Introduce Virtio-loopback epsilon release:

Epsilon release introduces a new compatibility layer which make virtio-loopback
design to work with QEMU and rust-vmm vhost-user backend without require any
changes.

Signed-off-by: Timos Ampelikiotis <t.ampelikiotis@virtualopensystems.com>
Change-Id: I52e57563e08a7d0bdc002f8e928ee61ba0c53dd9

1 files changed, 1471 insertions, 0 deletions

diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c
new file mode 100644
index 000000000..d18b5d05b
--- /dev/null
+++ b/migration/postcopy-ram.c
@@ -0,0 +1,1471 @@
+/*
+ * 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 "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"
+
+/* 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);
+}
+
+/* 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 (migrate_postcopy_blocktime() && mis &&
+        UFFD_FEATURE_THREAD_ID & supported_features) {
+        /* kernel supports that feature */
+        /* don't create blocktime_context if it exists */
+        if (!mis->blocktime_ctx) {
+            mis->blocktime_ctx = blocktime_context_new();
+        }
+
+        asked_features |= UFFD_FEATURE_THREAD_ID;
+    }
+#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;
+}
+
+/*
+ * 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->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.
+             */
+        }
+    }
+
+    if (mis->postcopy_tmp_page) {
+        munmap(mis->postcopy_tmp_page, mis->largest_page_size);
+        mis->postcopy_tmp_page = NULL;
+    }
+    if (mis->postcopy_tmp_zero_page) {
+        munmap(mis->postcopy_tmp_zero_page, mis->largest_page_size);
+        mis->postcopy_tmp_zero_page = NULL;
+    }
+    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 bool 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();
+
+    return true;
+}
+
+/*
+ * 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->fault_thread_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.
+             */
+            if (postcopy_pause_fault_thread(mis)) {
+                /* Continue to read the userfaultfd */
+            } else {
+                error_report("%s: paused but don't allow to continue",
+                             __func__);
+                break;
+            }
+        }
+
+        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 */
+                if (ret == -EIO && postcopy_pause_fault_thread(mis)) {
+                    /* We got reconnected somehow, try to continue */
+                    goto retry;
+                } else {
+                    /* This is a unavoidable fault */
+                    error_report("%s: postcopy_request_page() get %d",
+                                 __func__, ret);
+                    break;
+                }
+            }
+        }
+
+        /* 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;
+}
+
+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;
+    }
+
+    qemu_sem_init(&mis->fault_thread_sem, 0);
+    qemu_thread_create(&mis->fault_thread, "postcopy/fault",
+                       postcopy_ram_fault_thread, mis, QEMU_THREAD_JOINABLE);
+    qemu_sem_wait(&mis->fault_thread_sem);
+    qemu_sem_destroy(&mis->fault_thread_sem);
+    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;
+    }
+
+    mis->postcopy_tmp_page = mmap(NULL, mis->largest_page_size,
+                                  PROT_READ | PROT_WRITE, MAP_PRIVATE |
+                                  MAP_ANONYMOUS, -1, 0);
+    if (mis->postcopy_tmp_page == MAP_FAILED) {
+        mis->postcopy_tmp_page = NULL;
+        error_report("%s: Failed to map postcopy_tmp_page %s",
+                     __func__, strerror(errno));
+        return -1;
+    }
+
+    /*
+     * 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) {
+        int e = errno;
+        mis->postcopy_tmp_zero_page = NULL;
+        error_report("%s: Failed to map large zero page %s",
+                     __func__, strerror(e));
+        return -e;
+    }
+    memset(mis->postcopy_tmp_zero_page, '\0', mis->largest_page_size);
+
+    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_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;
+        }
+    }
+}