Currently virtio-blk doesn't set any QUEUE_ORDERED_ flag by default, which means it does not allow filesystems to use barriers. But the typical use case for virtio-blk is to use a backed that uses synchronous I/O, and in that case we can simply set QUEUE_ORDERED_DRAIN to make the block layer drain the request queue around barrier I/O and provide the semantics that the filesystems need. This is what the SCSI disk driver does for disks that have the write cache disabled.
With this patch we incorrectly advertise barrier support if someone configure qemu with write back caching. While this displays wrong information in the guest there is nothing that guest could have done even if we rightfully told it that we do not support any barriers. Signed-off-by: Christoph Hellwig <[email protected]> Index: linux-2.6/drivers/block/virtio_blk.c =================================================================== --- linux-2.6.orig/drivers/block/virtio_blk.c 2009-08-20 17:41:37.019718433 -0300 +++ linux-2.6/drivers/block/virtio_blk.c 2009-08-20 17:45:40.511747922 -0300 @@ -336,9 +336,16 @@ 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 barriers are supported, tell block layer that queue is ordered. + * + * If no barriers are supported assume the host uses synchronous + * writes and just drain the the queue before and after the barrier. + */ 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)) -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html
