1 files changed, 23 insertions, 3 deletions

diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index 258bc2ae288..2d6191aa594 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c
@@ -366,12 +366,32 @@ 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 (virtio_has_feature(vdev, VIRTIO_BLK_F_FLUSH))
+        if (virtio_has_feature(vdev, VIRTIO_BLK_F_FLUSH)) {
+                /*
+                 * If the FLUSH feature is supported we do have support for
+                 * flushing a volatile write cache on the host.  Use that
+                 * to implement write barrier support.
+                 */
                 blk_queue_ordered(q, QUEUE_ORDERED_DRAIN_FLUSH,
                                   virtblk_prepare_flush);
-        else if (virtio_has_feature(vdev, VIRTIO_BLK_F_BARRIER))
+        } else if (virtio_has_feature(vdev, VIRTIO_BLK_F_BARRIER)) {
+                /*
+                 * If the BARRIER feature is supported the host expects us
+                 * to order request by tags.  This implies there is not
+                 * volatile write cache on the host, and that the host
+                 * never re-orders outstanding I/O.  This feature is not
+                 * useful for real life scenarious and deprecated.
+                 */
                 blk_queue_ordered(q, QUEUE_ORDERED_TAG, NULL);
+        } else {
+                /*
+                 * If the FLUSH feature is not supported we must assume that
+                 * the host does not perform any kind of volatile write
+                 * caching. We still need to drain the queue to provider
+                 * proper barrier semantics.
+                 */
+                blk_queue_ordered(q, 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))