qemu

postcopy-ram.c (54175B)

---

 /*
  * Postcopy migration for RAM
  *
  * Copyright 2013-2015 Red Hat, Inc. and/or its affiliates
  *
  * Authors:
  *  Dave Gilbert  <dgilbert@redhat.com>
  *
  * This work is licensed under the terms of the GNU GPL, version 2 or later.
  * See the COPYING file in the top-level directory.
  *
  */
 
 /*
  * Postcopy is a migration technique where the execution flips from the
  * source to the destination before all the data has been copied.
  */
 
 #include "qemu/osdep.h"
 #include "qemu/rcu.h"
 #include "qemu/madvise.h"
 #include "exec/target_page.h"
 #include "migration.h"
 #include "qemu-file.h"
 #include "savevm.h"
 #include "postcopy-ram.h"
 #include "ram.h"
 #include "qapi/error.h"
 #include "qemu/notify.h"
 #include "qemu/rcu.h"
 #include "sysemu/sysemu.h"
 #include "qemu/error-report.h"
 #include "trace.h"
 #include "hw/boards.h"
 #include "exec/ramblock.h"
 #include "socket.h"
 #include "qemu-file.h"
 #include "yank_functions.h"
 #include "tls.h"
 
 /* Arbitrary limit on size of each discard command,
  * keeps them around ~200 bytes
  */
 #define MAX_DISCARDS_PER_COMMAND 12
 
 struct PostcopyDiscardState {
     const char *ramblock_name;
     uint16_t cur_entry;
     /*
      * Start and length of a discard range (bytes)
      */
     uint64_t start_list[MAX_DISCARDS_PER_COMMAND];
     uint64_t length_list[MAX_DISCARDS_PER_COMMAND];
     unsigned int nsentwords;
     unsigned int nsentcmds;
 };
 
 static NotifierWithReturnList postcopy_notifier_list;
 
 void postcopy_infrastructure_init(void)
 {
     notifier_with_return_list_init(&postcopy_notifier_list);
 }
 
 void postcopy_add_notifier(NotifierWithReturn *nn)
 {
     notifier_with_return_list_add(&postcopy_notifier_list, nn);
 }
 
 void postcopy_remove_notifier(NotifierWithReturn *n)
 {
     notifier_with_return_remove(n);
 }
 
 int postcopy_notify(enum PostcopyNotifyReason reason, Error **errp)
 {
     struct PostcopyNotifyData pnd;
     pnd.reason = reason;
     pnd.errp = errp;
 
     return notifier_with_return_list_notify(&postcopy_notifier_list,
                                             &pnd);
 }
 
 /*
  * NOTE: this routine is not thread safe, we can't call it concurrently. But it
  * should be good enough for migration's purposes.
  */
 void postcopy_thread_create(MigrationIncomingState *mis,
                             QemuThread *thread, const char *name,
                             void *(*fn)(void *), int joinable)
 {
     qemu_sem_init(&mis->thread_sync_sem, 0);
     qemu_thread_create(thread, name, fn, mis, joinable);
     qemu_sem_wait(&mis->thread_sync_sem);
     qemu_sem_destroy(&mis->thread_sync_sem);
 }
 
 /* Postcopy needs to detect accesses to pages that haven't yet been copied
  * across, and efficiently map new pages in, the techniques for doing this
  * are target OS specific.
  */
 #if defined(__linux__)
 
 #include <poll.h>
 #include <sys/ioctl.h>
 #include <sys/syscall.h>
 #include <asm/types.h> /* for __u64 */
 #endif
 
 #if defined(__linux__) && defined(__NR_userfaultfd) && defined(CONFIG_EVENTFD)
 #include <sys/eventfd.h>
 #include <linux/userfaultfd.h>
 
 typedef struct PostcopyBlocktimeContext {
     /* time when page fault initiated per vCPU */
     uint32_t *page_fault_vcpu_time;
     /* page address per vCPU */
     uintptr_t *vcpu_addr;
     uint32_t total_blocktime;
     /* blocktime per vCPU */
     uint32_t *vcpu_blocktime;
     /* point in time when last page fault was initiated */
     uint32_t last_begin;
     /* number of vCPU are suspended */
     int smp_cpus_down;
     uint64_t start_time;
 
     /*
      * Handler for exit event, necessary for
      * releasing whole blocktime_ctx
      */
     Notifier exit_notifier;
 } PostcopyBlocktimeContext;
 
 static void destroy_blocktime_context(struct PostcopyBlocktimeContext *ctx)
 {
     g_free(ctx->page_fault_vcpu_time);
     g_free(ctx->vcpu_addr);
     g_free(ctx->vcpu_blocktime);
     g_free(ctx);
 }
 
 static void migration_exit_cb(Notifier *n, void *data)
 {
     PostcopyBlocktimeContext *ctx = container_of(n, PostcopyBlocktimeContext,
                                                  exit_notifier);
     destroy_blocktime_context(ctx);
 }
 
 static struct PostcopyBlocktimeContext *blocktime_context_new(void)
 {
     MachineState *ms = MACHINE(qdev_get_machine());
     unsigned int smp_cpus = ms->smp.cpus;
     PostcopyBlocktimeContext *ctx = g_new0(PostcopyBlocktimeContext, 1);
     ctx->page_fault_vcpu_time = g_new0(uint32_t, smp_cpus);
     ctx->vcpu_addr = g_new0(uintptr_t, smp_cpus);
     ctx->vcpu_blocktime = g_new0(uint32_t, smp_cpus);
 
     ctx->exit_notifier.notify = migration_exit_cb;
     ctx->start_time = qemu_clock_get_ms(QEMU_CLOCK_REALTIME);
     qemu_add_exit_notifier(&ctx->exit_notifier);
     return ctx;
 }
 
 static uint32List *get_vcpu_blocktime_list(PostcopyBlocktimeContext *ctx)
 {
     MachineState *ms = MACHINE(qdev_get_machine());
     uint32List *list = NULL;
     int i;
 
     for (i = ms->smp.cpus - 1; i >= 0; i--) {
         QAPI_LIST_PREPEND(list, ctx->vcpu_blocktime[i]);
     }
 
     return list;
 }
 
 /*
  * This function just populates MigrationInfo from postcopy's
  * blocktime context. It will not populate MigrationInfo,
  * unless postcopy-blocktime capability was set.
  *
  * @info: pointer to MigrationInfo to populate
  */
 void fill_destination_postcopy_migration_info(MigrationInfo *info)
 {
     MigrationIncomingState *mis = migration_incoming_get_current();
     PostcopyBlocktimeContext *bc = mis->blocktime_ctx;
 
     if (!bc) {
         return;
     }
 
     info->has_postcopy_blocktime = true;
     info->postcopy_blocktime = bc->total_blocktime;
     info->has_postcopy_vcpu_blocktime = true;
     info->postcopy_vcpu_blocktime = get_vcpu_blocktime_list(bc);
 }
 
 static uint32_t get_postcopy_total_blocktime(void)
 {
     MigrationIncomingState *mis = migration_incoming_get_current();
     PostcopyBlocktimeContext *bc = mis->blocktime_ctx;
 
     if (!bc) {
         return 0;
     }
 
     return bc->total_blocktime;
 }
 
 /**
  * receive_ufd_features: check userfault fd features, to request only supported
  * features in the future.
  *
  * Returns: true on success
  *
  * __NR_userfaultfd - should be checked before
  *  @features: out parameter will contain uffdio_api.features provided by kernel
  *              in case of success
  */
 static bool receive_ufd_features(uint64_t *features)
 {
     struct uffdio_api api_struct = {0};
     int ufd;
     bool ret = true;
 
     /* if we are here __NR_userfaultfd should exists */
     ufd = syscall(__NR_userfaultfd, O_CLOEXEC);
     if (ufd == -1) {
         error_report("%s: syscall __NR_userfaultfd failed: %s", __func__,
                      strerror(errno));
         return false;
     }
 
     /* ask features */
     api_struct.api = UFFD_API;
     api_struct.features = 0;
     if (ioctl(ufd, UFFDIO_API, &api_struct)) {
         error_report("%s: UFFDIO_API failed: %s", __func__,
                      strerror(errno));
         ret = false;
         goto release_ufd;
     }
 
     *features = api_struct.features;
 
 release_ufd:
     close(ufd);
     return ret;
 }
 
 /**
  * request_ufd_features: this function should be called only once on a newly
  * opened ufd, subsequent calls will lead to error.
  *
  * Returns: true on success
  *
  * @ufd: fd obtained from userfaultfd syscall
  * @features: bit mask see UFFD_API_FEATURES
  */
 static bool request_ufd_features(int ufd, uint64_t features)
 {
     struct uffdio_api api_struct = {0};
     uint64_t ioctl_mask;
 
     api_struct.api = UFFD_API;
     api_struct.features = features;
     if (ioctl(ufd, UFFDIO_API, &api_struct)) {
         error_report("%s failed: UFFDIO_API failed: %s", __func__,
                      strerror(errno));
         return false;
     }
 
     ioctl_mask = (__u64)1 << _UFFDIO_REGISTER |
                  (__u64)1 << _UFFDIO_UNREGISTER;
     if ((api_struct.ioctls & ioctl_mask) != ioctl_mask) {
         error_report("Missing userfault features: %" PRIx64,
                      (uint64_t)(~api_struct.ioctls & ioctl_mask));
         return false;
     }
 
     return true;
 }
 
 static bool ufd_check_and_apply(int ufd, MigrationIncomingState *mis)
 {
     uint64_t asked_features = 0;
     static uint64_t supported_features;
 
     /*
      * it's not possible to
      * request UFFD_API twice per one fd
      * userfault fd features is persistent
      */
     if (!supported_features) {
         if (!receive_ufd_features(&supported_features)) {
             error_report("%s failed", __func__);
             return false;
         }
     }
 
 #ifdef UFFD_FEATURE_THREAD_ID
     if (UFFD_FEATURE_THREAD_ID & supported_features) {
         asked_features |= UFFD_FEATURE_THREAD_ID;
         if (migrate_postcopy_blocktime()) {
             if (!mis->blocktime_ctx) {
                 mis->blocktime_ctx = blocktime_context_new();
             }
         }
     }
 #endif
 
     /*
      * request features, even if asked_features is 0, due to
      * kernel expects UFFD_API before UFFDIO_REGISTER, per
      * userfault file descriptor
      */
     if (!request_ufd_features(ufd, asked_features)) {
         error_report("%s failed: features %" PRIu64, __func__,
                      asked_features);
         return false;
     }
 
     if (qemu_real_host_page_size() != ram_pagesize_summary()) {
         bool have_hp = false;
         /* We've got a huge page */
 #ifdef UFFD_FEATURE_MISSING_HUGETLBFS
         have_hp = supported_features & UFFD_FEATURE_MISSING_HUGETLBFS;
 #endif
         if (!have_hp) {
             error_report("Userfault on this host does not support huge pages");
             return false;
         }
     }
     return true;
 }
 
 /* Callback from postcopy_ram_supported_by_host block iterator.
  */
 static int test_ramblock_postcopiable(RAMBlock *rb, void *opaque)
 {
     const char *block_name = qemu_ram_get_idstr(rb);
     ram_addr_t length = qemu_ram_get_used_length(rb);
     size_t pagesize = qemu_ram_pagesize(rb);
 
     if (length % pagesize) {
         error_report("Postcopy requires RAM blocks to be a page size multiple,"
                      " block %s is 0x" RAM_ADDR_FMT " bytes with a "
                      "page size of 0x%zx", block_name, length, pagesize);
         return 1;
     }
     return 0;
 }
 
 /*
  * Note: This has the side effect of munlock'ing all of RAM, that's
  * normally fine since if the postcopy succeeds it gets turned back on at the
  * end.
  */
 bool postcopy_ram_supported_by_host(MigrationIncomingState *mis)
 {
     long pagesize = qemu_real_host_page_size();
     int ufd = -1;
     bool ret = false; /* Error unless we change it */
     void *testarea = NULL;
     struct uffdio_register reg_struct;
     struct uffdio_range range_struct;
     uint64_t feature_mask;
     Error *local_err = NULL;
 
     if (qemu_target_page_size() > pagesize) {
         error_report("Target page size bigger than host page size");
         goto out;
     }
 
     ufd = syscall(__NR_userfaultfd, O_CLOEXEC);
     if (ufd == -1) {
         error_report("%s: userfaultfd not available: %s", __func__,
                      strerror(errno));
         goto out;
     }
 
     /* Give devices a chance to object */
     if (postcopy_notify(POSTCOPY_NOTIFY_PROBE, &local_err)) {
         error_report_err(local_err);
         goto out;
     }
 
     /* Version and features check */
     if (!ufd_check_and_apply(ufd, mis)) {
         goto out;
     }
 
     /* We don't support postcopy with shared RAM yet */
     if (foreach_not_ignored_block(test_ramblock_postcopiable, NULL)) {
         goto out;
     }
 
     /*
      * userfault and mlock don't go together; we'll put it back later if
      * it was enabled.
      */
     if (munlockall()) {
         error_report("%s: munlockall: %s", __func__,  strerror(errno));
         goto out;
     }
 
     /*
      *  We need to check that the ops we need are supported on anon memory
      *  To do that we need to register a chunk and see the flags that
      *  are returned.
      */
     testarea = mmap(NULL, pagesize, PROT_READ | PROT_WRITE, MAP_PRIVATE |
                                     MAP_ANONYMOUS, -1, 0);
     if (testarea == MAP_FAILED) {
         error_report("%s: Failed to map test area: %s", __func__,
                      strerror(errno));
         goto out;
     }
     g_assert(QEMU_PTR_IS_ALIGNED(testarea, pagesize));
 
     reg_struct.range.start = (uintptr_t)testarea;
     reg_struct.range.len = pagesize;
     reg_struct.mode = UFFDIO_REGISTER_MODE_MISSING;
 
     if (ioctl(ufd, UFFDIO_REGISTER, &reg_struct)) {
         error_report("%s userfault register: %s", __func__, strerror(errno));
         goto out;
     }
 
     range_struct.start = (uintptr_t)testarea;
     range_struct.len = pagesize;
     if (ioctl(ufd, UFFDIO_UNREGISTER, &range_struct)) {
         error_report("%s userfault unregister: %s", __func__, strerror(errno));
         goto out;
     }
 
     feature_mask = (__u64)1 << _UFFDIO_WAKE |
                    (__u64)1 << _UFFDIO_COPY |
                    (__u64)1 << _UFFDIO_ZEROPAGE;
     if ((reg_struct.ioctls & feature_mask) != feature_mask) {
         error_report("Missing userfault map features: %" PRIx64,
                      (uint64_t)(~reg_struct.ioctls & feature_mask));
         goto out;
     }
 
     /* Success! */
     ret = true;
 out:
     if (testarea) {
         munmap(testarea, pagesize);
     }
     if (ufd != -1) {
         close(ufd);
     }
     return ret;
 }
 
 /*
  * Setup an area of RAM so that it *can* be used for postcopy later; this
  * must be done right at the start prior to pre-copy.
  * opaque should be the MIS.
  */
 static int init_range(RAMBlock *rb, void *opaque)
 {
     const char *block_name = qemu_ram_get_idstr(rb);
     void *host_addr = qemu_ram_get_host_addr(rb);
     ram_addr_t offset = qemu_ram_get_offset(rb);
     ram_addr_t length = qemu_ram_get_used_length(rb);
     trace_postcopy_init_range(block_name, host_addr, offset, length);
 
     /*
      * Save the used_length before running the guest. In case we have to
      * resize RAM blocks when syncing RAM block sizes from the source during
      * precopy, we'll update it manually via the ram block notifier.
      */
     rb->postcopy_length = length;
 
     /*
      * We need the whole of RAM to be truly empty for postcopy, so things
      * like ROMs and any data tables built during init must be zero'd
      * - we're going to get the copy from the source anyway.
      * (Precopy will just overwrite this data, so doesn't need the discard)
      */
     if (ram_discard_range(block_name, 0, length)) {
         return -1;
     }
 
     return 0;
 }
 
 /*
  * At the end of migration, undo the effects of init_range
  * opaque should be the MIS.
  */
 static int cleanup_range(RAMBlock *rb, void *opaque)
 {
     const char *block_name = qemu_ram_get_idstr(rb);
     void *host_addr = qemu_ram_get_host_addr(rb);
     ram_addr_t offset = qemu_ram_get_offset(rb);
     ram_addr_t length = rb->postcopy_length;
     MigrationIncomingState *mis = opaque;
     struct uffdio_range range_struct;
     trace_postcopy_cleanup_range(block_name, host_addr, offset, length);
 
     /*
      * We turned off hugepage for the precopy stage with postcopy enabled
      * we can turn it back on now.
      */
     qemu_madvise(host_addr, length, QEMU_MADV_HUGEPAGE);
 
     /*
      * We can also turn off userfault now since we should have all the
      * pages.   It can be useful to leave it on to debug postcopy
      * if you're not sure it's always getting every page.
      */
     range_struct.start = (uintptr_t)host_addr;
     range_struct.len = length;
 
     if (ioctl(mis->userfault_fd, UFFDIO_UNREGISTER, &range_struct)) {
         error_report("%s: userfault unregister %s", __func__, strerror(errno));
 
         return -1;
     }
 
     return 0;
 }
 
 /*
  * Initialise postcopy-ram, setting the RAM to a state where we can go into
  * postcopy later; must be called prior to any precopy.
  * called from arch_init's similarly named ram_postcopy_incoming_init
  */
 int postcopy_ram_incoming_init(MigrationIncomingState *mis)
 {
     if (foreach_not_ignored_block(init_range, NULL)) {
         return -1;
     }
 
     return 0;
 }
 
 static void postcopy_temp_pages_cleanup(MigrationIncomingState *mis)
 {
     int i;
 
     if (mis->postcopy_tmp_pages) {
         for (i = 0; i < mis->postcopy_channels; i++) {
             if (mis->postcopy_tmp_pages[i].tmp_huge_page) {
                 munmap(mis->postcopy_tmp_pages[i].tmp_huge_page,
                        mis->largest_page_size);
                 mis->postcopy_tmp_pages[i].tmp_huge_page = NULL;
             }
         }
         g_free(mis->postcopy_tmp_pages);
         mis->postcopy_tmp_pages = NULL;
     }
 
     if (mis->postcopy_tmp_zero_page) {
         munmap(mis->postcopy_tmp_zero_page, mis->largest_page_size);
         mis->postcopy_tmp_zero_page = NULL;
     }
 }
 
 /*
  * At the end of a migration where postcopy_ram_incoming_init was called.
  */
 int postcopy_ram_incoming_cleanup(MigrationIncomingState *mis)
 {
     trace_postcopy_ram_incoming_cleanup_entry();
 
     if (mis->postcopy_prio_thread_created) {
         qemu_thread_join(&mis->postcopy_prio_thread);
         mis->postcopy_prio_thread_created = false;
     }
 
     if (mis->have_fault_thread) {
         Error *local_err = NULL;
 
         /* Let the fault thread quit */
         qatomic_set(&mis->fault_thread_quit, 1);
         postcopy_fault_thread_notify(mis);
         trace_postcopy_ram_incoming_cleanup_join();
         qemu_thread_join(&mis->fault_thread);
 
         if (postcopy_notify(POSTCOPY_NOTIFY_INBOUND_END, &local_err)) {
             error_report_err(local_err);
             return -1;
         }
 
         if (foreach_not_ignored_block(cleanup_range, mis)) {
             return -1;
         }
 
         trace_postcopy_ram_incoming_cleanup_closeuf();
         close(mis->userfault_fd);
         close(mis->userfault_event_fd);
         mis->have_fault_thread = false;
     }
 
     if (enable_mlock) {
         if (os_mlock() < 0) {
             error_report("mlock: %s", strerror(errno));
             /*
              * It doesn't feel right to fail at this point, we have a valid
              * VM state.
              */
         }
     }
 
     postcopy_temp_pages_cleanup(mis);
 
     trace_postcopy_ram_incoming_cleanup_blocktime(
             get_postcopy_total_blocktime());
 
     trace_postcopy_ram_incoming_cleanup_exit();
     return 0;
 }
 
 /*
  * Disable huge pages on an area
  */
 static int nhp_range(RAMBlock *rb, void *opaque)
 {
     const char *block_name = qemu_ram_get_idstr(rb);
     void *host_addr = qemu_ram_get_host_addr(rb);
     ram_addr_t offset = qemu_ram_get_offset(rb);
     ram_addr_t length = rb->postcopy_length;
     trace_postcopy_nhp_range(block_name, host_addr, offset, length);
 
     /*
      * Before we do discards we need to ensure those discards really
      * do delete areas of the page, even if THP thinks a hugepage would
      * be a good idea, so force hugepages off.
      */
     qemu_madvise(host_addr, length, QEMU_MADV_NOHUGEPAGE);
 
     return 0;
 }
 
 /*
  * Userfault requires us to mark RAM as NOHUGEPAGE prior to discard
  * however leaving it until after precopy means that most of the precopy
  * data is still THPd
  */
 int postcopy_ram_prepare_discard(MigrationIncomingState *mis)
 {
     if (foreach_not_ignored_block(nhp_range, mis)) {
         return -1;
     }
 
     postcopy_state_set(POSTCOPY_INCOMING_DISCARD);
 
     return 0;
 }
 
 /*
  * Mark the given area of RAM as requiring notification to unwritten areas
  * Used as a  callback on foreach_not_ignored_block.
  *   host_addr: Base of area to mark
  *   offset: Offset in the whole ram arena
  *   length: Length of the section
  *   opaque: MigrationIncomingState pointer
  * Returns 0 on success
  */
 static int ram_block_enable_notify(RAMBlock *rb, void *opaque)
 {
     MigrationIncomingState *mis = opaque;
     struct uffdio_register reg_struct;
 
     reg_struct.range.start = (uintptr_t)qemu_ram_get_host_addr(rb);
     reg_struct.range.len = rb->postcopy_length;
     reg_struct.mode = UFFDIO_REGISTER_MODE_MISSING;
 
     /* Now tell our userfault_fd that it's responsible for this area */
     if (ioctl(mis->userfault_fd, UFFDIO_REGISTER, &reg_struct)) {
         error_report("%s userfault register: %s", __func__, strerror(errno));
         return -1;
     }
     if (!(reg_struct.ioctls & ((__u64)1 << _UFFDIO_COPY))) {
         error_report("%s userfault: Region doesn't support COPY", __func__);
         return -1;
     }
     if (reg_struct.ioctls & ((__u64)1 << _UFFDIO_ZEROPAGE)) {
         qemu_ram_set_uf_zeroable(rb);
     }
 
     return 0;
 }
 
 int postcopy_wake_shared(struct PostCopyFD *pcfd,
                          uint64_t client_addr,
                          RAMBlock *rb)
 {
     size_t pagesize = qemu_ram_pagesize(rb);
     struct uffdio_range range;
     int ret;
     trace_postcopy_wake_shared(client_addr, qemu_ram_get_idstr(rb));
     range.start = ROUND_DOWN(client_addr, pagesize);
     range.len = pagesize;
     ret = ioctl(pcfd->fd, UFFDIO_WAKE, &range);
     if (ret) {
         error_report("%s: Failed to wake: %zx in %s (%s)",
                      __func__, (size_t)client_addr, qemu_ram_get_idstr(rb),
                      strerror(errno));
     }
     return ret;
 }
 
 static int postcopy_request_page(MigrationIncomingState *mis, RAMBlock *rb,
                                  ram_addr_t start, uint64_t haddr)
 {
     void *aligned = (void *)(uintptr_t)ROUND_DOWN(haddr, qemu_ram_pagesize(rb));
 
     /*
      * Discarded pages (via RamDiscardManager) are never migrated. On unlikely
      * access, place a zeropage, which will also set the relevant bits in the
      * recv_bitmap accordingly, so we won't try placing a zeropage twice.
      *
      * Checking a single bit is sufficient to handle pagesize > TPS as either
      * all relevant bits are set or not.
      */
     assert(QEMU_IS_ALIGNED(start, qemu_ram_pagesize(rb)));
     if (ramblock_page_is_discarded(rb, start)) {
         bool received = ramblock_recv_bitmap_test_byte_offset(rb, start);
 
         return received ? 0 : postcopy_place_page_zero(mis, aligned, rb);
     }
 
     return migrate_send_rp_req_pages(mis, rb, start, haddr);
 }
 
 /*
  * Callback from shared fault handlers to ask for a page,
  * the page must be specified by a RAMBlock and an offset in that rb
  * Note: Only for use by shared fault handlers (in fault thread)
  */
 int postcopy_request_shared_page(struct PostCopyFD *pcfd, RAMBlock *rb,
                                  uint64_t client_addr, uint64_t rb_offset)
 {
     uint64_t aligned_rbo = ROUND_DOWN(rb_offset, qemu_ram_pagesize(rb));
     MigrationIncomingState *mis = migration_incoming_get_current();
 
     trace_postcopy_request_shared_page(pcfd->idstr, qemu_ram_get_idstr(rb),
                                        rb_offset);
     if (ramblock_recv_bitmap_test_byte_offset(rb, aligned_rbo)) {
         trace_postcopy_request_shared_page_present(pcfd->idstr,
                                         qemu_ram_get_idstr(rb), rb_offset);
         return postcopy_wake_shared(pcfd, client_addr, rb);
     }
     postcopy_request_page(mis, rb, aligned_rbo, client_addr);
     return 0;
 }
 
 static int get_mem_fault_cpu_index(uint32_t pid)
 {
     CPUState *cpu_iter;
 
     CPU_FOREACH(cpu_iter) {
         if (cpu_iter->thread_id == pid) {
             trace_get_mem_fault_cpu_index(cpu_iter->cpu_index, pid);
             return cpu_iter->cpu_index;
         }
     }
     trace_get_mem_fault_cpu_index(-1, pid);
     return -1;
 }
 
 static uint32_t get_low_time_offset(PostcopyBlocktimeContext *dc)
 {
     int64_t start_time_offset = qemu_clock_get_ms(QEMU_CLOCK_REALTIME) -
                                     dc->start_time;
     return start_time_offset < 1 ? 1 : start_time_offset & UINT32_MAX;
 }
 
 /*
  * This function is being called when pagefault occurs. It
  * tracks down vCPU blocking time.
  *
  * @addr: faulted host virtual address
  * @ptid: faulted process thread id
  * @rb: ramblock appropriate to addr
  */
 static void mark_postcopy_blocktime_begin(uintptr_t addr, uint32_t ptid,
                                           RAMBlock *rb)
 {
     int cpu, already_received;
     MigrationIncomingState *mis = migration_incoming_get_current();
     PostcopyBlocktimeContext *dc = mis->blocktime_ctx;
     uint32_t low_time_offset;
 
     if (!dc || ptid == 0) {
         return;
     }
     cpu = get_mem_fault_cpu_index(ptid);
     if (cpu < 0) {
         return;
     }
 
     low_time_offset = get_low_time_offset(dc);
     if (dc->vcpu_addr[cpu] == 0) {
         qatomic_inc(&dc->smp_cpus_down);
     }
 
     qatomic_xchg(&dc->last_begin, low_time_offset);
     qatomic_xchg(&dc->page_fault_vcpu_time[cpu], low_time_offset);
     qatomic_xchg(&dc->vcpu_addr[cpu], addr);
 
     /*
      * check it here, not at the beginning of the function,
      * due to, check could occur early than bitmap_set in
      * qemu_ufd_copy_ioctl
      */
     already_received = ramblock_recv_bitmap_test(rb, (void *)addr);
     if (already_received) {
         qatomic_xchg(&dc->vcpu_addr[cpu], 0);
         qatomic_xchg(&dc->page_fault_vcpu_time[cpu], 0);
         qatomic_dec(&dc->smp_cpus_down);
     }
     trace_mark_postcopy_blocktime_begin(addr, dc, dc->page_fault_vcpu_time[cpu],
                                         cpu, already_received);
 }
 
 /*
  *  This function just provide calculated blocktime per cpu and trace it.
  *  Total blocktime is calculated in mark_postcopy_blocktime_end.
  *
  *
  * Assume we have 3 CPU
  *
  *      S1        E1           S1               E1
  * -----***********------------xxx***************------------------------> CPU1
  *
  *             S2                E2
  * ------------****************xxx---------------------------------------> CPU2
  *
  *                         S3            E3
  * ------------------------****xxx********-------------------------------> CPU3
  *
  * We have sequence S1,S2,E1,S3,S1,E2,E3,E1
  * S2,E1 - doesn't match condition due to sequence S1,S2,E1 doesn't include CPU3
  * S3,S1,E2 - sequence includes all CPUs, in this case overlap will be S1,E2 -
  *            it's a part of total blocktime.
  * S1 - here is last_begin
  * Legend of the picture is following:
  *              * - means blocktime per vCPU
  *              x - means overlapped blocktime (total blocktime)
  *
  * @addr: host virtual address
  */
 static void mark_postcopy_blocktime_end(uintptr_t addr)
 {
     MigrationIncomingState *mis = migration_incoming_get_current();
     PostcopyBlocktimeContext *dc = mis->blocktime_ctx;
     MachineState *ms = MACHINE(qdev_get_machine());
     unsigned int smp_cpus = ms->smp.cpus;
     int i, affected_cpu = 0;
     bool vcpu_total_blocktime = false;
     uint32_t read_vcpu_time, low_time_offset;
 
     if (!dc) {
         return;
     }
 
     low_time_offset = get_low_time_offset(dc);
     /* lookup cpu, to clear it,
      * that algorithm looks straightforward, but it's not
      * optimal, more optimal algorithm is keeping tree or hash
      * where key is address value is a list of  */
     for (i = 0; i < smp_cpus; i++) {
         uint32_t vcpu_blocktime = 0;
 
         read_vcpu_time = qatomic_fetch_add(&dc->page_fault_vcpu_time[i], 0);
         if (qatomic_fetch_add(&dc->vcpu_addr[i], 0) != addr ||
             read_vcpu_time == 0) {
             continue;
         }
         qatomic_xchg(&dc->vcpu_addr[i], 0);
         vcpu_blocktime = low_time_offset - read_vcpu_time;
         affected_cpu += 1;
         /* we need to know is that mark_postcopy_end was due to
          * faulted page, another possible case it's prefetched
          * page and in that case we shouldn't be here */
         if (!vcpu_total_blocktime &&
             qatomic_fetch_add(&dc->smp_cpus_down, 0) == smp_cpus) {
             vcpu_total_blocktime = true;
         }
         /* continue cycle, due to one page could affect several vCPUs */
         dc->vcpu_blocktime[i] += vcpu_blocktime;
     }
 
     qatomic_sub(&dc->smp_cpus_down, affected_cpu);
     if (vcpu_total_blocktime) {
         dc->total_blocktime += low_time_offset - qatomic_fetch_add(
                 &dc->last_begin, 0);
     }
     trace_mark_postcopy_blocktime_end(addr, dc, dc->total_blocktime,
                                       affected_cpu);
 }
 
 static void postcopy_pause_fault_thread(MigrationIncomingState *mis)
 {
     trace_postcopy_pause_fault_thread();
     qemu_sem_wait(&mis->postcopy_pause_sem_fault);
     trace_postcopy_pause_fault_thread_continued();
 }
 
 /*
  * Handle faults detected by the USERFAULT markings
  */
 static void *postcopy_ram_fault_thread(void *opaque)
 {
     MigrationIncomingState *mis = opaque;
     struct uffd_msg msg;
     int ret;
     size_t index;
     RAMBlock *rb = NULL;
 
     trace_postcopy_ram_fault_thread_entry();
     rcu_register_thread();
     mis->last_rb = NULL; /* last RAMBlock we sent part of */
     qemu_sem_post(&mis->thread_sync_sem);
 
     struct pollfd *pfd;
     size_t pfd_len = 2 + mis->postcopy_remote_fds->len;
 
     pfd = g_new0(struct pollfd, pfd_len);
 
     pfd[0].fd = mis->userfault_fd;
     pfd[0].events = POLLIN;
     pfd[1].fd = mis->userfault_event_fd;
     pfd[1].events = POLLIN; /* Waiting for eventfd to go positive */
     trace_postcopy_ram_fault_thread_fds_core(pfd[0].fd, pfd[1].fd);
     for (index = 0; index < mis->postcopy_remote_fds->len; index++) {
         struct PostCopyFD *pcfd = &g_array_index(mis->postcopy_remote_fds,
                                                  struct PostCopyFD, index);
         pfd[2 + index].fd = pcfd->fd;
         pfd[2 + index].events = POLLIN;
         trace_postcopy_ram_fault_thread_fds_extra(2 + index, pcfd->idstr,
                                                   pcfd->fd);
     }
 
     while (true) {
         ram_addr_t rb_offset;
         int poll_result;
 
         /*
          * We're mainly waiting for the kernel to give us a faulting HVA,
          * however we can be told to quit via userfault_quit_fd which is
          * an eventfd
          */
 
         poll_result = poll(pfd, pfd_len, -1 /* Wait forever */);
         if (poll_result == -1) {
             error_report("%s: userfault poll: %s", __func__, strerror(errno));
             break;
         }
 
         if (!mis->to_src_file) {
             /*
              * Possibly someone tells us that the return path is
              * broken already using the event. We should hold until
              * the channel is rebuilt.
              */
             postcopy_pause_fault_thread(mis);
         }
 
         if (pfd[1].revents) {
             uint64_t tmp64 = 0;
 
             /* Consume the signal */
             if (read(mis->userfault_event_fd, &tmp64, 8) != 8) {
                 /* Nothing obviously nicer than posting this error. */
                 error_report("%s: read() failed", __func__);
             }
 
             if (qatomic_read(&mis->fault_thread_quit)) {
                 trace_postcopy_ram_fault_thread_quit();
                 break;
             }
         }
 
         if (pfd[0].revents) {
             poll_result--;
             ret = read(mis->userfault_fd, &msg, sizeof(msg));
             if (ret != sizeof(msg)) {
                 if (errno == EAGAIN) {
                     /*
                      * if a wake up happens on the other thread just after
                      * the poll, there is nothing to read.
                      */
                     continue;
                 }
                 if (ret < 0) {
                     error_report("%s: Failed to read full userfault "
                                  "message: %s",
                                  __func__, strerror(errno));
                     break;
                 } else {
                     error_report("%s: Read %d bytes from userfaultfd "
                                  "expected %zd",
                                  __func__, ret, sizeof(msg));
                     break; /* Lost alignment, don't know what we'd read next */
                 }
             }
             if (msg.event != UFFD_EVENT_PAGEFAULT) {
                 error_report("%s: Read unexpected event %ud from userfaultfd",
                              __func__, msg.event);
                 continue; /* It's not a page fault, shouldn't happen */
             }
 
             rb = qemu_ram_block_from_host(
                      (void *)(uintptr_t)msg.arg.pagefault.address,
                      true, &rb_offset);
             if (!rb) {
                 error_report("postcopy_ram_fault_thread: Fault outside guest: %"
                              PRIx64, (uint64_t)msg.arg.pagefault.address);
                 break;
             }
 
             rb_offset = ROUND_DOWN(rb_offset, qemu_ram_pagesize(rb));
             trace_postcopy_ram_fault_thread_request(msg.arg.pagefault.address,
                                                 qemu_ram_get_idstr(rb),
                                                 rb_offset,
                                                 msg.arg.pagefault.feat.ptid);
             mark_postcopy_blocktime_begin(
                     (uintptr_t)(msg.arg.pagefault.address),
                                 msg.arg.pagefault.feat.ptid, rb);
 
 retry:
             /*
              * Send the request to the source - we want to request one
              * of our host page sizes (which is >= TPS)
              */
             ret = postcopy_request_page(mis, rb, rb_offset,
                                         msg.arg.pagefault.address);
             if (ret) {
                 /* May be network failure, try to wait for recovery */
                 postcopy_pause_fault_thread(mis);
                 goto retry;
             }
         }
 
         /* Now handle any requests from external processes on shared memory */
         /* TODO: May need to handle devices deregistering during postcopy */
         for (index = 2; index < pfd_len && poll_result; index++) {
             if (pfd[index].revents) {
                 struct PostCopyFD *pcfd =
                     &g_array_index(mis->postcopy_remote_fds,
                                    struct PostCopyFD, index - 2);
 
                 poll_result--;
                 if (pfd[index].revents & POLLERR) {
                     error_report("%s: POLLERR on poll %zd fd=%d",
                                  __func__, index, pcfd->fd);
                     pfd[index].events = 0;
                     continue;
                 }
 
                 ret = read(pcfd->fd, &msg, sizeof(msg));
                 if (ret != sizeof(msg)) {
                     if (errno == EAGAIN) {
                         /*
                          * if a wake up happens on the other thread just after
                          * the poll, there is nothing to read.
                          */
                         continue;
                     }
                     if (ret < 0) {
                         error_report("%s: Failed to read full userfault "
                                      "message: %s (shared) revents=%d",
                                      __func__, strerror(errno),
                                      pfd[index].revents);
                         /*TODO: Could just disable this sharer */
                         break;
                     } else {
                         error_report("%s: Read %d bytes from userfaultfd "
                                      "expected %zd (shared)",
                                      __func__, ret, sizeof(msg));
                         /*TODO: Could just disable this sharer */
                         break; /*Lost alignment,don't know what we'd read next*/
                     }
                 }
                 if (msg.event != UFFD_EVENT_PAGEFAULT) {
                     error_report("%s: Read unexpected event %ud "
                                  "from userfaultfd (shared)",
                                  __func__, msg.event);
                     continue; /* It's not a page fault, shouldn't happen */
                 }
                 /* Call the device handler registered with us */
                 ret = pcfd->handler(pcfd, &msg);
                 if (ret) {
                     error_report("%s: Failed to resolve shared fault on %zd/%s",
                                  __func__, index, pcfd->idstr);
                     /* TODO: Fail? Disable this sharer? */
                 }
             }
         }
     }
     rcu_unregister_thread();
     trace_postcopy_ram_fault_thread_exit();
     g_free(pfd);
     return NULL;
 }
 
 static int postcopy_temp_pages_setup(MigrationIncomingState *mis)
 {
     PostcopyTmpPage *tmp_page;
     int err, i, channels;
     void *temp_page;
 
     if (migrate_postcopy_preempt()) {
         /* If preemption enabled, need extra channel for urgent requests */
         mis->postcopy_channels = RAM_CHANNEL_MAX;
     } else {
         /* Both precopy/postcopy on the same channel */
         mis->postcopy_channels = 1;
     }
 
     channels = mis->postcopy_channels;
     mis->postcopy_tmp_pages = g_malloc0_n(sizeof(PostcopyTmpPage), channels);
 
     for (i = 0; i < channels; i++) {
         tmp_page = &mis->postcopy_tmp_pages[i];
         temp_page = mmap(NULL, mis->largest_page_size, PROT_READ | PROT_WRITE,
                          MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
         if (temp_page == MAP_FAILED) {
             err = errno;
             error_report("%s: Failed to map postcopy_tmp_pages[%d]: %s",
                          __func__, i, strerror(err));
             /* Clean up will be done later */
             return -err;
         }
         tmp_page->tmp_huge_page = temp_page;
         /* Initialize default states for each tmp page */
         postcopy_temp_page_reset(tmp_page);
     }
 
     /*
      * Map large zero page when kernel can't use UFFDIO_ZEROPAGE for hugepages
      */
     mis->postcopy_tmp_zero_page = mmap(NULL, mis->largest_page_size,
                                        PROT_READ | PROT_WRITE,
                                        MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
     if (mis->postcopy_tmp_zero_page == MAP_FAILED) {
         err = errno;
         mis->postcopy_tmp_zero_page = NULL;
         error_report("%s: Failed to map large zero page %s",
                      __func__, strerror(err));
         return -err;
     }
 
     memset(mis->postcopy_tmp_zero_page, '\0', mis->largest_page_size);
 
     return 0;
 }
 
 int postcopy_ram_incoming_setup(MigrationIncomingState *mis)
 {
     /* Open the fd for the kernel to give us userfaults */
     mis->userfault_fd = syscall(__NR_userfaultfd, O_CLOEXEC | O_NONBLOCK);
     if (mis->userfault_fd == -1) {
         error_report("%s: Failed to open userfault fd: %s", __func__,
                      strerror(errno));
         return -1;
     }
 
     /*
      * Although the host check already tested the API, we need to
      * do the check again as an ABI handshake on the new fd.
      */
     if (!ufd_check_and_apply(mis->userfault_fd, mis)) {
         return -1;
     }
 
     /* Now an eventfd we use to tell the fault-thread to quit */
     mis->userfault_event_fd = eventfd(0, EFD_CLOEXEC);
     if (mis->userfault_event_fd == -1) {
         error_report("%s: Opening userfault_event_fd: %s", __func__,
                      strerror(errno));
         close(mis->userfault_fd);
         return -1;
     }
 
     postcopy_thread_create(mis, &mis->fault_thread, "fault-default",
                            postcopy_ram_fault_thread, QEMU_THREAD_JOINABLE);
     mis->have_fault_thread = true;
 
     /* Mark so that we get notified of accesses to unwritten areas */
     if (foreach_not_ignored_block(ram_block_enable_notify, mis)) {
         error_report("ram_block_enable_notify failed");
         return -1;
     }
 
     if (postcopy_temp_pages_setup(mis)) {
         /* Error dumped in the sub-function */
         return -1;
     }
 
     if (migrate_postcopy_preempt()) {
         /*
          * This thread needs to be created after the temp pages because
          * it'll fetch RAM_CHANNEL_POSTCOPY PostcopyTmpPage immediately.
          */
         postcopy_thread_create(mis, &mis->postcopy_prio_thread, "fault-fast",
                                postcopy_preempt_thread, QEMU_THREAD_JOINABLE);
         mis->postcopy_prio_thread_created = true;
     }
 
     trace_postcopy_ram_enable_notify();
 
     return 0;
 }
 
 static int qemu_ufd_copy_ioctl(MigrationIncomingState *mis, void *host_addr,
                                void *from_addr, uint64_t pagesize, RAMBlock *rb)
 {
     int userfault_fd = mis->userfault_fd;
     int ret;
 
     if (from_addr) {
         struct uffdio_copy copy_struct;
         copy_struct.dst = (uint64_t)(uintptr_t)host_addr;
         copy_struct.src = (uint64_t)(uintptr_t)from_addr;
         copy_struct.len = pagesize;
         copy_struct.mode = 0;
         ret = ioctl(userfault_fd, UFFDIO_COPY, &copy_struct);
     } else {
         struct uffdio_zeropage zero_struct;
         zero_struct.range.start = (uint64_t)(uintptr_t)host_addr;
         zero_struct.range.len = pagesize;
         zero_struct.mode = 0;
         ret = ioctl(userfault_fd, UFFDIO_ZEROPAGE, &zero_struct);
     }
     if (!ret) {
         qemu_mutex_lock(&mis->page_request_mutex);
         ramblock_recv_bitmap_set_range(rb, host_addr,
                                        pagesize / qemu_target_page_size());
         /*
          * If this page resolves a page fault for a previous recorded faulted
          * address, take a special note to maintain the requested page list.
          */
         if (g_tree_lookup(mis->page_requested, host_addr)) {
             g_tree_remove(mis->page_requested, host_addr);
             mis->page_requested_count--;
             trace_postcopy_page_req_del(host_addr, mis->page_requested_count);
         }
         qemu_mutex_unlock(&mis->page_request_mutex);
         mark_postcopy_blocktime_end((uintptr_t)host_addr);
     }
     return ret;
 }
 
 int postcopy_notify_shared_wake(RAMBlock *rb, uint64_t offset)
 {
     int i;
     MigrationIncomingState *mis = migration_incoming_get_current();
     GArray *pcrfds = mis->postcopy_remote_fds;
 
     for (i = 0; i < pcrfds->len; i++) {
         struct PostCopyFD *cur = &g_array_index(pcrfds, struct PostCopyFD, i);
         int ret = cur->waker(cur, rb, offset);
         if (ret) {
             return ret;
         }
     }
     return 0;
 }
 
 /*
  * Place a host page (from) at (host) atomically
  * returns 0 on success
  */
 int postcopy_place_page(MigrationIncomingState *mis, void *host, void *from,
                         RAMBlock *rb)
 {
     size_t pagesize = qemu_ram_pagesize(rb);
 
     /* copy also acks to the kernel waking the stalled thread up
      * TODO: We can inhibit that ack and only do it if it was requested
      * which would be slightly cheaper, but we'd have to be careful
      * of the order of updating our page state.
      */
     if (qemu_ufd_copy_ioctl(mis, host, from, pagesize, rb)) {
         int e = errno;
         error_report("%s: %s copy host: %p from: %p (size: %zd)",
                      __func__, strerror(e), host, from, pagesize);
 
         return -e;
     }
 
     trace_postcopy_place_page(host);
     return postcopy_notify_shared_wake(rb,
                                        qemu_ram_block_host_offset(rb, host));
 }
 
 /*
  * Place a zero page at (host) atomically
  * returns 0 on success
  */
 int postcopy_place_page_zero(MigrationIncomingState *mis, void *host,
                              RAMBlock *rb)
 {
     size_t pagesize = qemu_ram_pagesize(rb);
     trace_postcopy_place_page_zero(host);
 
     /* Normal RAMBlocks can zero a page using UFFDIO_ZEROPAGE
      * but it's not available for everything (e.g. hugetlbpages)
      */
     if (qemu_ram_is_uf_zeroable(rb)) {
         if (qemu_ufd_copy_ioctl(mis, host, NULL, pagesize, rb)) {
             int e = errno;
             error_report("%s: %s zero host: %p",
                          __func__, strerror(e), host);
 
             return -e;
         }
         return postcopy_notify_shared_wake(rb,
                                            qemu_ram_block_host_offset(rb,
                                                                       host));
     } else {
         return postcopy_place_page(mis, host, mis->postcopy_tmp_zero_page, rb);
     }
 }
 
 #else
 /* No target OS support, stubs just fail */
 void fill_destination_postcopy_migration_info(MigrationInfo *info)
 {
 }
 
 bool postcopy_ram_supported_by_host(MigrationIncomingState *mis)
 {
     error_report("%s: No OS support", __func__);
     return false;
 }
 
 int postcopy_ram_incoming_init(MigrationIncomingState *mis)
 {
     error_report("postcopy_ram_incoming_init: No OS support");
     return -1;
 }
 
 int postcopy_ram_incoming_cleanup(MigrationIncomingState *mis)
 {
     assert(0);
     return -1;
 }
 
 int postcopy_ram_prepare_discard(MigrationIncomingState *mis)
 {
     assert(0);
     return -1;
 }
 
 int postcopy_request_shared_page(struct PostCopyFD *pcfd, RAMBlock *rb,
                                  uint64_t client_addr, uint64_t rb_offset)
 {
     assert(0);
     return -1;
 }
 
 int postcopy_ram_incoming_setup(MigrationIncomingState *mis)
 {
     assert(0);
     return -1;
 }
 
 int postcopy_place_page(MigrationIncomingState *mis, void *host, void *from,
                         RAMBlock *rb)
 {
     assert(0);
     return -1;
 }
 
 int postcopy_place_page_zero(MigrationIncomingState *mis, void *host,
                         RAMBlock *rb)
 {
     assert(0);
     return -1;
 }
 
 int postcopy_wake_shared(struct PostCopyFD *pcfd,
                          uint64_t client_addr,
                          RAMBlock *rb)
 {
     assert(0);
     return -1;
 }
 #endif
 
 /* ------------------------------------------------------------------------- */
 void postcopy_temp_page_reset(PostcopyTmpPage *tmp_page)
 {
     tmp_page->target_pages = 0;
     tmp_page->host_addr = NULL;
     /*
      * This is set to true when reset, and cleared as long as we received any
      * of the non-zero small page within this huge page.
      */
     tmp_page->all_zero = true;
 }
 
 void postcopy_fault_thread_notify(MigrationIncomingState *mis)
 {
     uint64_t tmp64 = 1;
 
     /*
      * Wakeup the fault_thread.  It's an eventfd that should currently
      * be at 0, we're going to increment it to 1
      */
     if (write(mis->userfault_event_fd, &tmp64, 8) != 8) {
         /* Not much we can do here, but may as well report it */
         error_report("%s: incrementing failed: %s", __func__,
                      strerror(errno));
     }
 }
 
 /**
  * postcopy_discard_send_init: Called at the start of each RAMBlock before
  *   asking to discard individual ranges.
  *
  * @ms: The current migration state.
  * @offset: the bitmap offset of the named RAMBlock in the migration bitmap.
  * @name: RAMBlock that discards will operate on.
  */
 static PostcopyDiscardState pds = {0};
 void postcopy_discard_send_init(MigrationState *ms, const char *name)
 {
     pds.ramblock_name = name;
     pds.cur_entry = 0;
     pds.nsentwords = 0;
     pds.nsentcmds = 0;
 }
 
 /**
  * postcopy_discard_send_range: Called by the bitmap code for each chunk to
  *   discard. May send a discard message, may just leave it queued to
  *   be sent later.
  *
  * @ms: Current migration state.
  * @start,@length: a range of pages in the migration bitmap in the
  *   RAM block passed to postcopy_discard_send_init() (length=1 is one page)
  */
 void postcopy_discard_send_range(MigrationState *ms, unsigned long start,
                                  unsigned long length)
 {
     size_t tp_size = qemu_target_page_size();
     /* Convert to byte offsets within the RAM block */
     pds.start_list[pds.cur_entry] = start  * tp_size;
     pds.length_list[pds.cur_entry] = length * tp_size;
     trace_postcopy_discard_send_range(pds.ramblock_name, start, length);
     pds.cur_entry++;
     pds.nsentwords++;
 
     if (pds.cur_entry == MAX_DISCARDS_PER_COMMAND) {
         /* Full set, ship it! */
         qemu_savevm_send_postcopy_ram_discard(ms->to_dst_file,
                                               pds.ramblock_name,
                                               pds.cur_entry,
                                               pds.start_list,
                                               pds.length_list);
         pds.nsentcmds++;
         pds.cur_entry = 0;
     }
 }
 
 /**
  * postcopy_discard_send_finish: Called at the end of each RAMBlock by the
  * bitmap code. Sends any outstanding discard messages, frees the PDS
  *
  * @ms: Current migration state.
  */
 void postcopy_discard_send_finish(MigrationState *ms)
 {
     /* Anything unsent? */
     if (pds.cur_entry) {
         qemu_savevm_send_postcopy_ram_discard(ms->to_dst_file,
                                               pds.ramblock_name,
                                               pds.cur_entry,
                                               pds.start_list,
                                               pds.length_list);
         pds.nsentcmds++;
     }
 
     trace_postcopy_discard_send_finish(pds.ramblock_name, pds.nsentwords,
                                        pds.nsentcmds);
 }
 
 /*
  * Current state of incoming postcopy; note this is not part of
  * MigrationIncomingState since it's state is used during cleanup
  * at the end as MIS is being freed.
  */
 static PostcopyState incoming_postcopy_state;
 
 PostcopyState  postcopy_state_get(void)
 {
     return qatomic_mb_read(&incoming_postcopy_state);
 }
 
 /* Set the state and return the old state */
 PostcopyState postcopy_state_set(PostcopyState new_state)
 {
     return qatomic_xchg(&incoming_postcopy_state, new_state);
 }
 
 /* Register a handler for external shared memory postcopy
  * called on the destination.
  */
 void postcopy_register_shared_ufd(struct PostCopyFD *pcfd)
 {
     MigrationIncomingState *mis = migration_incoming_get_current();
 
     mis->postcopy_remote_fds = g_array_append_val(mis->postcopy_remote_fds,
                                                   *pcfd);
 }
 
 /* Unregister a handler for external shared memory postcopy
  */
 void postcopy_unregister_shared_ufd(struct PostCopyFD *pcfd)
 {
     guint i;
     MigrationIncomingState *mis = migration_incoming_get_current();
     GArray *pcrfds = mis->postcopy_remote_fds;
 
     if (!pcrfds) {
         /* migration has already finished and freed the array */
         return;
     }
     for (i = 0; i < pcrfds->len; i++) {
         struct PostCopyFD *cur = &g_array_index(pcrfds, struct PostCopyFD, i);
         if (cur->fd == pcfd->fd) {
             mis->postcopy_remote_fds = g_array_remove_index(pcrfds, i);
             return;
         }
     }
 }
 
 bool postcopy_preempt_new_channel(MigrationIncomingState *mis, QEMUFile *file)
 {
     /*
      * The new loading channel has its own threads, so it needs to be
      * blocked too.  It's by default true, just be explicit.
      */
     qemu_file_set_blocking(file, true);
     mis->postcopy_qemufile_dst = file;
     trace_postcopy_preempt_new_channel();
 
     /* Start the migration immediately */
     return true;
 }
 
 /*
  * Setup the postcopy preempt channel with the IOC.  If ERROR is specified,
  * setup the error instead.  This helper will free the ERROR if specified.
  */
 static void
 postcopy_preempt_send_channel_done(MigrationState *s,
                                    QIOChannel *ioc, Error *local_err)
 {
     if (local_err) {
         migrate_set_error(s, local_err);
         error_free(local_err);
     } else {
         migration_ioc_register_yank(ioc);
         s->postcopy_qemufile_src = qemu_file_new_output(ioc);
         trace_postcopy_preempt_new_channel();
     }
 
     /*
      * Kick the waiter in all cases.  The waiter should check upon
      * postcopy_qemufile_src to know whether it failed or not.
      */
     qemu_sem_post(&s->postcopy_qemufile_src_sem);
 }
 
 static void
 postcopy_preempt_tls_handshake(QIOTask *task, gpointer opaque)
 {
     g_autoptr(QIOChannel) ioc = QIO_CHANNEL(qio_task_get_source(task));
     MigrationState *s = opaque;
     Error *local_err = NULL;
 
     qio_task_propagate_error(task, &local_err);
     postcopy_preempt_send_channel_done(s, ioc, local_err);
 }
 
 static void
 postcopy_preempt_send_channel_new(QIOTask *task, gpointer opaque)
 {
     g_autoptr(QIOChannel) ioc = QIO_CHANNEL(qio_task_get_source(task));
     MigrationState *s = opaque;
     QIOChannelTLS *tioc;
     Error *local_err = NULL;
 
     if (qio_task_propagate_error(task, &local_err)) {
         goto out;
     }
 
     if (migrate_channel_requires_tls_upgrade(ioc)) {
         tioc = migration_tls_client_create(s, ioc, s->hostname, &local_err);
         if (!tioc) {
             goto out;
         }
         trace_postcopy_preempt_tls_handshake();
         qio_channel_set_name(QIO_CHANNEL(tioc), "migration-tls-preempt");
         qio_channel_tls_handshake(tioc, postcopy_preempt_tls_handshake,
                                   s, NULL, NULL);
         /* Setup the channel until TLS handshake finished */
         return;
     }
 
 out:
     /* This handles both good and error cases */
     postcopy_preempt_send_channel_done(s, ioc, local_err);
 }
 
 /* Returns 0 if channel established, -1 for error. */
 int postcopy_preempt_wait_channel(MigrationState *s)
 {
     /* If preempt not enabled, no need to wait */
     if (!migrate_postcopy_preempt()) {
         return 0;
     }
 
     /*
      * We need the postcopy preempt channel to be established before
      * starting doing anything.
      */
     qemu_sem_wait(&s->postcopy_qemufile_src_sem);
 
     return s->postcopy_qemufile_src ? 0 : -1;
 }
 
 int postcopy_preempt_setup(MigrationState *s, Error **errp)
 {
     if (!migrate_postcopy_preempt()) {
         return 0;
     }
 
     if (!migrate_multi_channels_is_allowed()) {
         error_setg(errp, "Postcopy preempt is not supported as current "
                    "migration stream does not support multi-channels.");
         return -1;
     }
 
     /* Kick an async task to connect */
     socket_send_channel_create(postcopy_preempt_send_channel_new, s);
 
     return 0;
 }
 
 static void postcopy_pause_ram_fast_load(MigrationIncomingState *mis)
 {
     trace_postcopy_pause_fast_load();
     qemu_mutex_unlock(&mis->postcopy_prio_thread_mutex);
     qemu_sem_wait(&mis->postcopy_pause_sem_fast_load);
     qemu_mutex_lock(&mis->postcopy_prio_thread_mutex);
     trace_postcopy_pause_fast_load_continued();
 }
 
 void *postcopy_preempt_thread(void *opaque)
 {
     MigrationIncomingState *mis = opaque;
     int ret;
 
     trace_postcopy_preempt_thread_entry();
 
     rcu_register_thread();
 
     qemu_sem_post(&mis->thread_sync_sem);
 
     /* Sending RAM_SAVE_FLAG_EOS to terminate this thread */
     qemu_mutex_lock(&mis->postcopy_prio_thread_mutex);
     while (1) {
         ret = ram_load_postcopy(mis->postcopy_qemufile_dst,
                                 RAM_CHANNEL_POSTCOPY);
         /* If error happened, go into recovery routine */
         if (ret) {
             postcopy_pause_ram_fast_load(mis);
         } else {
             /* We're done */
             break;
         }
     }
     qemu_mutex_unlock(&mis->postcopy_prio_thread_mutex);
 
     rcu_unregister_thread();
 
     trace_postcopy_preempt_thread_exit();
 
     return NULL;
 }