block: Introduce bdrv_co_flush_to_os

qcow2 has a writeback metadata cache, so flushing a qcow2 image actually
consists of writing back that cache to the protocol and only then flushes the
protocol in order to get everything stable on disk.

This introduces a separate bdrv_co_flush_to_os to reflect the split.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>

3 files changed, 27 insertions, 3 deletions

diff --git a/block.c b/block.c
index 6521eaa..b1a4629 100644
--- a/block.c
+++ b/block.c
@@ -2789,11 +2789,22 @@ static void coroutine_fn bdrv_flush_co_entry(void *opaque)
 
 int coroutine_fn bdrv_co_flush(BlockDriverState *bs)
 {
+    int ret;
+
     if (bs->open_flags & BDRV_O_NO_FLUSH) {
         return 0;
     } else if (!bs->drv) {
         return 0;
-    } else if (bs->drv->bdrv_co_flush_to_disk) {
+    }
+
+    if (bs->drv->bdrv_co_flush_to_os) {
+        ret = bs->drv->bdrv_co_flush_to_os(bs);
+        if (ret < 0) {
+            return ret;
+        }
+    }
+
+    if (bs->drv->bdrv_co_flush_to_disk) {
         return bs->drv->bdrv_co_flush_to_disk(bs);
     } else if (bs->drv->bdrv_aio_flush) {
         BlockDriverAIOCB *acb;
diff --git a/block/qcow2.c b/block/qcow2.c
index f7f73fe..5c784ee 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -1105,7 +1105,7 @@ fail:
     return ret;
 }
 
-static int qcow2_co_flush(BlockDriverState *bs)
+static int qcow2_co_flush_to_os(BlockDriverState *bs)
 {
     BDRVQcowState *s = bs->opaque;
     int ret;
@@ -1124,6 +1124,11 @@ static int qcow2_co_flush(BlockDriverState *bs)
     }
     qemu_co_mutex_unlock(&s->lock);
 
+    return 0;
+}
+
+static int qcow2_co_flush_to_disk(BlockDriverState *bs)
+{
     return bdrv_co_flush(bs->file);
 }
 
@@ -1245,7 +1250,8 @@ static BlockDriver bdrv_qcow2 = {
 
     .bdrv_co_readv          = qcow2_co_readv,
     .bdrv_co_writev         = qcow2_co_writev,
-    .bdrv_co_flush_to_disk  = qcow2_co_flush,
+    .bdrv_co_flush_to_os    = qcow2_co_flush_to_os,
+    .bdrv_co_flush_to_disk  = qcow2_co_flush_to_disk,
 
     .bdrv_co_discard        = qcow2_co_discard,
     .bdrv_truncate          = qcow2_truncate,
diff --git a/block_int.h b/block_int.h
index 5aadc1f..1ec4921 100644
--- a/block_int.h
+++ b/block_int.h
@@ -93,6 +93,13 @@ struct BlockDriver {
      */
     int coroutine_fn (*bdrv_co_flush_to_disk)(BlockDriverState *bs);
 
+    /*
+     * Flushes all internal caches to the OS. The data may still sit in a
+     * writeback cache of the host OS, but it will survive a crash of the qemu
+     * process.
+     */
+    int coroutine_fn (*bdrv_co_flush_to_os)(BlockDriverState *bs);
+
     int (*bdrv_aio_multiwrite)(BlockDriverState *bs, BlockRequest *reqs,
         int num_reqs);
     int (*bdrv_merge_requests)(BlockDriverState *bs, BlockRequest* a,