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Date: Wed, 19 May 2010 12:40:09 +0200
From: Christoph Hellwig <hch@lst.de>
Message-ID: <20100519104009.GA8813@lst.de>
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Subject: [Qemu-devel] [PATCH] virtio-blk: fix barrier support
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To: qemu-devel@nongnu.org

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>

Index: qemu/hw/virtio-blk.c
===================================================================
--- qemu.orig/hw/virtio-blk.c	2010-05-19 11:05:23.259005741 +0200
+++ qemu/hw/virtio-blk.c	2010-05-19 11:08:09.797255846 +0200
@@ -238,10 +238,20 @@ static void do_multiwrite(BlockDriverSta
     }
 }
 
-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);
@@ -314,7 +324,8 @@ static void virtio_blk_handle_request(Vi
     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) {