virtio-blk: fix barrier support

Before issuing the barrier to the block driver we need to flush our oustanding
queue of write requests, as the flush is supposed to be issued after them.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
(cherry picked from commit 618fbb84299780af96e3d4c4b6f2148656fe3708)

diff --git a/hw/virtio-blk.c b/hw/virtio-blk.c
index 2fd9b3fba7a357ec5d2538ec0fe4814e2a1edaad..0871d202417124ccd854b8698fd8a483f826e2da 100644 (file)
--- a/hw/virtio-blk.c
+++ b/hw/virtio-blk.c
@@ -278,10 +278,20 @@ static void do_multiwrite(BlockDriverState *bs, BlockRequest *blkreq,
     }
 }
 
-static void virtio_blk_handle_flush(VirtIOBlockReq *req)
+static void virtio_blk_handle_flush(BlockRequest *blkreq, int *num_writes,
+    VirtIOBlockReq *req, BlockDriverState **old_bs)
 {
     BlockDriverAIOCB *acb;
 
+    /*
+     * Make sure all outstanding writes are posted to the backing device.
+     */
+    if (*old_bs != NULL) {
+        do_multiwrite(*old_bs, blkreq, *num_writes);
+    }
+    *num_writes = 0;
+    *old_bs = req->dev->bs;
+
     acb = bdrv_aio_flush(req->dev->bs, virtio_blk_flush_complete, req);
     if (!acb) {
         virtio_blk_req_complete(req, VIRTIO_BLK_S_IOERR);
@@ -344,7 +354,8 @@ static void virtio_blk_handle_request(VirtIOBlockReq *req,
     req->in = (void *)req->elem.in_sg[req->elem.in_num - 1].iov_base;
 
     if (req->out->type & VIRTIO_BLK_T_FLUSH) {
-        virtio_blk_handle_flush(req);
+        virtio_blk_handle_flush(mrb->blkreq, &mrb->num_writes,
+            req, &mrb->old_bs);
     } else if (req->out->type & VIRTIO_BLK_T_SCSI_CMD) {
         virtio_blk_handle_scsi(req);
     } else if (req->out->type & VIRTIO_BLK_T_OUT) {