_into_cluster(s, l2_offset)) {
             qcow2_signal_corruption(bs, true, -1, -1, "L2 table offset %#"
                                     PRIx64 " unaligned (L1 index: %#x)",
                                     l2_offset, i);
             ret = -EIO;
             goto fail;
         }
 
         ret = qcow2_get_refcount(bs, l2_offset >> s->cluster_bits,
                                  &l2_refcount);
         if (ret < 0) {
             goto fail;
         }
 
         for (slice = 0; slice < n_slices; slice++) {
             uint64_t slice_offset = l2_offset + slice * slice_size2;
             bool l2_dirty = false;
             if (is_active_l1) {
                 /* get active L2 tables from cache */
                 ret = qcow2_cache_get(bs, s->l2_table_cache, slice_offset,
                                       (void **)&l2_slice);
             } else {
                 /* load inactive L2 tables from disk */
                 ret = bdrv_pread(bs->file, slice_offset, slice_size2,
                                  l2_slice, 0);
             }
             if (ret < 0) {
                 goto fail;
             }
 
             for (j = 0; j < s->l2_slice_size; j++) {
                 uint64_t l2_entry = get_l2_entry(s, l2_slice, j);
                 int64_t offset = l2_entry & L2E_OFFSET_MASK;
                 QCow2ClusterType cluster_type =
                     qcow2_get_cluster_type(bs, l2_entry);
 
                 if (cluster_type != QCOW2_CLUSTER_ZERO_PLAIN &&
                     cluster_type != QCOW2_CLUSTER_ZERO_ALLOC) {
                     continue;
                 }
 
                 if (cluster_type == QCOW2_CLUSTER_ZERO_PLAIN) {
                     if (!bs->backing) {
                         /*
                          * not backed; therefore we can simply deallocate the
                          * cluster. No need to call set_l2_bitmap(), this
                          * function doesn't support images with subclusters.
                          */
                         set_l2_entry(s, l2_slice, j, 0);
                         l2_dirty = true;
                         continue;
                     }
 
                     offset = qcow2_alloc_clusters(bs, s->cluster_size);
                     if (offset < 0) {
                         ret = offset;
                         goto fail;
                     }
 
                     /* The offset must fit in the offset field */
                     assert((offset & L2E_OFFSET_MASK) == offset);
 
                     if (l2_refcount > 1) {
                         /* For shared L2 tables, set the refcount accordingly
                          * (it is already 1 and needs to be l2_refcount) */
                         ret = qcow2_update_cluster_refcount(
                             bs, offset >> s->cluster_bits,
                             refcount_diff(1, l2_refcount), false,
                             QCOW2_DISCARD_OTHER);
                         if (ret < 0) {
                             qcow2_free_clusters(bs, offset, s->cluster_size,
                                                 QCOW2_DISCARD_OTHER);
                             goto fail;
                         }
                     }
                 }
 
                 if (offset_into_cluster(s, offset)) {
                     int l2_index = slice * s->l2_slice_size + j;
                     qcow2_signal_corruption(
                         bs, true, -1, -1,
                         "Cluster allocation offset "
                         "%#" PRIx64 " unaligned (L2 offset: %#"
                         PRIx64 ", L2 index: %#x)", offset,
                         l2_offset, l2_index);
                     if (cluster_type == QCOW2_CLUSTER_ZERO_PLAIN) {
                         qcow2_free_clusters(bs, offset, s->cluster_size,
                                             QCOW2_DISCARD_ALWAYS);
                     }
                     ret = -EIO;
                     goto fail;
                 }
 
                 ret = qcow2_pre_write_overlap_check(bs, 0, offset,
                                                     s->cluster_size, true);
                 if (ret < 0) {
                     if (cluster_type == QCOW2_CLUSTER_ZERO_PLAIN) {
                         qcow2_free_clusters(bs, offset, s->cluster_size,
                                             QCOW2_DISCARD_ALWAYS);
                     }
                     goto fail;
                 }
 
                 ret = bdrv_pwrite_zeroes(s->data_file, offset,
                                          s->cluster_size, 0);
                 if (ret < 0) {
                     if (cluster_type == QCOW2_CLUSTER_ZERO_PLAIN) {
                         qcow2_free_clusters(bs, offset, s->cluster_size,
                                             QCOW2_DISCARD_ALWAYS);
                     }
                     goto fail;
                 }
 
                 if (l2_refcount == 1) {
                     set_l2_entry(s, l2_slice, j, offset | QCOW_OFLAG_COPIED);
                 } else {
                     set_l2_entry(s, l2_slice, j, offset);
                 }
                 /*
                  * No need to call set_l2_bitmap() after set_l2_entry() because
                  * this function doesn't support images with subclusters.
                  */
                 l2_dirty = true;
             }
 
             if (is_active_l1) {
                 if (l2_dirty) {
                     qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_slice);
                     qcow2_cache_depends_on_flush(s->l2_table_cache);
                 }
                 qcow2_cache_put(s->l2_table_cache, (void **) &l2_slice);
             } else {
                 if (l2_dirty) {
                     ret = qcow2_pre_write_overlap_check(
                         bs, QCOW2_OL_INACTIVE_L2 | QCOW2_OL_ACTIVE_L2,
                         slice_offset, slice_size2, false);
                     if (ret < 0) {
                         goto fail;
                     }
 
                     ret = bdrv_pwrite(bs->file, slice_offset, slice_size2,
                                       l2_slice, 0);
                     if (ret < 0) {
                         goto fail;
                     }
                 }
             }
         }
 
         (*visited_l1_entries)++;
         if (status_cb) {
             status_cb(bs, *visited_l1_entries, l1_entries, cb_opaque);
         }
     }
 
     ret = 0;
 
 fail:
     if (l2_slice) {
         if (!is_active_l1) {
             qemu_vfree(l2_slice);
         } else {
             qcow2_cache_put(s->l2_table_cache, (void **) &l2_slice);
         }
     }
     return ret;
 }
 
 /*
  * For backed images, expands all zero clusters on the image. For non-backed
  * images, deallocates all non-pre-allocated zero clusters (and claims the
  * allocation for pre-allocated ones). This is important for downgrading to a
  * qcow2 version which doesn't yet support metadata zero clusters.
  */
 int qcow2_expand_zero_clusters(BlockDriverState *bs,
                                BlockDriverAmendStatusCB *status_cb,
                                void *cb_opaque)
 {
     BDRVQcow2State *s = bs->opaque;
     uint64_t *l1_table = NULL;
     int64_t l1_entries = 0, visited_l1_entries = 0;
     int ret;
     int i, j;
 
     if (status_cb) {
         l1_entries = s->l1_size;
         for (i = 0; i < s->nb_snapshots; i++) {
             l1_entries += s->snapshots[i].l1_size;
         }
     }
 
     ret = expand_zero_clusters_in_l1(bs, s->l1_table, s->l1_size,
                                      &visited_l1_entries, l1_entries,
                                      status_cb, cb_opaque);
     if (ret < 0) {
         goto fail;
     }
 
     /* Inactive L1 tables may point to active L2 tables - therefore it is
      * necessary to flush the L2 table cache before trying to access the L2
      * tables pointed to by inactive L1 entries (else we might try to expand
      * zero clusters that have already been expanded); furthermore, it is also
      * necessary to empty the L2 table cache, since it may contain tables which
      * are now going to be modified directly on disk, bypassing the cache.
      * qcow2_cache_empty() does both for us. */
     ret = qcow2_cache_empty(bs, s->l2_table_cache);
     if (ret < 0) {
         goto fail;
     }
 
     for (i = 0; i < s->nb_snapshots; i++) {
         int l1_size2;
         uint64_t *new_l1_table;
         Error *local_err = NULL;
 
         ret = qcow2_validate_table(bs, s->snapshots[i].l1_table_offset,
                                    s->snapshots[i].l1_size, L1E_SIZE,
                                    QCOW_MAX_L1_SIZE, "Snapshot L1 table",
                                    &local_err);
         if (ret < 0) {
             error_report_err(local_err);
             goto fail;
         }
 
         l1_size2 = s->snapshots[i].l1_size * L1E_SIZE;
         new_l1_table = g_try_realloc(l1_table, l1_size2);
 
         if (!new_l1_table) {
             ret = -ENOMEM;
             goto fail;
         }
 
         l1_table = new_l1_table;
 
         ret = bdrv_pread(bs->file, s->snapshots[i].l1_table_offset, l1_size2,
                          l1_table, 0);
         if (ret < 0) {
             goto fail;
         }
 
         for (j = 0; j < s->snapshots[i].l1_size; j++) {
             be64_to_cpus(&l1_table[j]);
         }
 
         ret = expand_zero_clusters_in_l1(bs, l1_table, s->snapshots[i].l1_size,
                                          &visited_l1_entries, l1_entries,
                                          status_cb, cb_opaque);
         if (ret < 0) {
             goto fail;
         }
     }
 
     ret = 0;
 
 fail:
     g_free(l1_table);
     return ret;
 }
 
 void qcow2_parse_compressed_l2_entry(BlockDriverState *bs, uint64_t l2_entry,
                                      uint64_t *coffset, int *csize)
 {
     BDRVQcow2State *s = bs->opaque;
     int nb_csectors;
 
     assert(qcow2_get_cluster_type(bs, l2_entry) == QCOW2_CLUSTER_COMPRESSED);
 
     *coffset = l2_entry & s->cluster_offset_mask;
 
     nb_csectors = ((l2_entry >> s->csize_shift) & s->csize_mask) + 1;
     *csize = nb_csectors * QCOW2_COMPRESSED_SECTOR_SIZE -
         (*coffset & (QCOW2_COMPRESSED_SECTOR_SIZE - 1));
 }