===================================================================
@@ -366,12 +366,32 @@ static int __devinit virtblk_probe(struc
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))