[PATCH] virtio-blk: fallback to draining the queue if barrier ops are not supported

From: Avi Kivity <avi@redhat.com>
To: Christoph Hellwig <hch@lst.de>
Cc: Matthew Tippett <tippettm@gmail.com>,
	Dustin Kirkland <dustin.kirkland@gmail.com>,
	Anthony Liguori <anthony@codemonkey.ws>, RW <kvm@tauceti.net>,
	kvm@vger.kernel.org
Subject: [PATCH] virtio-blk: fallback to draining the queue if barrier ops are not supported
Date: Wed, 14 Oct 2009 21:03:08 +0900	[thread overview]
Message-ID: <1255521788-24346-1-git-send-email-avi@redhat.com> (raw)
In-Reply-To: <4AD5B00D.102@redhat.com>

Early implementations of virtio devices did not support barrier operations,
but did commit the data to disk.  In such cases, drain the queue to emulate
barrier operations.

Signed-off-by: Avi Kivity <avi@redhat.com>
---
 drivers/block/virtio_blk.c |    6 +++++-
 1 files changed, 5 insertions(+), 1 deletions(-)

diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index 43f1938..2627cc3 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c
@@ -354,12 +354,16 @@ static int __devinit virtblk_probe(struct virtio_device *vdev)
 	vblk->disk->driverfs_dev = &vdev->dev;
 	index++;
 
-	/* If barriers are supported, tell block layer that queue is ordered */
+	/* If barriers are supported, tell block layer that queue is ordered;
+	 * otherwise just drain the queue.
+	 */
 	if (virtio_has_feature(vdev, VIRTIO_BLK_F_FLUSH))
 		blk_queue_ordered(vblk->disk->queue, QUEUE_ORDERED_DRAIN_FLUSH,
 				  virtblk_prepare_flush);
 	else if (virtio_has_feature(vdev, VIRTIO_BLK_F_BARRIER))
 		blk_queue_ordered(vblk->disk->queue, QUEUE_ORDERED_TAG, NULL);
+	else
+		blk_queue_ordered(vblk->disk->queue, QUEUE_ORDERED_DRAIN, NULL);
 
 	/* If disk is read-only in the host, the guest should obey */
 	if (virtio_has_feature(vdev, VIRTIO_BLK_F_RO))
-- 
1.6.2.5

