qemu

block-backend.c (70171B)

---

 /*
  * QEMU Block backends
  *
  * Copyright (C) 2014-2016 Red Hat, Inc.
  *
  * Authors:
  *  Markus Armbruster <armbru@redhat.com>,
  *
  * This work is licensed under the terms of the GNU LGPL, version 2.1
  * or later.  See the COPYING.LIB file in the top-level directory.
  */
 
 #include "qemu/osdep.h"
 #include "sysemu/block-backend.h"
 #include "block/block_int.h"
 #include "block/blockjob.h"
 #include "block/coroutines.h"
 #include "block/throttle-groups.h"
 #include "hw/qdev-core.h"
 #include "sysemu/blockdev.h"
 #include "sysemu/runstate.h"
 #include "sysemu/replay.h"
 #include "qapi/error.h"
 #include "qapi/qapi-events-block.h"
 #include "qemu/id.h"
 #include "qemu/main-loop.h"
 #include "qemu/option.h"
 #include "trace.h"
 #include "migration/misc.h"
 
 /* Number of coroutines to reserve per attached device model */
 #define COROUTINE_POOL_RESERVATION 64
 
 #define NOT_DONE 0x7fffffff /* used while emulated sync operation in progress */
 
 static AioContext *blk_aiocb_get_aio_context(BlockAIOCB *acb);
 
 typedef struct BlockBackendAioNotifier {
     void (*attached_aio_context)(AioContext *new_context, void *opaque);
     void (*detach_aio_context)(void *opaque);
     void *opaque;
     QLIST_ENTRY(BlockBackendAioNotifier) list;
 } BlockBackendAioNotifier;
 
 struct BlockBackend {
     char *name;
     int refcnt;
     BdrvChild *root;
     AioContext *ctx;
     DriveInfo *legacy_dinfo;    /* null unless created by drive_new() */
     QTAILQ_ENTRY(BlockBackend) link;         /* for block_backends */
     QTAILQ_ENTRY(BlockBackend) monitor_link; /* for monitor_block_backends */
     BlockBackendPublic public;
 
     DeviceState *dev;           /* attached device model, if any */
     const BlockDevOps *dev_ops;
     void *dev_opaque;
 
     /* If the BDS tree is removed, some of its options are stored here (which
      * can be used to restore those options in the new BDS on insert) */
     BlockBackendRootState root_state;
 
     bool enable_write_cache;
 
     /* I/O stats (display with "info blockstats"). */
     BlockAcctStats stats;
 
     BlockdevOnError on_read_error, on_write_error;
     bool iostatus_enabled;
     BlockDeviceIoStatus iostatus;
 
     uint64_t perm;
     uint64_t shared_perm;
     bool disable_perm;
 
     bool allow_aio_context_change;
     bool allow_write_beyond_eof;
 
     /* Protected by BQL */
     NotifierList remove_bs_notifiers, insert_bs_notifiers;
     QLIST_HEAD(, BlockBackendAioNotifier) aio_notifiers;
 
     int quiesce_counter;
     CoQueue queued_requests;
     bool disable_request_queuing;
 
     VMChangeStateEntry *vmsh;
     bool force_allow_inactivate;
 
     /* Number of in-flight aio requests.  BlockDriverState also counts
      * in-flight requests but aio requests can exist even when blk->root is
      * NULL, so we cannot rely on its counter for that case.
      * Accessed with atomic ops.
      */
     unsigned int in_flight;
 };
 
 typedef struct BlockBackendAIOCB {
     BlockAIOCB common;
     BlockBackend *blk;
     int ret;
 } BlockBackendAIOCB;
 
 static const AIOCBInfo block_backend_aiocb_info = {
     .get_aio_context = blk_aiocb_get_aio_context,
     .aiocb_size = sizeof(BlockBackendAIOCB),
 };
 
 static void drive_info_del(DriveInfo *dinfo);
 static BlockBackend *bdrv_first_blk(BlockDriverState *bs);
 
 /* All BlockBackends. Protected by BQL. */
 static QTAILQ_HEAD(, BlockBackend) block_backends =
     QTAILQ_HEAD_INITIALIZER(block_backends);
 
 /*
  * All BlockBackends referenced by the monitor and which are iterated through by
  * blk_next(). Protected by BQL.
  */
 static QTAILQ_HEAD(, BlockBackend) monitor_block_backends =
     QTAILQ_HEAD_INITIALIZER(monitor_block_backends);
 
 static void blk_root_inherit_options(BdrvChildRole role, bool parent_is_format,
                                      int *child_flags, QDict *child_options,
                                      int parent_flags, QDict *parent_options)
 {
     /* We're not supposed to call this function for root nodes */
     abort();
 }
 static void blk_root_drained_begin(BdrvChild *child);
 static bool blk_root_drained_poll(BdrvChild *child);
 static void blk_root_drained_end(BdrvChild *child, int *drained_end_counter);
 
 static void blk_root_change_media(BdrvChild *child, bool load);
 static void blk_root_resize(BdrvChild *child);
 
 static bool blk_root_change_aio_ctx(BdrvChild *child, AioContext *ctx,
                                     GHashTable *visited, Transaction *tran,
                                     Error **errp);
 
 static char *blk_root_get_parent_desc(BdrvChild *child)
 {
     BlockBackend *blk = child->opaque;
     g_autofree char *dev_id = NULL;
 
     if (blk->name) {
         return g_strdup_printf("block device '%s'", blk->name);
     }
 
     dev_id = blk_get_attached_dev_id(blk);
     if (*dev_id) {
         return g_strdup_printf("block device '%s'", dev_id);
     } else {
         /* TODO Callback into the BB owner for something more detailed */
         return g_strdup("an unnamed block device");
     }
 }
 
 static const char *blk_root_get_name(BdrvChild *child)
 {
     return blk_name(child->opaque);
 }
 
 static void blk_vm_state_changed(void *opaque, bool running, RunState state)
 {
     Error *local_err = NULL;
     BlockBackend *blk = opaque;
 
     if (state == RUN_STATE_INMIGRATE) {
         return;
     }
 
     qemu_del_vm_change_state_handler(blk->vmsh);
     blk->vmsh = NULL;
     blk_set_perm(blk, blk->perm, blk->shared_perm, &local_err);
     if (local_err) {
         error_report_err(local_err);
     }
 }
 
 /*
  * Notifies the user of the BlockBackend that migration has completed. qdev
  * devices can tighten their permissions in response (specifically revoke
  * shared write permissions that we needed for storage migration).
  *
  * If an error is returned, the VM cannot be allowed to be resumed.
  */
 static void blk_root_activate(BdrvChild *child, Error **errp)
 {
     BlockBackend *blk = child->opaque;
     Error *local_err = NULL;
     uint64_t saved_shared_perm;
 
     if (!blk->disable_perm) {
         return;
     }
 
     blk->disable_perm = false;
 
     /*
      * blk->shared_perm contains the permissions we want to share once
      * migration is really completely done.  For now, we need to share
      * all; but we also need to retain blk->shared_perm, which is
      * overwritten by a successful blk_set_perm() call.  Save it and
      * restore it below.
      */
     saved_shared_perm = blk->shared_perm;
 
     blk_set_perm(blk, blk->perm, BLK_PERM_ALL, &local_err);
     if (local_err) {
         error_propagate(errp, local_err);
         blk->disable_perm = true;
         return;
     }
     blk->shared_perm = saved_shared_perm;
 
     if (runstate_check(RUN_STATE_INMIGRATE)) {
         /* Activation can happen when migration process is still active, for
          * example when nbd_server_add is called during non-shared storage
          * migration. Defer the shared_perm update to migration completion. */
         if (!blk->vmsh) {
             blk->vmsh = qemu_add_vm_change_state_handler(blk_vm_state_changed,
                                                          blk);
         }
         return;
     }
 
     blk_set_perm(blk, blk->perm, blk->shared_perm, &local_err);
     if (local_err) {
         error_propagate(errp, local_err);
         blk->disable_perm = true;
         return;
     }
 }
 
 void blk_set_force_allow_inactivate(BlockBackend *blk)
 {
     GLOBAL_STATE_CODE();
     blk->force_allow_inactivate = true;
 }
 
 static bool blk_can_inactivate(BlockBackend *blk)
 {
     /* If it is a guest device, inactivate is ok. */
     if (blk->dev || blk_name(blk)[0]) {
         return true;
     }
 
     /* Inactivating means no more writes to the image can be done,
      * even if those writes would be changes invisible to the
      * guest.  For block job BBs that satisfy this, we can just allow
      * it.  This is the case for mirror job source, which is required
      * by libvirt non-shared block migration. */
     if (!(blk->perm & (BLK_PERM_WRITE | BLK_PERM_WRITE_UNCHANGED))) {
         return true;
     }
 
     return blk->force_allow_inactivate;
 }
 
 static int blk_root_inactivate(BdrvChild *child)
 {
     BlockBackend *blk = child->opaque;
 
     if (blk->disable_perm) {
         return 0;
     }
 
     if (!blk_can_inactivate(blk)) {
         return -EPERM;
     }
 
     blk->disable_perm = true;
     if (blk->root) {
         bdrv_child_try_set_perm(blk->root, 0, BLK_PERM_ALL, &error_abort);
     }
 
     return 0;
 }
 
 static void blk_root_attach(BdrvChild *child)
 {
     BlockBackend *blk = child->opaque;
     BlockBackendAioNotifier *notifier;
 
     trace_blk_root_attach(child, blk, child->bs);
 
     QLIST_FOREACH(notifier, &blk->aio_notifiers, list) {
         bdrv_add_aio_context_notifier(child->bs,
                 notifier->attached_aio_context,
                 notifier->detach_aio_context,
                 notifier->opaque);
     }
 }
 
 static void blk_root_detach(BdrvChild *child)
 {
     BlockBackend *blk = child->opaque;
     BlockBackendAioNotifier *notifier;
 
     trace_blk_root_detach(child, blk, child->bs);
 
     QLIST_FOREACH(notifier, &blk->aio_notifiers, list) {
         bdrv_remove_aio_context_notifier(child->bs,
                 notifier->attached_aio_context,
                 notifier->detach_aio_context,
                 notifier->opaque);
     }
 }
 
 static AioContext *blk_root_get_parent_aio_context(BdrvChild *c)
 {
     BlockBackend *blk = c->opaque;
     IO_CODE();
 
     return blk_get_aio_context(blk);
 }
 
 static const BdrvChildClass child_root = {
     .inherit_options    = blk_root_inherit_options,
 
     .change_media       = blk_root_change_media,
     .resize             = blk_root_resize,
     .get_name           = blk_root_get_name,
     .get_parent_desc    = blk_root_get_parent_desc,
 
     .drained_begin      = blk_root_drained_begin,
     .drained_poll       = blk_root_drained_poll,
     .drained_end        = blk_root_drained_end,
 
     .activate           = blk_root_activate,
     .inactivate         = blk_root_inactivate,
 
     .attach             = blk_root_attach,
     .detach             = blk_root_detach,
 
     .change_aio_ctx     = blk_root_change_aio_ctx,
 
     .get_parent_aio_context = blk_root_get_parent_aio_context,
 };
 
 /*
  * Create a new BlockBackend with a reference count of one.
  *
  * @perm is a bitmasks of BLK_PERM_* constants which describes the permissions
  * to request for a block driver node that is attached to this BlockBackend.
  * @shared_perm is a bitmask which describes which permissions may be granted
  * to other users of the attached node.
  * Both sets of permissions can be changed later using blk_set_perm().
  *
  * Return the new BlockBackend on success, null on failure.
  */
 BlockBackend *blk_new(AioContext *ctx, uint64_t perm, uint64_t shared_perm)
 {
     BlockBackend *blk;
 
     GLOBAL_STATE_CODE();
 
     blk = g_new0(BlockBackend, 1);
     blk->refcnt = 1;
     blk->ctx = ctx;
     blk->perm = perm;
     blk->shared_perm = shared_perm;
     blk_set_enable_write_cache(blk, true);
 
     blk->on_read_error = BLOCKDEV_ON_ERROR_REPORT;
     blk->on_write_error = BLOCKDEV_ON_ERROR_ENOSPC;
 
     block_acct_init(&blk->stats);
 
     qemu_co_queue_init(&blk->queued_requests);
     notifier_list_init(&blk->remove_bs_notifiers);
     notifier_list_init(&blk->insert_bs_notifiers);
     QLIST_INIT(&blk->aio_notifiers);
 
     QTAILQ_INSERT_TAIL(&block_backends, blk, link);
     return blk;
 }
 
 /*
  * Create a new BlockBackend connected to an existing BlockDriverState.
  *
  * @perm is a bitmasks of BLK_PERM_* constants which describes the
  * permissions to request for @bs that is attached to this
  * BlockBackend.  @shared_perm is a bitmask which describes which
  * permissions may be granted to other users of the attached node.
  * Both sets of permissions can be changed later using blk_set_perm().
  *
  * Return the new BlockBackend on success, null on failure.
  */
 BlockBackend *blk_new_with_bs(BlockDriverState *bs, uint64_t perm,
                               uint64_t shared_perm, Error **errp)
 {
     BlockBackend *blk = blk_new(bdrv_get_aio_context(bs), perm, shared_perm);
 
     GLOBAL_STATE_CODE();
 
     if (blk_insert_bs(blk, bs, errp) < 0) {
         blk_unref(blk);
         return NULL;
     }
     return blk;
 }
 
 /*
  * Creates a new BlockBackend, opens a new BlockDriverState, and connects both.
  * The new BlockBackend is in the main AioContext.
  *
  * Just as with bdrv_open(), after having called this function the reference to
  * @options belongs to the block layer (even on failure).
  *
  * TODO: Remove @filename and @flags; it should be possible to specify a whole
  * BDS tree just by specifying the @options QDict (or @reference,
  * alternatively). At the time of adding this function, this is not possible,
  * though, so callers of this function have to be able to specify @filename and
  * @flags.
  */
 BlockBackend *blk_new_open(const char *filename, const char *reference,
                            QDict *options, int flags, Error **errp)
 {
     BlockBackend *blk;
     BlockDriverState *bs;
     uint64_t perm = 0;
     uint64_t shared = BLK_PERM_ALL;
 
     GLOBAL_STATE_CODE();
 
     /*
      * blk_new_open() is mainly used in .bdrv_create implementations and the
      * tools where sharing isn't a major concern because the BDS stays private
      * and the file is generally not supposed to be used by a second process,
      * so we just request permission according to the flags.
      *
      * The exceptions are xen_disk and blockdev_init(); in these cases, the
      * caller of blk_new_open() doesn't make use of the permissions, but they
      * shouldn't hurt either. We can still share everything here because the
      * guest devices will add their own blockers if they can't share.
      */
     if ((flags & BDRV_O_NO_IO) == 0) {
         perm |= BLK_PERM_CONSISTENT_READ;
         if (flags & BDRV_O_RDWR) {
             perm |= BLK_PERM_WRITE;
         }
     }
     if (flags & BDRV_O_RESIZE) {
         perm |= BLK_PERM_RESIZE;
     }
     if (flags & BDRV_O_NO_SHARE) {
         shared = BLK_PERM_CONSISTENT_READ | BLK_PERM_WRITE_UNCHANGED;
     }
 
     blk = blk_new(qemu_get_aio_context(), perm, shared);
     bs = bdrv_open(filename, reference, options, flags, errp);
     if (!bs) {
         blk_unref(blk);
         return NULL;
     }
 
     blk->root = bdrv_root_attach_child(bs, "root", &child_root,
                                        BDRV_CHILD_FILTERED | BDRV_CHILD_PRIMARY,
                                        perm, shared, blk, errp);
     if (!blk->root) {
         blk_unref(blk);
         return NULL;
     }
 
     return blk;
 }
 
 static void blk_delete(BlockBackend *blk)
 {
     assert(!blk->refcnt);
     assert(!blk->name);
     assert(!blk->dev);
     if (blk->public.throttle_group_member.throttle_state) {
         blk_io_limits_disable(blk);
     }
     if (blk->root) {
         blk_remove_bs(blk);
     }
     if (blk->vmsh) {
         qemu_del_vm_change_state_handler(blk->vmsh);
         blk->vmsh = NULL;
     }
     assert(QLIST_EMPTY(&blk->remove_bs_notifiers.notifiers));
     assert(QLIST_EMPTY(&blk->insert_bs_notifiers.notifiers));
     assert(QLIST_EMPTY(&blk->aio_notifiers));
     QTAILQ_REMOVE(&block_backends, blk, link);
     drive_info_del(blk->legacy_dinfo);
     block_acct_cleanup(&blk->stats);
     g_free(blk);
 }
 
 static void drive_info_del(DriveInfo *dinfo)
 {
     if (!dinfo) {
         return;
     }
     qemu_opts_del(dinfo->opts);
     g_free(dinfo);
 }
 
 int blk_get_refcnt(BlockBackend *blk)
 {
     GLOBAL_STATE_CODE();
     return blk ? blk->refcnt : 0;
 }
 
 /*
  * Increment @blk's reference count.
  * @blk must not be null.
  */
 void blk_ref(BlockBackend *blk)
 {
     assert(blk->refcnt > 0);
     GLOBAL_STATE_CODE();
     blk->refcnt++;
 }
 
 /*
  * Decrement @blk's reference count.
  * If this drops it to zero, destroy @blk.
  * For convenience, do nothing if @blk is null.
  */
 void blk_unref(BlockBackend *blk)
 {
     GLOBAL_STATE_CODE();
     if (blk) {
         assert(blk->refcnt > 0);
         if (blk->refcnt > 1) {
             blk->refcnt--;
         } else {
             blk_drain(blk);
             /* blk_drain() cannot resurrect blk, nobody held a reference */
             assert(blk->refcnt == 1);
             blk->refcnt = 0;
             blk_delete(blk);
         }
     }
 }
 
 /*
  * Behaves similarly to blk_next() but iterates over all BlockBackends, even the
  * ones which are hidden (i.e. are not referenced by the monitor).
  */
 BlockBackend *blk_all_next(BlockBackend *blk)
 {
     GLOBAL_STATE_CODE();
     return blk ? QTAILQ_NEXT(blk, link)
                : QTAILQ_FIRST(&block_backends);
 }
 
 void blk_remove_all_bs(void)
 {
     BlockBackend *blk = NULL;
 
     GLOBAL_STATE_CODE();
 
     while ((blk = blk_all_next(blk)) != NULL) {
         AioContext *ctx = blk_get_aio_context(blk);
 
         aio_context_acquire(ctx);
         if (blk->root) {
             blk_remove_bs(blk);
         }
         aio_context_release(ctx);
     }
 }
 
 /*
  * Return the monitor-owned BlockBackend after @blk.
  * If @blk is null, return the first one.
  * Else, return @blk's next sibling, which may be null.
  *
  * To iterate over all BlockBackends, do
  * for (blk = blk_next(NULL); blk; blk = blk_next(blk)) {
  *     ...
  * }
  */
 BlockBackend *blk_next(BlockBackend *blk)
 {
     GLOBAL_STATE_CODE();
     return blk ? QTAILQ_NEXT(blk, monitor_link)
                : QTAILQ_FIRST(&monitor_block_backends);
 }
 
 /* Iterates over all top-level BlockDriverStates, i.e. BDSs that are owned by
  * the monitor or attached to a BlockBackend */
 BlockDriverState *bdrv_next(BdrvNextIterator *it)
 {
     BlockDriverState *bs, *old_bs;
 
     /* Must be called from the main loop */
     assert(qemu_get_current_aio_context() == qemu_get_aio_context());
 
     /* First, return all root nodes of BlockBackends. In order to avoid
      * returning a BDS twice when multiple BBs refer to it, we only return it
      * if the BB is the first one in the parent list of the BDS. */
     if (it->phase == BDRV_NEXT_BACKEND_ROOTS) {
         BlockBackend *old_blk = it->blk;
 
         old_bs = old_blk ? blk_bs(old_blk) : NULL;
 
         do {
             it->blk = blk_all_next(it->blk);
             bs = it->blk ? blk_bs(it->blk) : NULL;
         } while (it->blk && (bs == NULL || bdrv_first_blk(bs) != it->blk));
 
         if (it->blk) {
             blk_ref(it->blk);
         }
         blk_unref(old_blk);
 
         if (bs) {
             bdrv_ref(bs);
             bdrv_unref(old_bs);
             return bs;
         }
         it->phase = BDRV_NEXT_MONITOR_OWNED;
     } else {
         old_bs = it->bs;
     }
 
     /* Then return the monitor-owned BDSes without a BB attached. Ignore all
      * BDSes that are attached to a BlockBackend here; they have been handled
      * by the above block already */
     do {
         it->bs = bdrv_next_monitor_owned(it->bs);
         bs = it->bs;
     } while (bs && bdrv_has_blk(bs));
 
     if (bs) {
         bdrv_ref(bs);
     }
     bdrv_unref(old_bs);
 
     return bs;
 }
 
 static void bdrv_next_reset(BdrvNextIterator *it)
 {
     *it = (BdrvNextIterator) {
         .phase = BDRV_NEXT_BACKEND_ROOTS,
     };
 }
 
 BlockDriverState *bdrv_first(BdrvNextIterator *it)
 {
     GLOBAL_STATE_CODE();
     bdrv_next_reset(it);
     return bdrv_next(it);
 }
 
 /* Must be called when aborting a bdrv_next() iteration before
  * bdrv_next() returns NULL */
 void bdrv_next_cleanup(BdrvNextIterator *it)
 {
     /* Must be called from the main loop */
     assert(qemu_get_current_aio_context() == qemu_get_aio_context());
 
     if (it->phase == BDRV_NEXT_BACKEND_ROOTS) {
         if (it->blk) {
             bdrv_unref(blk_bs(it->blk));
             blk_unref(it->blk);
         }
     } else {
         bdrv_unref(it->bs);
     }
 
     bdrv_next_reset(it);
 }
 
 /*
  * Add a BlockBackend into the list of backends referenced by the monitor, with
  * the given @name acting as the handle for the monitor.
  * Strictly for use by blockdev.c.
  *
  * @name must not be null or empty.
  *
  * Returns true on success and false on failure. In the latter case, an Error
  * object is returned through @errp.
  */
 bool monitor_add_blk(BlockBackend *blk, const char *name, Error **errp)
 {
     assert(!blk->name);
     assert(name && name[0]);
     GLOBAL_STATE_CODE();
 
     if (!id_wellformed(name)) {
         error_setg(errp, "Invalid device name");
         return false;
     }
     if (blk_by_name(name)) {
         error_setg(errp, "Device with id '%s' already exists", name);
         return false;
     }
     if (bdrv_find_node(name)) {
         error_setg(errp,
                    "Device name '%s' conflicts with an existing node name",
                    name);
         return false;
     }
 
     blk->name = g_strdup(name);
     QTAILQ_INSERT_TAIL(&monitor_block_backends, blk, monitor_link);
     return true;
 }
 
 /*
  * Remove a BlockBackend from the list of backends referenced by the monitor.
  * Strictly for use by blockdev.c.
  */
 void monitor_remove_blk(BlockBackend *blk)
 {
     GLOBAL_STATE_CODE();
 
     if (!blk->name) {
         return;
     }
 
     QTAILQ_REMOVE(&monitor_block_backends, blk, monitor_link);
     g_free(blk->name);
     blk->name = NULL;
 }
 
 /*
  * Return @blk's name, a non-null string.
  * Returns an empty string iff @blk is not referenced by the monitor.
  */
 const char *blk_name(const BlockBackend *blk)
 {
     IO_CODE();
     return blk->name ?: "";
 }
 
 /*
  * Return the BlockBackend with name @name if it exists, else null.
  * @name must not be null.
  */
 BlockBackend *blk_by_name(const char *name)
 {
     BlockBackend *blk = NULL;
 
     GLOBAL_STATE_CODE();
     assert(name);
     while ((blk = blk_next(blk)) != NULL) {
         if (!strcmp(name, blk->name)) {
             return blk;
         }
     }
     return NULL;
 }
 
 /*
  * Return the BlockDriverState attached to @blk if any, else null.
  */
 BlockDriverState *blk_bs(BlockBackend *blk)
 {
     IO_CODE();
     return blk->root ? blk->root->bs : NULL;
 }
 
 static BlockBackend *bdrv_first_blk(BlockDriverState *bs)
 {
     BdrvChild *child;
 
     GLOBAL_STATE_CODE();
 
     QLIST_FOREACH(child, &bs->parents, next_parent) {
         if (child->klass == &child_root) {
             return child->opaque;
         }
     }
 
     return NULL;
 }
 
 /*
  * Returns true if @bs has an associated BlockBackend.
  */
 bool bdrv_has_blk(BlockDriverState *bs)
 {
     GLOBAL_STATE_CODE();
     return bdrv_first_blk(bs) != NULL;
 }
 
 /*
  * Returns true if @bs has only BlockBackends as parents.
  */
 bool bdrv_is_root_node(BlockDriverState *bs)
 {
     BdrvChild *c;
 
     GLOBAL_STATE_CODE();
     QLIST_FOREACH(c, &bs->parents, next_parent) {
         if (c->klass != &child_root) {
             return false;
         }
     }
 
     return true;
 }
 
 /*
  * Return @blk's DriveInfo if any, else null.
  */
 DriveInfo *blk_legacy_dinfo(BlockBackend *blk)
 {
     GLOBAL_STATE_CODE();
     return blk->legacy_dinfo;
 }
 
 /*
  * Set @blk's DriveInfo to @dinfo, and return it.
  * @blk must not have a DriveInfo set already.
  * No other BlockBackend may have the same DriveInfo set.
  */
 DriveInfo *blk_set_legacy_dinfo(BlockBackend *blk, DriveInfo *dinfo)
 {
     assert(!blk->legacy_dinfo);
     GLOBAL_STATE_CODE();
     return blk->legacy_dinfo = dinfo;
 }
 
 /*
  * Return the BlockBackend with DriveInfo @dinfo.
  * It must exist.
  */
 BlockBackend *blk_by_legacy_dinfo(DriveInfo *dinfo)
 {
     BlockBackend *blk = NULL;
     GLOBAL_STATE_CODE();
 
     while ((blk = blk_next(blk)) != NULL) {
         if (blk->legacy_dinfo == dinfo) {
             return blk;
         }
     }
     abort();
 }
 
 /*
  * Returns a pointer to the publicly accessible fields of @blk.
  */
 BlockBackendPublic *blk_get_public(BlockBackend *blk)
 {
     GLOBAL_STATE_CODE();
     return &blk->public;
 }
 
 /*
  * Returns a BlockBackend given the associated @public fields.
  */
 BlockBackend *blk_by_public(BlockBackendPublic *public)
 {
     GLOBAL_STATE_CODE();
     return container_of(public, BlockBackend, public);
 }
 
 /*
  * Disassociates the currently associated BlockDriverState from @blk.
  */
 void blk_remove_bs(BlockBackend *blk)
 {
     ThrottleGroupMember *tgm = &blk->public.throttle_group_member;
     BdrvChild *root;
 
     GLOBAL_STATE_CODE();
 
     notifier_list_notify(&blk->remove_bs_notifiers, blk);
     if (tgm->throttle_state) {
         BlockDriverState *bs = blk_bs(blk);
 
         /*
          * Take a ref in case blk_bs() changes across bdrv_drained_begin(), for
          * example, if a temporary filter node is removed by a blockjob.
          */
         bdrv_ref(bs);
         bdrv_drained_begin(bs);
         throttle_group_detach_aio_context(tgm);
         throttle_group_attach_aio_context(tgm, qemu_get_aio_context());
         bdrv_drained_end(bs);
         bdrv_unref(bs);
     }
 
     blk_update_root_state(blk);
 
     /* bdrv_root_unref_child() will cause blk->root to become stale and may
      * switch to a completion coroutine later on. Let's drain all I/O here
      * to avoid that and a potential QEMU crash.
      */
     blk_drain(blk);
     root = blk->root;
     blk->root = NULL;
     bdrv_root_unref_child(root);
 }
 
 /*
  * Associates a new BlockDriverState with @blk.
  */
 int blk_insert_bs(BlockBackend *blk, BlockDriverState *bs, Error **errp)
 {
     ThrottleGroupMember *tgm = &blk->public.throttle_group_member;
     GLOBAL_STATE_CODE();
     bdrv_ref(bs);
     blk->root = bdrv_root_attach_child(bs, "root", &child_root,
                                        BDRV_CHILD_FILTERED | BDRV_CHILD_PRIMARY,
                                        blk->perm, blk->shared_perm,
                                        blk, errp);
     if (blk->root == NULL) {
         return -EPERM;
     }
 
     notifier_list_notify(&blk->insert_bs_notifiers, blk);
     if (tgm->throttle_state) {
         throttle_group_detach_aio_context(tgm);
         throttle_group_attach_aio_context(tgm, bdrv_get_aio_context(bs));
     }
 
     return 0;
 }
 
 /*
  * Change BlockDriverState associated with @blk.
  */
 int blk_replace_bs(BlockBackend *blk, BlockDriverState *new_bs, Error **errp)
 {
     GLOBAL_STATE_CODE();
     return bdrv_replace_child_bs(blk->root, new_bs, errp);
 }
 
 /*
  * Sets the permission bitmasks that the user of the BlockBackend needs.
  */
 int blk_set_perm(BlockBackend *blk, uint64_t perm, uint64_t shared_perm,
                  Error **errp)
 {
     int ret;
     GLOBAL_STATE_CODE();
 
     if (blk->root && !blk->disable_perm) {
         ret = bdrv_child_try_set_perm(blk->root, perm, shared_perm, errp);
         if (ret < 0) {
             return ret;
         }
     }
 
     blk->perm = perm;
     blk->shared_perm = shared_perm;
 
     return 0;
 }
 
 void blk_get_perm(BlockBackend *blk, uint64_t *perm, uint64_t *shared_perm)
 {
     GLOBAL_STATE_CODE();
     *perm = blk->perm;
     *shared_perm = blk->shared_perm;
 }
 
 /*
  * Attach device model @dev to @blk.
  * Return 0 on success, -EBUSY when a device model is attached already.
  */
 int blk_attach_dev(BlockBackend *blk, DeviceState *dev)
 {
     GLOBAL_STATE_CODE();
     if (blk->dev) {
         return -EBUSY;
     }
 
     /* While migration is still incoming, we don't need to apply the
      * permissions of guest device BlockBackends. We might still have a block
      * job or NBD server writing to the image for storage migration. */
     if (runstate_check(RUN_STATE_INMIGRATE)) {
         blk->disable_perm = true;
     }
 
     blk_ref(blk);
     blk->dev = dev;
     blk_iostatus_reset(blk);
 
     return 0;
 }
 
 /*
  * Detach device model @dev from @blk.
  * @dev must be currently attached to @blk.
  */
 void blk_detach_dev(BlockBackend *blk, DeviceState *dev)
 {
     assert(blk->dev == dev);
     GLOBAL_STATE_CODE();
     blk->dev = NULL;
     blk->dev_ops = NULL;
     blk->dev_opaque = NULL;
     blk_set_perm(blk, 0, BLK_PERM_ALL, &error_abort);
     blk_unref(blk);
 }
 
 /*
  * Return the device model attached to @blk if any, else null.
  */
 DeviceState *blk_get_attached_dev(BlockBackend *blk)
 {
     GLOBAL_STATE_CODE();
     return blk->dev;
 }
 
 /* Return the qdev ID, or if no ID is assigned the QOM path, of the block
  * device attached to the BlockBackend. */
 char *blk_get_attached_dev_id(BlockBackend *blk)
 {
     DeviceState *dev = blk->dev;
     IO_CODE();
 
     if (!dev) {
         return g_strdup("");
     } else if (dev->id) {
         return g_strdup(dev->id);
     }
 
     return object_get_canonical_path(OBJECT(dev)) ?: g_strdup("");
 }
 
 /*
  * Return the BlockBackend which has the device model @dev attached if it
  * exists, else null.
  *
  * @dev must not be null.
  */
 BlockBackend *blk_by_dev(void *dev)
 {
     BlockBackend *blk = NULL;
 
     GLOBAL_STATE_CODE();
 
     assert(dev != NULL);
     while ((blk = blk_all_next(blk)) != NULL) {
         if (blk->dev == dev) {
             return blk;
         }
     }
     return NULL;
 }
 
 /*
  * Set @blk's device model callbacks to @ops.
  * @opaque is the opaque argument to pass to the callbacks.
  * This is for use by device models.
  */
 void blk_set_dev_ops(BlockBackend *blk, const BlockDevOps *ops,
                      void *opaque)
 {
     GLOBAL_STATE_CODE();
     blk->dev_ops = ops;
     blk->dev_opaque = opaque;
 
     /* Are we currently quiesced? Should we enforce this right now? */
     if (blk->quiesce_counter && ops && ops->drained_begin) {
         ops->drained_begin(opaque);
     }
 }
 
 /*
  * Notify @blk's attached device model of media change.
  *
  * If @load is true, notify of media load. This action can fail, meaning that
  * the medium cannot be loaded. @errp is set then.
  *
  * If @load is false, notify of media eject. This can never fail.
  *
  * Also send DEVICE_TRAY_MOVED events as appropriate.
  */
 void blk_dev_change_media_cb(BlockBackend *blk, bool load, Error **errp)
 {
     GLOBAL_STATE_CODE();
     if (blk->dev_ops && blk->dev_ops->change_media_cb) {
         bool tray_was_open, tray_is_open;
         Error *local_err = NULL;
 
         tray_was_open = blk_dev_is_tray_open(blk);
         blk->dev_ops->change_media_cb(blk->dev_opaque, load, &local_err);
         if (local_err) {
             assert(load == true);
             error_propagate(errp, local_err);
             return;
         }
         tray_is_open = blk_dev_is_tray_open(blk);
 
         if (tray_was_open != tray_is_open) {
             char *id = blk_get_attached_dev_id(blk);
             qapi_event_send_device_tray_moved(blk_name(blk), id, tray_is_open);
             g_free(id);
         }
     }
 }
 
 static void blk_root_change_media(BdrvChild *child, bool load)
 {
     blk_dev_change_media_cb(child->opaque, load, NULL);
 }
 
 /*
  * Does @blk's attached device model have removable media?
  * %true if no device model is attached.
  */
 bool blk_dev_has_removable_media(BlockBackend *blk)
 {
     GLOBAL_STATE_CODE();
     return !blk->dev || (blk->dev_ops && blk->dev_ops->change_media_cb);
 }
 
 /*
  * Does @blk's attached device model have a tray?
  */
 bool blk_dev_has_tray(BlockBackend *blk)
 {
     IO_CODE();
     return blk->dev_ops && blk->dev_ops->is_tray_open;
 }
 
 /*
  * Notify @blk's attached device model of a media eject request.
  * If @force is true, the medium is about to be yanked out forcefully.
  */
 void blk_dev_eject_request(BlockBackend *blk, bool force)
 {
     GLOBAL_STATE_CODE();
     if (blk->dev_ops && blk->dev_ops->eject_request_cb) {
         blk->dev_ops->eject_request_cb(blk->dev_opaque, force);
     }
 }
 
 /*
  * Does @blk's attached device model have a tray, and is it open?
  */
 bool blk_dev_is_tray_open(BlockBackend *blk)
 {
     IO_CODE();
     if (blk_dev_has_tray(blk)) {
         return blk->dev_ops->is_tray_open(blk->dev_opaque);
     }
     return false;
 }
 
 /*
  * Does @blk's attached device model have the medium locked?
  * %false if the device model has no such lock.
  */
 bool blk_dev_is_medium_locked(BlockBackend *blk)
 {
     GLOBAL_STATE_CODE();
     if (blk->dev_ops && blk->dev_ops->is_medium_locked) {
         return blk->dev_ops->is_medium_locked(blk->dev_opaque);
     }
     return false;
 }
 
 /*
  * Notify @blk's attached device model of a backend size change.
  */
 static void blk_root_resize(BdrvChild *child)
 {
     BlockBackend *blk = child->opaque;
 
     if (blk->dev_ops && blk->dev_ops->resize_cb) {
         blk->dev_ops->resize_cb(blk->dev_opaque);
     }
 }
 
 void blk_iostatus_enable(BlockBackend *blk)
 {
     GLOBAL_STATE_CODE();
     blk->iostatus_enabled = true;
     blk->iostatus = BLOCK_DEVICE_IO_STATUS_OK;
 }
 
 /* The I/O status is only enabled if the drive explicitly
  * enables it _and_ the VM is configured to stop on errors */
 bool blk_iostatus_is_enabled(const BlockBackend *blk)
 {
     IO_CODE();
     return (blk->iostatus_enabled &&
            (blk->on_write_error == BLOCKDEV_ON_ERROR_ENOSPC ||
             blk->on_write_error == BLOCKDEV_ON_ERROR_STOP   ||
             blk->on_read_error == BLOCKDEV_ON_ERROR_STOP));
 }
 
 BlockDeviceIoStatus blk_iostatus(const BlockBackend *blk)
 {
     GLOBAL_STATE_CODE();
     return blk->iostatus;
 }
 
 void blk_iostatus_disable(BlockBackend *blk)
 {
     GLOBAL_STATE_CODE();
     blk->iostatus_enabled = false;
 }
 
 void blk_iostatus_reset(BlockBackend *blk)
 {
     GLOBAL_STATE_CODE();
     if (blk_iostatus_is_enabled(blk)) {
         blk->iostatus = BLOCK_DEVICE_IO_STATUS_OK;
     }
 }
 
 void blk_iostatus_set_err(BlockBackend *blk, int error)
 {
     IO_CODE();
     assert(blk_iostatus_is_enabled(blk));
     if (blk->iostatus == BLOCK_DEVICE_IO_STATUS_OK) {
         blk->iostatus = error == ENOSPC ? BLOCK_DEVICE_IO_STATUS_NOSPACE :
                                           BLOCK_DEVICE_IO_STATUS_FAILED;
     }
 }
 
 void blk_set_allow_write_beyond_eof(BlockBackend *blk, bool allow)
 {
     IO_CODE();
     blk->allow_write_beyond_eof = allow;
 }
 
 void blk_set_allow_aio_context_change(BlockBackend *blk, bool allow)
 {
     IO_CODE();
     blk->allow_aio_context_change = allow;
 }
 
 void blk_set_disable_request_queuing(BlockBackend *blk, bool disable)
 {
     IO_CODE();
     blk->disable_request_queuing = disable;
 }
 
 static int blk_check_byte_request(BlockBackend *blk, int64_t offset,
                                   int64_t bytes)
 {
     int64_t len;
 
     if (bytes < 0) {
         return -EIO;
     }
 
     if (!blk_is_available(blk)) {
         return -ENOMEDIUM;
     }
 
     if (offset < 0) {
         return -EIO;
     }
 
     if (!blk->allow_write_beyond_eof) {
         len = blk_getlength(blk);
         if (len < 0) {
             return len;
         }
 
         if (offset > len || len - offset < bytes) {
             return -EIO;
         }
     }
 
     return 0;
 }
 
 /* To be called between exactly one pair of blk_inc/dec_in_flight() */
 static void coroutine_fn blk_wait_while_drained(BlockBackend *blk)
 {
     assert(blk->in_flight > 0);
 
     if (blk->quiesce_counter && !blk->disable_request_queuing) {
         blk_dec_in_flight(blk);
         qemu_co_queue_wait(&blk->queued_requests, NULL);
         blk_inc_in_flight(blk);
     }
 }
 
 /* To be called between exactly one pair of blk_inc/dec_in_flight() */
 static int coroutine_fn
 blk_co_do_preadv_part(BlockBackend *blk, int64_t offset, int64_t bytes,
                       QEMUIOVector *qiov, size_t qiov_offset,
                       BdrvRequestFlags flags)
 {
     int ret;
     BlockDriverState *bs;
     IO_CODE();
 
     blk_wait_while_drained(blk);
 
     /* Call blk_bs() only after waiting, the graph may have changed */
     bs = blk_bs(blk);
     trace_blk_co_preadv(blk, bs, offset, bytes, flags);
 
     ret = blk_check_byte_request(blk, offset, bytes);
     if (ret < 0) {
         return ret;
     }
 
     bdrv_inc_in_flight(bs);
 
     /* throttling disk I/O */
     if (blk->public.throttle_group_member.throttle_state) {
         throttle_group_co_io_limits_intercept(&blk->public.throttle_group_member,
                 bytes, false);
     }
 
     ret = bdrv_co_preadv_part(blk->root, offset, bytes, qiov, qiov_offset,
                               flags);
     bdrv_dec_in_flight(bs);
     return ret;
 }
 
 int coroutine_fn blk_co_pread(BlockBackend *blk, int64_t offset, int64_t bytes,
                               void *buf, BdrvRequestFlags flags)
 {
     QEMUIOVector qiov = QEMU_IOVEC_INIT_BUF(qiov, buf, bytes);
     IO_OR_GS_CODE();
 
     assert(bytes <= SIZE_MAX);
 
     return blk_co_preadv(blk, offset, bytes, &qiov, flags);
 }
 
 int coroutine_fn blk_co_preadv(BlockBackend *blk, int64_t offset,
                                int64_t bytes, QEMUIOVector *qiov,
                                BdrvRequestFlags flags)
 {
     int ret;
     IO_OR_GS_CODE();
 
     blk_inc_in_flight(blk);
     ret = blk_co_do_preadv_part(blk, offset, bytes, qiov, 0, flags);
     blk_dec_in_flight(blk);
 
     return ret;
 }
 
 int coroutine_fn blk_co_preadv_part(BlockBackend *blk, int64_t offset,
                                     int64_t bytes, QEMUIOVector *qiov,
                                     size_t qiov_offset, BdrvRequestFlags flags)
 {
     int ret;
     IO_OR_GS_CODE();
 
     blk_inc_in_flight(blk);
     ret = blk_co_do_preadv_part(blk, offset, bytes, qiov, qiov_offset, flags);
     blk_dec_in_flight(blk);
 
     return ret;
 }
 
 /* To be called between exactly one pair of blk_inc/dec_in_flight() */
 static int coroutine_fn
 blk_co_do_pwritev_part(BlockBackend *blk, int64_t offset, int64_t bytes,
                        QEMUIOVector *qiov, size_t qiov_offset,
                        BdrvRequestFlags flags)
 {
     int ret;
     BlockDriverState *bs;
     IO_CODE();
 
     blk_wait_while_drained(blk);
 
     /* Call blk_bs() only after waiting, the graph may have changed */
     bs = blk_bs(blk);
     trace_blk_co_pwritev(blk, bs, offset, bytes, flags);
 
     ret = blk_check_byte_request(blk, offset, bytes);
     if (ret < 0) {
         return ret;
     }
 
     bdrv_inc_in_flight(bs);
     /* throttling disk I/O */
     if (blk->public.throttle_group_member.throttle_state) {
         throttle_group_co_io_limits_intercept(&blk->public.throttle_group_member,
                 bytes, true);
     }
 
     if (!blk->enable_write_cache) {
         flags |= BDRV_REQ_FUA;
     }
 
     ret = bdrv_co_pwritev_part(blk->root, offset, bytes, qiov, qiov_offset,
                                flags);
     bdrv_dec_in_flight(bs);
     return ret;
 }
 
 int coroutine_fn blk_co_pwritev_part(BlockBackend *blk, int64_t offset,
                                      int64_t bytes,
                                      QEMUIOVector *qiov, size_t qiov_offset,
                                      BdrvRequestFlags flags)
 {
     int ret;
     IO_OR_GS_CODE();
 
     blk_inc_in_flight(blk);
     ret = blk_co_do_pwritev_part(blk, offset, bytes, qiov, qiov_offset, flags);
     blk_dec_in_flight(blk);
 
     return ret;
 }
 
 int coroutine_fn blk_co_pwrite(BlockBackend *blk, int64_t offset, int64_t bytes,
                                const void *buf, BdrvRequestFlags flags)
 {
     QEMUIOVector qiov = QEMU_IOVEC_INIT_BUF(qiov, buf, bytes);
     IO_OR_GS_CODE();
 
     assert(bytes <= SIZE_MAX);
 
     return blk_co_pwritev(blk, offset, bytes, &qiov, flags);
 }
 
 int coroutine_fn blk_co_pwritev(BlockBackend *blk, int64_t offset,
                                 int64_t bytes, QEMUIOVector *qiov,
                                 BdrvRequestFlags flags)
 {
     IO_OR_GS_CODE();
     return blk_co_pwritev_part(blk, offset, bytes, qiov, 0, flags);
 }
 
 typedef struct BlkRwCo {
     BlockBackend *blk;
     int64_t offset;
     void *iobuf;
     int ret;
     BdrvRequestFlags flags;
 } BlkRwCo;
 
 int blk_make_zero(BlockBackend *blk, BdrvRequestFlags flags)
 {
     GLOBAL_STATE_CODE();
     return bdrv_make_zero(blk->root, flags);
 }
 
 void blk_inc_in_flight(BlockBackend *blk)
 {
     IO_CODE();
     qatomic_inc(&blk->in_flight);
 }
 
 void blk_dec_in_flight(BlockBackend *blk)
 {
     IO_CODE();
     qatomic_dec(&blk->in_flight);
     aio_wait_kick();
 }
 
 static void error_callback_bh(void *opaque)
 {
     struct BlockBackendAIOCB *acb = opaque;
 
     blk_dec_in_flight(acb->blk);
     acb->common.cb(acb->common.opaque, acb->ret);
     qemu_aio_unref(acb);
 }
 
 BlockAIOCB *blk_abort_aio_request(BlockBackend *blk,
                                   BlockCompletionFunc *cb,
                                   void *opaque, int ret)
 {
     struct BlockBackendAIOCB *acb;
     IO_CODE();
 
     blk_inc_in_flight(blk);
     acb = blk_aio_get(&block_backend_aiocb_info, blk, cb, opaque);
     acb->blk = blk;
     acb->ret = ret;
 
     replay_bh_schedule_oneshot_event(blk_get_aio_context(blk),
                                      error_callback_bh, acb);
     return &acb->common;
 }
 
 typedef struct BlkAioEmAIOCB {
     BlockAIOCB common;
     BlkRwCo rwco;
     int64_t bytes;
     bool has_returned;
 } BlkAioEmAIOCB;
 
 static AioContext *blk_aio_em_aiocb_get_aio_context(BlockAIOCB *acb_)
 {
     BlkAioEmAIOCB *acb = container_of(acb_, BlkAioEmAIOCB, common);
 
     return blk_get_aio_context(acb->rwco.blk);
 }
 
 static const AIOCBInfo blk_aio_em_aiocb_info = {
     .aiocb_size         = sizeof(BlkAioEmAIOCB),
     .get_aio_context    = blk_aio_em_aiocb_get_aio_context,
 };
 
 static void blk_aio_complete(BlkAioEmAIOCB *acb)
 {
     if (acb->has_returned) {
         acb->common.cb(acb->common.opaque, acb->rwco.ret);
         blk_dec_in_flight(acb->rwco.blk);
         qemu_aio_unref(acb);
     }
 }
 
 static void blk_aio_complete_bh(void *opaque)
 {
     BlkAioEmAIOCB *acb = opaque;
     assert(acb->has_returned);
     blk_aio_complete(acb);
 }
 
 static BlockAIOCB *blk_aio_prwv(BlockBackend *blk, int64_t offset,
                                 int64_t bytes,
                                 void *iobuf, CoroutineEntry co_entry,
                                 BdrvRequestFlags flags,
                                 BlockCompletionFunc *cb, void *opaque)
 {
     BlkAioEmAIOCB *acb;
     Coroutine *co;
 
     blk_inc_in_flight(blk);
     acb = blk_aio_get(&blk_aio_em_aiocb_info, blk, cb, opaque);
     acb->rwco = (BlkRwCo) {
         .blk    = blk,
         .offset = offset,
         .iobuf  = iobuf,
         .flags  = flags,
         .ret    = NOT_DONE,
     };
     acb->bytes = bytes;
     acb->has_returned = false;
 
     co = qemu_coroutine_create(co_entry, acb);
     bdrv_coroutine_enter(blk_bs(blk), co);
 
     acb->has_returned = true;
     if (acb->rwco.ret != NOT_DONE) {
         replay_bh_schedule_oneshot_event(blk_get_aio_context(blk),
                                          blk_aio_complete_bh, acb);
     }
 
     return &acb->common;
 }
 
 static void coroutine_fn blk_aio_read_entry(void *opaque)
 {
     BlkAioEmAIOCB *acb = opaque;
     BlkRwCo *rwco = &acb->rwco;
     QEMUIOVector *qiov = rwco->iobuf;
 
     assert(qiov->size == acb->bytes);
     rwco->ret = blk_co_do_preadv_part(rwco->blk, rwco->offset, acb->bytes, qiov,
                                       0, rwco->flags);
     blk_aio_complete(acb);
 }
 
 static void coroutine_fn blk_aio_write_entry(void *opaque)
 {
     BlkAioEmAIOCB *acb = opaque;
     BlkRwCo *rwco = &acb->rwco;
     QEMUIOVector *qiov = rwco->iobuf;
 
     assert(!qiov || qiov->size == acb->bytes);
     rwco->ret = blk_co_do_pwritev_part(rwco->blk, rwco->offset, acb->bytes,
                                        qiov, 0, rwco->flags);
     blk_aio_complete(acb);
 }
 
 BlockAIOCB *blk_aio_pwrite_zeroes(BlockBackend *blk, int64_t offset,
                                   int64_t bytes, BdrvRequestFlags flags,
                                   BlockCompletionFunc *cb, void *opaque)
 {
     IO_CODE();
     return blk_aio_prwv(blk, offset, bytes, NULL, blk_aio_write_entry,
                         flags | BDRV_REQ_ZERO_WRITE, cb, opaque);
 }
 
 int64_t blk_getlength(BlockBackend *blk)
 {
     IO_CODE();
     if (!blk_is_available(blk)) {
         return -ENOMEDIUM;
     }
 
     return bdrv_getlength(blk_bs(blk));
 }
 
 void blk_get_geometry(BlockBackend *blk, uint64_t *nb_sectors_ptr)
 {
     IO_CODE();
     if (!blk_bs(blk)) {
         *nb_sectors_ptr = 0;
     } else {
         bdrv_get_geometry(blk_bs(blk), nb_sectors_ptr);
     }
 }
 
 int64_t blk_nb_sectors(BlockBackend *blk)
 {
     IO_CODE();
     if (!blk_is_available(blk)) {
         return -ENOMEDIUM;
     }
 
     return bdrv_nb_sectors(blk_bs(blk));
 }
 
 BlockAIOCB *blk_aio_preadv(BlockBackend *blk, int64_t offset,
                            QEMUIOVector *qiov, BdrvRequestFlags flags,
                            BlockCompletionFunc *cb, void *opaque)
 {
     IO_CODE();
     assert((uint64_t)qiov->size <= INT64_MAX);
     return blk_aio_prwv(blk, offset, qiov->size, qiov,
                         blk_aio_read_entry, flags, cb, opaque);
 }
 
 BlockAIOCB *blk_aio_pwritev(BlockBackend *blk, int64_t offset,
                             QEMUIOVector *qiov, BdrvRequestFlags flags,
                             BlockCompletionFunc *cb, void *opaque)
 {
     IO_CODE();
     assert((uint64_t)qiov->size <= INT64_MAX);
     return blk_aio_prwv(blk, offset, qiov->size, qiov,
                         blk_aio_write_entry, flags, cb, opaque);
 }
 
 void blk_aio_cancel(BlockAIOCB *acb)
 {
     GLOBAL_STATE_CODE();
     bdrv_aio_cancel(acb);
 }
 
 void blk_aio_cancel_async(BlockAIOCB *acb)
 {
     IO_CODE();
     bdrv_aio_cancel_async(acb);
 }
 
 /* To be called between exactly one pair of blk_inc/dec_in_flight() */
 static int coroutine_fn
 blk_co_do_ioctl(BlockBackend *blk, unsigned long int req, void *buf)
 {
     IO_CODE();
 
     blk_wait_while_drained(blk);
 
     if (!blk_is_available(blk)) {
         return -ENOMEDIUM;
     }
 
     return bdrv_co_ioctl(blk_bs(blk), req, buf);
 }
 
 int coroutine_fn blk_co_ioctl(BlockBackend *blk, unsigned long int req,
                               void *buf)
 {
     int ret;
     IO_OR_GS_CODE();
 
     blk_inc_in_flight(blk);
     ret = blk_co_do_ioctl(blk, req, buf);
     blk_dec_in_flight(blk);
 
     return ret;
 }
 
 static void coroutine_fn blk_aio_ioctl_entry(void *opaque)
 {
     BlkAioEmAIOCB *acb = opaque;
     BlkRwCo *rwco = &acb->rwco;
 
     rwco->ret = blk_co_do_ioctl(rwco->blk, rwco->offset, rwco->iobuf);
 
     blk_aio_complete(acb);
 }
 
 BlockAIOCB *blk_aio_ioctl(BlockBackend *blk, unsigned long int req, void *buf,
                           BlockCompletionFunc *cb, void *opaque)
 {
     IO_CODE();
     return blk_aio_prwv(blk, req, 0, buf, blk_aio_ioctl_entry, 0, cb, opaque);
 }
 
 /* To be called between exactly one pair of blk_inc/dec_in_flight() */
 static int coroutine_fn
 blk_co_do_pdiscard(BlockBackend *blk, int64_t offset, int64_t bytes)
 {
     int ret;
     IO_CODE();
 
     blk_wait_while_drained(blk);
 
     ret = blk_check_byte_request(blk, offset, bytes);
     if (ret < 0) {
         return ret;
     }
 
     return bdrv_co_pdiscard(blk->root, offset, bytes);
 }
 
 static void coroutine_fn blk_aio_pdiscard_entry(void *opaque)
 {
     BlkAioEmAIOCB *acb = opaque;
     BlkRwCo *rwco = &acb->rwco;
 
     rwco->ret = blk_co_do_pdiscard(rwco->blk, rwco->offset, acb->bytes);
     blk_aio_complete(acb);
 }
 
 BlockAIOCB *blk_aio_pdiscard(BlockBackend *blk,
                              int64_t offset, int64_t bytes,
                              BlockCompletionFunc *cb, void *opaque)
 {
     IO_CODE();
     return blk_aio_prwv(blk, offset, bytes, NULL, blk_aio_pdiscard_entry, 0,
                         cb, opaque);
 }
 
 int coroutine_fn blk_co_pdiscard(BlockBackend *blk, int64_t offset,
                                  int64_t bytes)
 {
     int ret;
     IO_OR_GS_CODE();
 
     blk_inc_in_flight(blk);
     ret = blk_co_do_pdiscard(blk, offset, bytes);
     blk_dec_in_flight(blk);
 
     return ret;
 }
 
 /* To be called between exactly one pair of blk_inc/dec_in_flight() */
 static int coroutine_fn blk_co_do_flush(BlockBackend *blk)
 {
     blk_wait_while_drained(blk);
     IO_CODE();
 
     if (!blk_is_available(blk)) {
         return -ENOMEDIUM;
     }
 
     return bdrv_co_flush(blk_bs(blk));
 }
 
 static void coroutine_fn blk_aio_flush_entry(void *opaque)
 {
     BlkAioEmAIOCB *acb = opaque;
     BlkRwCo *rwco = &acb->rwco;
 
     rwco->ret = blk_co_do_flush(rwco->blk);
     blk_aio_complete(acb);
 }
 
 BlockAIOCB *blk_aio_flush(BlockBackend *blk,
                           BlockCompletionFunc *cb, void *opaque)
 {
     IO_CODE();
     return blk_aio_prwv(blk, 0, 0, NULL, blk_aio_flush_entry, 0, cb, opaque);
 }
 
 int coroutine_fn blk_co_flush(BlockBackend *blk)
 {
     int ret;
     IO_OR_GS_CODE();
 
     blk_inc_in_flight(blk);
     ret = blk_co_do_flush(blk);
     blk_dec_in_flight(blk);
 
     return ret;
 }
 
 void blk_drain(BlockBackend *blk)
 {
     BlockDriverState *bs = blk_bs(blk);
     GLOBAL_STATE_CODE();
 
     if (bs) {
         bdrv_ref(bs);
         bdrv_drained_begin(bs);
     }
 
     /* We may have -ENOMEDIUM completions in flight */
     AIO_WAIT_WHILE(blk_get_aio_context(blk),
                    qatomic_mb_read(&blk->in_flight) > 0);
 
     if (bs) {
         bdrv_drained_end(bs);
         bdrv_unref(bs);
     }
 }
 
 void blk_drain_all(void)
 {
     BlockBackend *blk = NULL;
 
     GLOBAL_STATE_CODE();
 
     bdrv_drain_all_begin();
 
     while ((blk = blk_all_next(blk)) != NULL) {
         AioContext *ctx = blk_get_aio_context(blk);
 
         aio_context_acquire(ctx);
 
         /* We may have -ENOMEDIUM completions in flight */
         AIO_WAIT_WHILE(ctx, qatomic_mb_read(&blk->in_flight) > 0);
 
         aio_context_release(ctx);
     }
 
     bdrv_drain_all_end();
 }
 
 void blk_set_on_error(BlockBackend *blk, BlockdevOnError on_read_error,
                       BlockdevOnError on_write_error)
 {
     GLOBAL_STATE_CODE();
     blk->on_read_error = on_read_error;
     blk->on_write_error = on_write_error;
 }
 
 BlockdevOnError blk_get_on_error(BlockBackend *blk, bool is_read)
 {
     IO_CODE();
     return is_read ? blk->on_read_error : blk->on_write_error;
 }
 
 BlockErrorAction blk_get_error_action(BlockBackend *blk, bool is_read,
                                       int error)
 {
     BlockdevOnError on_err = blk_get_on_error(blk, is_read);
     IO_CODE();
 
     switch (on_err) {
     case BLOCKDEV_ON_ERROR_ENOSPC:
         return (error == ENOSPC) ?
                BLOCK_ERROR_ACTION_STOP : BLOCK_ERROR_ACTION_REPORT;
     case BLOCKDEV_ON_ERROR_STOP:
         return BLOCK_ERROR_ACTION_STOP;
     case BLOCKDEV_ON_ERROR_REPORT:
         return BLOCK_ERROR_ACTION_REPORT;
     case BLOCKDEV_ON_ERROR_IGNORE:
         return BLOCK_ERROR_ACTION_IGNORE;
     case BLOCKDEV_ON_ERROR_AUTO:
     default:
         abort();
     }
 }
 
 static void send_qmp_error_event(BlockBackend *blk,
                                  BlockErrorAction action,
                                  bool is_read, int error)
 {
     IoOperationType optype;
     BlockDriverState *bs = blk_bs(blk);
 
     optype = is_read ? IO_OPERATION_TYPE_READ : IO_OPERATION_TYPE_WRITE;
     qapi_event_send_block_io_error(blk_name(blk), !!bs,
                                    bs ? bdrv_get_node_name(bs) : NULL, optype,
                                    action, blk_iostatus_is_enabled(blk),
                                    error == ENOSPC, strerror(error));
 }
 
 /* This is done by device models because, while the block layer knows
  * about the error, it does not know whether an operation comes from
  * the device or the block layer (from a job, for example).
  */
 void blk_error_action(BlockBackend *blk, BlockErrorAction action,
                       bool is_read, int error)
 {
     assert(error >= 0);
     IO_CODE();
 
     if (action == BLOCK_ERROR_ACTION_STOP) {
         /* First set the iostatus, so that "info block" returns an iostatus
          * that matches the events raised so far (an additional error iostatus
          * is fine, but not a lost one).
          */
         blk_iostatus_set_err(blk, error);
 
         /* Then raise the request to stop the VM and the event.
          * qemu_system_vmstop_request_prepare has two effects.  First,
          * it ensures that the STOP event always comes after the
          * BLOCK_IO_ERROR event.  Second, it ensures that even if management
          * can observe the STOP event and do a "cont" before the STOP
          * event is issued, the VM will not stop.  In this case, vm_start()
          * also ensures that the STOP/RESUME pair of events is emitted.
          */
         qemu_system_vmstop_request_prepare();
         send_qmp_error_event(blk, action, is_read, error);
         qemu_system_vmstop_request(RUN_STATE_IO_ERROR);
     } else {
         send_qmp_error_event(blk, action, is_read, error);
     }
 }
 
 /*
  * Returns true if the BlockBackend can support taking write permissions
  * (because its root node is not read-only).
  */
 bool blk_supports_write_perm(BlockBackend *blk)
 {
     BlockDriverState *bs = blk_bs(blk);
     GLOBAL_STATE_CODE();
 
     if (bs) {
         return !bdrv_is_read_only(bs);
     } else {
         return blk->root_state.open_flags & BDRV_O_RDWR;
     }
 }
 
 /*
  * Returns true if the BlockBackend can be written to in its current
  * configuration (i.e. if write permission have been requested)
  */
 bool blk_is_writable(BlockBackend *blk)
 {
     IO_CODE();
     return blk->perm & BLK_PERM_WRITE;
 }
 
 bool blk_is_sg(BlockBackend *blk)
 {
     BlockDriverState *bs = blk_bs(blk);
     GLOBAL_STATE_CODE();
 
     if (!bs) {
         return false;
     }
 
     return bdrv_is_sg(bs);
 }
 
 bool blk_enable_write_cache(BlockBackend *blk)
 {
     IO_CODE();
     return blk->enable_write_cache;
 }
 
 void blk_set_enable_write_cache(BlockBackend *blk, bool wce)
 {
     IO_CODE();
     blk->enable_write_cache = wce;
 }
 
 void blk_activate(BlockBackend *blk, Error **errp)
 {
     BlockDriverState *bs = blk_bs(blk);
     GLOBAL_STATE_CODE();
 
     if (!bs) {
         error_setg(errp, "Device '%s' has no medium", blk->name);
         return;
     }
 
     bdrv_activate(bs, errp);
 }
 
 bool blk_is_inserted(BlockBackend *blk)
 {
     BlockDriverState *bs = blk_bs(blk);
     IO_CODE();
 
     return bs && bdrv_is_inserted(bs);
 }
 
 bool blk_is_available(BlockBackend *blk)
 {
     IO_CODE();
     return blk_is_inserted(blk) && !blk_dev_is_tray_open(blk);
 }
 
 void blk_lock_medium(BlockBackend *blk, bool locked)
 {
     BlockDriverState *bs = blk_bs(blk);
     IO_CODE();
 
     if (bs) {
         bdrv_lock_medium(bs, locked);
     }
 }
 
 void blk_eject(BlockBackend *blk, bool eject_flag)
 {
     BlockDriverState *bs = blk_bs(blk);
     char *id;
     IO_CODE();
 
     if (bs) {
         bdrv_eject(bs, eject_flag);
     }
 
     /* Whether or not we ejected on the backend,
      * the frontend experienced a tray event. */
     id = blk_get_attached_dev_id(blk);
     qapi_event_send_device_tray_moved(blk_name(blk), id,
                                       eject_flag);
     g_free(id);
 }
 
 int blk_get_flags(BlockBackend *blk)
 {
     BlockDriverState *bs = blk_bs(blk);
     GLOBAL_STATE_CODE();
 
     if (bs) {
         return bdrv_get_flags(bs);
     } else {
         return blk->root_state.open_flags;
     }
 }
 
 /* Returns the minimum request alignment, in bytes; guaranteed nonzero */
 uint32_t blk_get_request_alignment(BlockBackend *blk)
 {
     BlockDriverState *bs = blk_bs(blk);
     IO_CODE();
     return bs ? bs->bl.request_alignment : BDRV_SECTOR_SIZE;
 }
 
 /* Returns the maximum hardware transfer length, in bytes; guaranteed nonzero */
 uint64_t blk_get_max_hw_transfer(BlockBackend *blk)
 {
     BlockDriverState *bs = blk_bs(blk);
     uint64_t max = INT_MAX;
     IO_CODE();
 
     if (bs) {
         max = MIN_NON_ZERO(max, bs->bl.max_hw_transfer);
         max = MIN_NON_ZERO(max, bs->bl.max_transfer);
     }
     return ROUND_DOWN(max, blk_get_request_alignment(blk));
 }
 
 /* Returns the maximum transfer length, in bytes; guaranteed nonzero */
 uint32_t blk_get_max_transfer(BlockBackend *blk)
 {
     BlockDriverState *bs = blk_bs(blk);
     uint32_t max = INT_MAX;
     IO_CODE();
 
     if (bs) {
         max = MIN_NON_ZERO(max, bs->bl.max_transfer);
     }
     return ROUND_DOWN(max, blk_get_request_alignment(blk));
 }
 
 int blk_get_max_hw_iov(BlockBackend *blk)
 {
     IO_CODE();
     return MIN_NON_ZERO(blk->root->bs->bl.max_hw_iov,
                         blk->root->bs->bl.max_iov);
 }
 
 int blk_get_max_iov(BlockBackend *blk)
 {
     IO_CODE();
     return blk->root->bs->bl.max_iov;
 }
 
 void *blk_try_blockalign(BlockBackend *blk, size_t size)
 {
     IO_CODE();
     return qemu_try_blockalign(blk ? blk_bs(blk) : NULL, size);
 }
 
 void *blk_blockalign(BlockBackend *blk, size_t size)
 {
     IO_CODE();
     return qemu_blockalign(blk ? blk_bs(blk) : NULL, size);
 }
 
 bool blk_op_is_blocked(BlockBackend *blk, BlockOpType op, Error **errp)
 {
     BlockDriverState *bs = blk_bs(blk);
     GLOBAL_STATE_CODE();
 
     if (!bs) {
         return false;
     }
 
     return bdrv_op_is_blocked(bs, op, errp);
 }
 
 void blk_op_unblock(BlockBackend *blk, BlockOpType op, Error *reason)
 {
     BlockDriverState *bs = blk_bs(blk);
     GLOBAL_STATE_CODE();
 
     if (bs) {
         bdrv_op_unblock(bs, op, reason);
     }
 }
 
 void blk_op_block_all(BlockBackend *blk, Error *reason)
 {
     BlockDriverState *bs = blk_bs(blk);
     GLOBAL_STATE_CODE();
 
     if (bs) {
         bdrv_op_block_all(bs, reason);
     }
 }
 
 void blk_op_unblock_all(BlockBackend *blk, Error *reason)
 {
     BlockDriverState *bs = blk_bs(blk);
     GLOBAL_STATE_CODE();
 
     if (bs) {
         bdrv_op_unblock_all(bs, reason);
     }
 }
 
 AioContext *blk_get_aio_context(BlockBackend *blk)
 {
     BlockDriverState *bs = blk_bs(blk);
     IO_CODE();
 
     if (bs) {
         AioContext *ctx = bdrv_get_aio_context(blk_bs(blk));
         assert(ctx == blk->ctx);
     }
 
     return blk->ctx;
 }
 
 static AioContext *blk_aiocb_get_aio_context(BlockAIOCB *acb)
 {
     BlockBackendAIOCB *blk_acb = DO_UPCAST(BlockBackendAIOCB, common, acb);
     return blk_get_aio_context(blk_acb->blk);
 }
 
 static int blk_do_set_aio_context(BlockBackend *blk, AioContext *new_context,
                                   bool update_root_node, Error **errp)
 {
     BlockDriverState *bs = blk_bs(blk);
     ThrottleGroupMember *tgm = &blk->public.throttle_group_member;
     int ret;
 
     if (bs) {
         bdrv_ref(bs);
 
         if (update_root_node) {
             /*
              * update_root_node MUST be false for blk_root_set_aio_ctx_commit(),
              * as we are already in the commit function of a transaction.
              */
             ret = bdrv_try_change_aio_context(bs, new_context, blk->root, errp);
             if (ret < 0) {
                 bdrv_unref(bs);
                 return ret;
             }
         }
         /*
          * Make blk->ctx consistent with the root node before we invoke any
          * other operations like drain that might inquire blk->ctx
          */
         blk->ctx = new_context;
         if (tgm->throttle_state) {
             bdrv_drained_begin(bs);
             throttle_group_detach_aio_context(tgm);
             throttle_group_attach_aio_context(tgm, new_context);
             bdrv_drained_end(bs);
         }
 
         bdrv_unref(bs);
     } else {
         blk->ctx = new_context;
     }
 
     return 0;
 }
 
 int blk_set_aio_context(BlockBackend *blk, AioContext *new_context,
                         Error **errp)
 {
     GLOBAL_STATE_CODE();
     return blk_do_set_aio_context(blk, new_context, true, errp);
 }
 
 typedef struct BdrvStateBlkRootContext {
     AioContext *new_ctx;
     BlockBackend *blk;
 } BdrvStateBlkRootContext;
 
 static void blk_root_set_aio_ctx_commit(void *opaque)
 {
     BdrvStateBlkRootContext *s = opaque;
     BlockBackend *blk = s->blk;
 
     blk_do_set_aio_context(blk, s->new_ctx, false, &error_abort);
 }
 
 static TransactionActionDrv set_blk_root_context = {
     .commit = blk_root_set_aio_ctx_commit,
     .clean = g_free,
 };
 
 static bool blk_root_change_aio_ctx(BdrvChild *child, AioContext *ctx,
                                     GHashTable *visited, Transaction *tran,
                                     Error **errp)
 {
     BlockBackend *blk = child->opaque;
     BdrvStateBlkRootContext *s;
 
     if (!blk->allow_aio_context_change) {
         /*
          * Manually created BlockBackends (those with a name) that are not
          * attached to anything can change their AioContext without updating
          * their user; return an error for others.
          */
         if (!blk->name || blk->dev) {
             /* TODO Add BB name/QOM path */
             error_setg(errp, "Cannot change iothread of active block backend");
             return false;
         }
     }
 
     s = g_new(BdrvStateBlkRootContext, 1);
     *s = (BdrvStateBlkRootContext) {
         .new_ctx = ctx,
         .blk = blk,
     };
 
     tran_add(tran, &set_blk_root_context, s);
     return true;
 }
 
 void blk_add_aio_context_notifier(BlockBackend *blk,
         void (*attached_aio_context)(AioContext *new_context, void *opaque),
         void (*detach_aio_context)(void *opaque), void *opaque)
 {
     BlockBackendAioNotifier *notifier;
     BlockDriverState *bs = blk_bs(blk);
     GLOBAL_STATE_CODE();
 
     notifier = g_new(BlockBackendAioNotifier, 1);
     notifier->attached_aio_context = attached_aio_context;
     notifier->detach_aio_context = detach_aio_context;
     notifier->opaque = opaque;
     QLIST_INSERT_HEAD(&blk->aio_notifiers, notifier, list);
 
     if (bs) {
         bdrv_add_aio_context_notifier(bs, attached_aio_context,
                                       detach_aio_context, opaque);
     }
 }
 
 void blk_remove_aio_context_notifier(BlockBackend *blk,
                                      void (*attached_aio_context)(AioContext *,
                                                                   void *),
                                      void (*detach_aio_context)(void *),
                                      void *opaque)
 {
     BlockBackendAioNotifier *notifier;
     BlockDriverState *bs = blk_bs(blk);
 
     GLOBAL_STATE_CODE();
 
     if (bs) {
         bdrv_remove_aio_context_notifier(bs, attached_aio_context,
                                          detach_aio_context, opaque);
     }
 
     QLIST_FOREACH(notifier, &blk->aio_notifiers, list) {
         if (notifier->attached_aio_context == attached_aio_context &&
             notifier->detach_aio_context == detach_aio_context &&
             notifier->opaque == opaque) {
             QLIST_REMOVE(notifier, list);
             g_free(notifier);
             return;
         }
     }
 
     abort();
 }
 
 void blk_add_remove_bs_notifier(BlockBackend *blk, Notifier *notify)
 {
     GLOBAL_STATE_CODE();
     notifier_list_add(&blk->remove_bs_notifiers, notify);
 }
 
 void blk_add_insert_bs_notifier(BlockBackend *blk, Notifier *notify)
 {
     GLOBAL_STATE_CODE();
     notifier_list_add(&blk->insert_bs_notifiers, notify);
 }
 
 void blk_io_plug(BlockBackend *blk)
 {
     BlockDriverState *bs = blk_bs(blk);
     IO_CODE();
 
     if (bs) {
         bdrv_io_plug(bs);
     }
 }
 
 void blk_io_unplug(BlockBackend *blk)
 {
     BlockDriverState *bs = blk_bs(blk);
     IO_CODE();
 
     if (bs) {
         bdrv_io_unplug(bs);
     }
 }
 
 BlockAcctStats *blk_get_stats(BlockBackend *blk)
 {
     IO_CODE();
     return &blk->stats;
 }
 
 void *blk_aio_get(const AIOCBInfo *aiocb_info, BlockBackend *blk,
                   BlockCompletionFunc *cb, void *opaque)
 {
     IO_CODE();
     return qemu_aio_get(aiocb_info, blk_bs(blk), cb, opaque);
 }
 
 int coroutine_fn blk_co_pwrite_zeroes(BlockBackend *blk, int64_t offset,
                                       int64_t bytes, BdrvRequestFlags flags)
 {
     IO_OR_GS_CODE();
     return blk_co_pwritev(blk, offset, bytes, NULL,
                           flags | BDRV_REQ_ZERO_WRITE);
 }
 
 int coroutine_fn blk_co_pwrite_compressed(BlockBackend *blk, int64_t offset,
                                           int64_t bytes, const void *buf)
 {
     QEMUIOVector qiov = QEMU_IOVEC_INIT_BUF(qiov, buf, bytes);
     IO_OR_GS_CODE();
     return blk_co_pwritev_part(blk, offset, bytes, &qiov, 0,
                                BDRV_REQ_WRITE_COMPRESSED);
 }
 
 int coroutine_fn blk_co_truncate(BlockBackend *blk, int64_t offset, bool exact,
                                  PreallocMode prealloc, BdrvRequestFlags flags,
                                  Error **errp)
 {
     IO_OR_GS_CODE();
     if (!blk_is_available(blk)) {
         error_setg(errp, "No medium inserted");
         return -ENOMEDIUM;
     }
 
     return bdrv_co_truncate(blk->root, offset, exact, prealloc, flags, errp);
 }
 
 int blk_save_vmstate(BlockBackend *blk, const uint8_t *buf,
                      int64_t pos, int size)
 {
     int ret;
     GLOBAL_STATE_CODE();
 
     if (!blk_is_available(blk)) {
         return -ENOMEDIUM;
     }
 
     ret = bdrv_save_vmstate(blk_bs(blk), buf, pos, size);
     if (ret < 0) {
         return ret;
     }
 
     if (ret == size && !blk->enable_write_cache) {
         ret = bdrv_flush(blk_bs(blk));
     }
 
     return ret < 0 ? ret : size;
 }
 
 int blk_load_vmstate(BlockBackend *blk, uint8_t *buf, int64_t pos, int size)
 {
     GLOBAL_STATE_CODE();
     if (!blk_is_available(blk)) {
         return -ENOMEDIUM;
     }
 
     return bdrv_load_vmstate(blk_bs(blk), buf, pos, size);
 }
 
 int blk_probe_blocksizes(BlockBackend *blk, BlockSizes *bsz)
 {
     GLOBAL_STATE_CODE();
     if (!blk_is_available(blk)) {
         return -ENOMEDIUM;
     }
 
     return bdrv_probe_blocksizes(blk_bs(blk), bsz);
 }
 
 int blk_probe_geometry(BlockBackend *blk, HDGeometry *geo)
 {
     GLOBAL_STATE_CODE();
     if (!blk_is_available(blk)) {
         return -ENOMEDIUM;
     }
 
     return bdrv_probe_geometry(blk_bs(blk), geo);
 }
 
 /*
  * Updates the BlockBackendRootState object with data from the currently
  * attached BlockDriverState.
  */
 void blk_update_root_state(BlockBackend *blk)
 {
     GLOBAL_STATE_CODE();
     assert(blk->root);
 
     blk->root_state.open_flags    = blk->root->bs->open_flags;
     blk->root_state.detect_zeroes = blk->root->bs->detect_zeroes;
 }
 
 /*
  * Returns the detect-zeroes setting to be used for bdrv_open() of a
  * BlockDriverState which is supposed to inherit the root state.
  */
 bool blk_get_detect_zeroes_from_root_state(BlockBackend *blk)
 {
     GLOBAL_STATE_CODE();
     return blk->root_state.detect_zeroes;
 }
 
 /*
  * Returns the flags to be used for bdrv_open() of a BlockDriverState which is
  * supposed to inherit the root state.
  */
 int blk_get_open_flags_from_root_state(BlockBackend *blk)
 {
     GLOBAL_STATE_CODE();
     return blk->root_state.open_flags;
 }
 
 BlockBackendRootState *blk_get_root_state(BlockBackend *blk)
 {
     GLOBAL_STATE_CODE();
     return &blk->root_state;
 }
 
 int blk_commit_all(void)
 {
     BlockBackend *blk = NULL;
     GLOBAL_STATE_CODE();
 
     while ((blk = blk_all_next(blk)) != NULL) {
         AioContext *aio_context = blk_get_aio_context(blk);
         BlockDriverState *unfiltered_bs = bdrv_skip_filters(blk_bs(blk));
 
         aio_context_acquire(aio_context);
         if (blk_is_inserted(blk) && bdrv_cow_child(unfiltered_bs)) {
             int ret;
 
             ret = bdrv_commit(unfiltered_bs);
             if (ret < 0) {
                 aio_context_release(aio_context);
                 return ret;
             }
         }
         aio_context_release(aio_context);
     }
     return 0;
 }
 
 
 /* throttling disk I/O limits */
 void blk_set_io_limits(BlockBackend *blk, ThrottleConfig *cfg)
 {
     GLOBAL_STATE_CODE();
     throttle_group_config(&blk->public.throttle_group_member, cfg);
 }
 
 void blk_io_limits_disable(BlockBackend *blk)
 {
     BlockDriverState *bs = blk_bs(blk);
     ThrottleGroupMember *tgm = &blk->public.throttle_group_member;
     assert(tgm->throttle_state);
     GLOBAL_STATE_CODE();
     if (bs) {
         bdrv_ref(bs);
         bdrv_drained_begin(bs);
     }
     throttle_group_unregister_tgm(tgm);
     if (bs) {
         bdrv_drained_end(bs);
         bdrv_unref(bs);
     }
 }
 
 /* should be called before blk_set_io_limits if a limit is set */
 void blk_io_limits_enable(BlockBackend *blk, const char *group)
 {
     assert(!blk->public.throttle_group_member.throttle_state);
     GLOBAL_STATE_CODE();
     throttle_group_register_tgm(&blk->public.throttle_group_member,
                                 group, blk_get_aio_context(blk));
 }
 
 void blk_io_limits_update_group(BlockBackend *blk, const char *group)
 {
     GLOBAL_STATE_CODE();
     /* this BB is not part of any group */
     if (!blk->public.throttle_group_member.throttle_state) {
         return;
     }
 
     /* this BB is a part of the same group than the one we want */
     if (!g_strcmp0(throttle_group_get_name(&blk->public.throttle_group_member),
                 group)) {
         return;
     }
 
     /* need to change the group this bs belong to */
     blk_io_limits_disable(blk);
     blk_io_limits_enable(blk, group);
 }
 
 static void blk_root_drained_begin(BdrvChild *child)
 {
     BlockBackend *blk = child->opaque;
     ThrottleGroupMember *tgm = &blk->public.throttle_group_member;
 
     if (++blk->quiesce_counter == 1) {
         if (blk->dev_ops && blk->dev_ops->drained_begin) {
             blk->dev_ops->drained_begin(blk->dev_opaque);
         }
     }
 
     /* Note that blk->root may not be accessible here yet if we are just
      * attaching to a BlockDriverState that is drained. Use child instead. */
 
     if (qatomic_fetch_inc(&tgm->io_limits_disabled) == 0) {
         throttle_group_restart_tgm(tgm);
     }
 }
 
 static bool blk_root_drained_poll(BdrvChild *child)
 {
     BlockBackend *blk = child->opaque;
     bool busy = false;
     assert(blk->quiesce_counter);
 
     if (blk->dev_ops && blk->dev_ops->drained_poll) {
         busy = blk->dev_ops->drained_poll(blk->dev_opaque);
     }
     return busy || !!blk->in_flight;
 }
 
 static void blk_root_drained_end(BdrvChild *child, int *drained_end_counter)
 {
     BlockBackend *blk = child->opaque;
     assert(blk->quiesce_counter);
 
     assert(blk->public.throttle_group_member.io_limits_disabled);
     qatomic_dec(&blk->public.throttle_group_member.io_limits_disabled);
 
     if (--blk->quiesce_counter == 0) {
         if (blk->dev_ops && blk->dev_ops->drained_end) {
             blk->dev_ops->drained_end(blk->dev_opaque);
         }
         while (qemu_co_enter_next(&blk->queued_requests, NULL)) {
             /* Resume all queued requests */
         }
     }
 }
 
 bool blk_register_buf(BlockBackend *blk, void *host, size_t size, Error **errp)
 {
     BlockDriverState *bs = blk_bs(blk);
 
     GLOBAL_STATE_CODE();
 
     if (bs) {
         return bdrv_register_buf(bs, host, size, errp);
     }
     return true;
 }
 
 void blk_unregister_buf(BlockBackend *blk, void *host, size_t size)
 {
     BlockDriverState *bs = blk_bs(blk);
 
     GLOBAL_STATE_CODE();
 
     if (bs) {
         bdrv_unregister_buf(bs, host, size);
     }
 }
 
 int coroutine_fn blk_co_copy_range(BlockBackend *blk_in, int64_t off_in,
                                    BlockBackend *blk_out, int64_t off_out,
                                    int64_t bytes, BdrvRequestFlags read_flags,
                                    BdrvRequestFlags write_flags)
 {
     int r;
     IO_CODE();
 
     r = blk_check_byte_request(blk_in, off_in, bytes);
     if (r) {
         return r;
     }
     r = blk_check_byte_request(blk_out, off_out, bytes);
     if (r) {
         return r;
     }
     return bdrv_co_copy_range(blk_in->root, off_in,
                               blk_out->root, off_out,
                               bytes, read_flags, write_flags);
 }
 
 const BdrvChild *blk_root(BlockBackend *blk)
 {
     GLOBAL_STATE_CODE();
     return blk->root;
 }
 
 int blk_make_empty(BlockBackend *blk, Error **errp)
 {
     GLOBAL_STATE_CODE();
     if (!blk_is_available(blk)) {
         error_setg(errp, "No medium inserted");
         return -ENOMEDIUM;
     }
 
     return bdrv_make_empty(blk->root, errp);
 }