[Qemu-devel] [RFC v2 24/33] migration: synchronize dirty bitmap for resume

[Peter Xu <peterx@redhat.com>]

This patch implements the first part of core RAM resume logic for
postcopy. ram_resume_prepare() is provided for the work.

When the migration is interrupted by network failure, the dirty bitmap
on the source side will be meaningless, because even the dirty bit is
cleared, it is still possible that the sent page was lost along the way
to destination. Here instead of continue the migration with the old
dirty bitmap on source, we ask the destination side to send back its
received bitmap, then invert it to be our initial dirty bitmap.

The source side send thread will issue the MIG_CMD_RECV_BITMAP requests,
once per ramblock, to ask for the received bitmap. On destination side,
MIG_RP_MSG_RECV_BITMAP will be issued, along with the requested bitmap.
Data will be received on the return-path thread of source, and the main
migration thread will be notified when all the ramblock bitmaps are
synchronized.

Signed-off-by: Peter Xu <peterx@redhat.com>
---
 migration/migration.c  |  4 +++
 migration/migration.h  |  1 +
 migration/ram.c        | 67 ++++++++++++++++++++++++++++++++++++++++++++++++++
 migration/trace-events |  4 +++
 4 files changed, 76 insertions(+)

diff --git a/migration/migration.c b/migration/migration.c
index 19b7f3a5..19aed72 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -2605,6 +2605,8 @@ static void migration_instance_finalize(Object *obj)
 
     g_free(params->tls_hostname);
     g_free(params->tls_creds);
+
+    qemu_sem_destroy(&ms->rp_state.rp_sem);
 }
 
 static void migration_instance_init(Object *obj)
@@ -2629,6 +2631,8 @@ static void migration_instance_init(Object *obj)
     params->has_downtime_limit = true;
     params->has_x_checkpoint_delay = true;
     params->has_block_incremental = true;
+
+    qemu_sem_init(&ms->rp_state.rp_sem, 1);
 }
 
 /*
diff --git a/migration/migration.h b/migration/migration.h
index a3a0582..d041369 100644
--- a/migration/migration.h
+++ b/migration/migration.h
@@ -107,6 +107,7 @@ struct MigrationState
         QEMUFile     *from_dst_file;
         QemuThread    rp_thread;
         bool          error;
+        QemuSemaphore rp_sem;
     } rp_state;
 
     double mbps;
diff --git a/migration/ram.c b/migration/ram.c
index 5d938e3..afabcf5 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -47,6 +47,7 @@
 #include "exec/target_page.h"
 #include "qemu/rcu_queue.h"
 #include "migration/colo.h"
+#include "savevm.h"
 
 /***********************************************************/
 /* ram save/restore */
@@ -295,6 +296,8 @@ struct RAMState {
     RAMBlock *last_req_rb;
     /* Queue of outstanding page requests from the destination */
     QemuMutex src_page_req_mutex;
+    /* Ramblock counts to sync dirty bitmap. Only used for recovery */
+    int ramblock_to_sync;
     QSIMPLEQ_HEAD(src_page_requests, RAMSrcPageRequest) src_page_requests;
 };
 typedef struct RAMState RAMState;
@@ -2770,6 +2773,56 @@ static int ram_load(QEMUFile *f, void *opaque, int version_id)
     return ret;
 }
 
+/* Sync all the dirty bitmap with destination VM.  */
+static int ram_dirty_bitmap_sync_all(MigrationState *s, RAMState *rs)
+{
+    RAMBlock *block;
+    QEMUFile *file = s->to_dst_file;
+    int ramblock_count = 0;
+
+    trace_ram_dirty_bitmap_sync_start();
+
+    /*
+     * We do this in such order:
+     *
+     * 1. calculate block count
+     * 2. fill in the count to N
+     * 3. send MIG_CMD_RECV_BITMAP requests
+     * 4. wait on the semaphore until N -> 0
+     */
+
+    RAMBLOCK_FOREACH(block) {
+        ramblock_count++;
+    }
+
+    atomic_set(&rs->ramblock_to_sync, ramblock_count);
+
+    RAMBLOCK_FOREACH(block) {
+        qemu_savevm_send_recv_bitmap(file, block->idstr);
+    }
+
+    trace_ram_dirty_bitmap_sync_wait();
+
+    /* Wait until all the ramblocks' dirty bitmap synced */
+    while (atomic_read(&rs->ramblock_to_sync)) {
+        qemu_sem_wait(&s->rp_state.rp_sem);
+    }
+
+    trace_ram_dirty_bitmap_sync_complete();
+
+    return 0;
+}
+
+static void ram_dirty_bitmap_reload_notify(MigrationState *s)
+{
+    atomic_dec(&ram_state->ramblock_to_sync);
+    if (ram_state->ramblock_to_sync == 0) {
+        /* Make sure the other thread gets the latest */
+        trace_ram_dirty_bitmap_sync_notify();
+        qemu_sem_post(&s->rp_state.rp_sem);
+    }
+}
+
 /*
  * Read the received bitmap, revert it as the initial dirty bitmap.
  * This is only used when the postcopy migration is paused but wants
@@ -2841,12 +2894,25 @@ int ram_dirty_bitmap_reload(MigrationState *s, RAMBlock *block)
 
     trace_ram_dirty_bitmap_reload(block->idstr);
 
+    /*
+     * We succeeded to sync bitmap for current ramblock. If this is
+     * the last one to sync, we need to notify the main send thread.
+     */
+    ram_dirty_bitmap_reload_notify(s);
+
     ret = 0;
 out:
     free(le_bitmap);
     return ret;
 }
 
+static int ram_resume_prepare(MigrationState *s, void *opaque)
+{
+    RAMState *rs = *(RAMState **)opaque;
+
+    return ram_dirty_bitmap_sync_all(s, rs);
+}
+
 static SaveVMHandlers savevm_ram_handlers = {
     .save_setup = ram_save_setup,
     .save_live_iterate = ram_save_iterate,
@@ -2857,6 +2923,7 @@ static SaveVMHandlers savevm_ram_handlers = {
     .save_cleanup = ram_save_cleanup,
     .load_setup = ram_load_setup,
     .load_cleanup = ram_load_cleanup,
+    .resume_prepare = ram_resume_prepare,
 };
 
 void ram_mig_init(void)
diff --git a/migration/trace-events b/migration/trace-events
index 61b0d49..8962916 100644
--- a/migration/trace-events
+++ b/migration/trace-events
@@ -81,6 +81,10 @@ ram_postcopy_send_discard_bitmap(void) ""
 ram_save_page(const char *rbname, uint64_t offset, void *host) "%s: offset: 0x%" PRIx64 " host: %p"
 ram_save_queue_pages(const char *rbname, size_t start, size_t len) "%s: start: 0x%zx len: 0x%zx"
 ram_dirty_bitmap_reload(char *str) "%s"
+ram_dirty_bitmap_sync_start(void) ""
+ram_dirty_bitmap_sync_wait(void) ""
+ram_dirty_bitmap_sync_notify(void) ""
+ram_dirty_bitmap_sync_complete(void) ""
 
 # migration/migration.c
 await_return_path_close_on_source_close(void) ""
-- 
2.7.4