block: add flush callback

This patch adds callback for flush request. This callback is responsible
for flushing whole block devices stack. bdrv_flush function does not
proceed to underlying devices. It should be performed by this callback
function, if needed.
Signed-off-by: NPavel Dovgalyuk <pavel.dovgaluk@ispras.ru>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/io.c

@@ -2333,6 +2333,13 @@ int coroutine_fn bdrv_co_flush(BlockDriverState *bs)
     }
 
     tracked_request_begin(&req, bs, 0, 0, BDRV_TRACKED_FLUSH);
+
+    /* Write back all layers by calling one driver function */
+    if (bs->drv->bdrv_co_flush) {
+        ret = bs->drv->bdrv_co_flush(bs);
+        goto out;
+    }
+
     /* Write back cached data to the OS even with cache=unsafe */
     BLKDBG_EVENT(bs->file, BLKDBG_FLUSH_TO_OS);
     if (bs->drv->bdrv_co_flush_to_os) {

include/block/block_int.h

@@ -175,6 +175,13 @@ struct BlockDriver {
     void (*bdrv_invalidate_cache)(BlockDriverState *bs, Error **errp);
     int (*bdrv_inactivate)(BlockDriverState *bs);
 
+    /*
+     * Flushes all data for all layers by calling bdrv_co_flush for underlying
+     * layers, if needed. This function is needed for deterministic
+     * synchronization of the flush finishing callback.
+     */
+    int coroutine_fn (*bdrv_co_flush)(BlockDriverState *bs);
+
     /*
      * Flushes all data that was already written to the OS all the way down to
      * the disk (for example raw-posix calls fsync()).