diff options
| -rw-r--r-- | drivers/block/virtio_blk.c | 26 |
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)) |