From 10bc310c27af1ed358e62351e7ac1d0110c3da27 Mon Sep 17 00:00:00 2001 From: Christoph Hellwig Date: Tue, 15 Jun 2010 14:43:48 +0200 Subject: virtio_blk: support barriers without FLUSH feature If we want to support barriers with the cache=writethrough mode in qemu we need to tell the block layer that we only need queue drains to implement a barrier. Follow the model set by SCSI and IDE and assume that there is no volatile write cache if the host doesn't advertize it. While this might imply working barriers on old qemu versions or other hypervisors that actually have a volatile write cache this is only a cosmetic issue - these hypervisors don't guarantee any data integrity with or without this patch, but with the patch we at least provide data ordering. Signed-off-by: Christoph Hellwig Signed-off-by: Rusty Russell --- drivers/block/virtio_blk.c | 26 +++++++++++++++++++++++--- 1 file 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)) -- cgit v1.2.3-70-g09d2