From: Michael S. Tsirkin <mst@redhat.com>
Date: Sun, 25 Oct 2009 13:28:53 +0000 (+0200)
Subject: virtio: order used ring after used index read
X-Git-Tag: v2.6.31.6~56
X-Git-Url: https://git.fsl.cs.sunysb.edu/?a=commitdiff_plain;h=48fe10afb355d88be5a9c0b7d5a198c5e5b9b4aa;p=unionfs-2.6.39.y.git

virtio: order used ring after used index read

commit 2d61ba95034f1abbdec7729d52c740870a5eddb6 upstream.

On SMP guests, reads from the ring might bypass used index reads. This
causes guest crashes because host writes to used index to signal ring
data readiness.  Fix this by inserting rmb before used ring reads.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
---

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index a882f260651..0ffff9ed4eb 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -281,6 +281,9 @@ static void *vring_get_buf(struct virtqueue *_vq, unsigned int *len)
 		return NULL;
 	}
 
+	/* Only get used array entries after they have been exposed by host. */
+	rmb();
+
 	i = vq->vring.used->ring[vq->last_used_idx%vq->vring.num].id;
 	*len = vq->vring.used->ring[vq->last_used_idx%vq->vring.num].len;