block: Inactivate BDS when migration completes

So far, live migration with shared storage meant that the image is in a
not-really-ready don't-touch-me state on the destination while the
source is still actively using it, but after completing the migration,
the image was fully opened on both sides. This is bad.

This patch adds a block driver callback to inactivate images on the
source before completing the migration. Inactivation means that it goes
to a state as if it was just live migrated to the qemu instance on the
source (i.e. BDRV_O_INACTIVE is set). You're then supposed to continue
either on the source or on the destination, which takes ownership of the
image.

A typical migration looks like this now with respect to disk images:

1. Destination qemu is started, the image is opened with
   BDRV_O_INACTIVE. The image is fully opened on the source.

2. Migration is about to complete. The source flushes the image and
   inactivates it. Now both sides have the image opened with
   BDRV_O_INACTIVE and are expecting the other side to still modify it.

3. One side (the destination on success) continues and calls
   bdrv_invalidate_all() in order to take ownership of the image again.
   This removes BDRV_O_INACTIVE on the resuming side; the flag remains
   set on the other side.

This ensures that the same image isn't written to by both instances
(unless both are resumed, but then you get what you deserve). This is
important because .bdrv_close for non-BDRV_O_INACTIVE images could write
to the image file, which is definitely forbidden while another host is
using the image.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>

5 files changed, 55 insertions, 0 deletions

diff --git a/block.c b/block.c
index 95b2967192..5709d3ddb4 100644
--- a/block.c
+++ b/block.c
@@ -3303,6 +3303,40 @@ void bdrv_invalidate_cache_all(Error **errp)
     }
 }
 
+static int bdrv_inactivate(BlockDriverState *bs)
+{
+    int ret;
+
+    if (bs->drv->bdrv_inactivate) {
+        ret = bs->drv->bdrv_inactivate(bs);
+        if (ret < 0) {
+            return ret;
+        }
+    }
+
+    bs->open_flags |= BDRV_O_INACTIVE;
+    return 0;
+}
+
+int bdrv_inactivate_all(void)
+{
+    BlockDriverState *bs;
+    int ret;
+
+    QTAILQ_FOREACH(bs, &bdrv_states, device_list) {
+        AioContext *aio_context = bdrv_get_aio_context(bs);
+
+        aio_context_acquire(aio_context);
+        ret = bdrv_inactivate(bs);
+        aio_context_release(aio_context);
+        if (ret < 0) {
+            return ret;
+        }
+    }
+
+    return 0;
+}
+
 /**************************************************************/
 /* removable device support */
 
diff --git a/include/block/block.h b/include/block/block.h
index 2b7d33c78e..25f36dcc74 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -369,6 +369,7 @@ BlockAIOCB *bdrv_aio_ioctl(BlockDriverState *bs,
 /* Invalidate any cached metadata used by image formats */
 void bdrv_invalidate_cache(BlockDriverState *bs, Error **errp);
 void bdrv_invalidate_cache_all(Error **errp);
+int bdrv_inactivate_all(void);
 
 /* Ensure contents are flushed to disk.  */
 int bdrv_flush(BlockDriverState *bs);
diff --git a/include/block/block_int.h b/include/block/block_int.h
index 256609dd3d..428fa3397e 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -172,6 +172,7 @@ struct BlockDriver {
      * Invalidate any cached meta-data.
      */
     void (*bdrv_invalidate_cache)(BlockDriverState *bs, Error **errp);
+    int (*bdrv_inactivate)(BlockDriverState *bs);
 
     /*
      * Flushes all data that was already written to the OS all the way down to
diff --git a/migration/migration.c b/migration/migration.c
index bc611e453b..aaca451cf8 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -1422,7 +1422,11 @@ static int postcopy_start(MigrationState *ms, bool *old_vm_running)
     *old_vm_running = runstate_is_running();
     global_state_store();
     ret = vm_stop_force_state(RUN_STATE_FINISH_MIGRATE);
+    if (ret < 0) {
+        goto fail;
+    }
 
+    ret = bdrv_inactivate_all();
     if (ret < 0) {
         goto fail;
     }
@@ -1542,6 +1546,9 @@ static void migration_completion(MigrationState *s, int current_active_state,
         if (!ret) {
             ret = vm_stop_force_state(RUN_STATE_FINISH_MIGRATE);
             if (ret >= 0) {
+                ret = bdrv_inactivate_all();
+            }
+            if (ret >= 0) {
                 qemu_file_set_rate_limit(s->file, INT64_MAX);
                 qemu_savevm_state_complete_precopy(s->file, false);
             }
diff --git a/qmp.c b/qmp.c
index 3ff6db79b9..53affe2cfd 100644
--- a/qmp.c
+++ b/qmp.c
@@ -192,6 +192,18 @@ void qmp_cont(Error **errp)
         }
     }
 
+    /* Continuing after completed migration. Images have been inactivated to
+     * allow the destination to take control. Need to get control back now. */
+    if (runstate_check(RUN_STATE_FINISH_MIGRATE) ||
+        runstate_check(RUN_STATE_POSTMIGRATE))
+    {
+        bdrv_invalidate_cache_all(&local_err);
+        if (local_err) {
+            error_propagate(errp, local_err);
+            return;
+        }
+    }
+
     if (runstate_check(RUN_STATE_INMIGRATE)) {
         autostart = 1;
     } else {