[Qemu-devel] [PATCH v6 20/28] 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.

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
---
 migration/migration.c  |  2 ++
 migration/migration.h  |  1 +
 migration/ram.c        | 47 +++++++++++++++++++++++++++++++++++++++++++++++
 migration/trace-events |  4 ++++
 4 files changed, 54 insertions(+)

diff --git a/migration/migration.c b/migration/migration.c
index f0a35f7dfd..c2ecebda02 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -2894,6 +2894,7 @@ static void migration_instance_finalize(Object *obj)
     qemu_sem_destroy(&ms->pause_sem);
     qemu_sem_destroy(&ms->postcopy_pause_sem);
     qemu_sem_destroy(&ms->postcopy_pause_rp_sem);
+    qemu_sem_destroy(&ms->rp_state.rp_sem);
 }
 
 static void migration_instance_init(Object *obj)
@@ -2925,6 +2926,7 @@ static void migration_instance_init(Object *obj)
 
     qemu_sem_init(&ms->postcopy_pause_sem, 0);
     qemu_sem_init(&ms->postcopy_pause_rp_sem, 0);
+    qemu_sem_init(&ms->rp_state.rp_sem, 0);
 }
 
 /*
diff --git a/migration/migration.h b/migration/migration.h
index bb81b6c926..88f5614b90 100644
--- a/migration/migration.h
+++ b/migration/migration.h
@@ -119,6 +119,7 @@ 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 463d5b3fe4..a2a4b05d5c 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -49,6 +49,7 @@
 #include "qemu/rcu_queue.h"
 #include "migration/colo.h"
 #include "migration/block.h"
+#include "savevm.h"
 
 /***********************************************************/
 /* ram save/restore */
@@ -3056,6 +3057,38 @@ static bool ram_has_postcopy(void *opaque)
     return migrate_postcopy_ram();
 }
 
+/* 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();
+
+    RAMBLOCK_FOREACH(block) {
+        qemu_savevm_send_recv_bitmap(file, block->idstr);
+        trace_ram_dirty_bitmap_request(block->idstr);
+        ramblock_count++;
+    }
+
+    trace_ram_dirty_bitmap_sync_wait();
+
+    /* Wait until all the ramblocks' dirty bitmap synced */
+    while (ramblock_count--) {
+        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)
+{
+    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
@@ -3130,12 +3163,25 @@ int ram_dirty_bitmap_reload(MigrationState *s, RAMBlock *block)
 
     trace_ram_dirty_bitmap_reload_complete(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,
@@ -3147,6 +3193,7 @@ 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 fe46b2c6c5..45b1d89217 100644
--- a/migration/trace-events
+++ b/migration/trace-events
@@ -82,8 +82,12 @@ ram_load_postcopy_loop(uint64_t addr, int flags) "@%" PRIx64 " %x"
 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_request(char *str) "%s"
 ram_dirty_bitmap_reload_begin(char *str) "%s"
 ram_dirty_bitmap_reload_complete(char *str) "%s"
+ram_dirty_bitmap_sync_start(void) ""
+ram_dirty_bitmap_sync_wait(void) ""
+ram_dirty_bitmap_sync_complete(void) ""
 
 # migration/migration.c
 await_return_path_close_on_source_close(void) ""
-- 
2.14.3