virtio: Force use of power-of-two for descriptor ring sizes

The virtio descriptor rings of size N-1 were nicely set up to be
aligned to an N-byte boundary.  But as Anthony Liguori points out, the
free-running indices used by virtio require that the sizes be a power
of 2, otherwise we get problems on wrap (demonstrated with lguest).

So we replace the clever "2^n-1" scheme with a simple "align to page
boundary" scheme: this means that all virtio rings take at least two
pages, but it's safer than guessing cache alignment.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

2 files changed, 9 insertions, 2 deletions

diff --git a/drivers/lguest/lguest_device.c b/drivers/lguest/lguest_device.c
index 8904f72..66f3872 100644
--- a/drivers/lguest/lguest_device.c
+++ b/drivers/lguest/lguest_device.c
@@ -200,7 +200,8 @@ static struct virtqueue *lg_find_vq(struct virtio_device *vdev,
 
 	/* Figure out how many pages the ring will take, and map that memory */
 	lvq->pages = lguest_map((unsigned long)lvq->config.pfn << PAGE_SHIFT,
-				DIV_ROUND_UP(vring_size(lvq->config.num),
+				DIV_ROUND_UP(vring_size(lvq->config.num,
+							PAGE_SIZE),
 					     PAGE_SIZE));
 	if (!lvq->pages) {
 		err = -ENOMEM;
diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 0e1bf05..1dc04b6 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -277,11 +277,17 @@ struct virtqueue *vring_new_virtqueue(unsigned int num,
 	struct vring_virtqueue *vq;
 	unsigned int i;
 
+	/* We assume num is a power of 2. */
+	if (num & (num - 1)) {
+		dev_warn(&vdev->dev, "Bad virtqueue length %u\n", num);
+		return NULL;
+	}
+
 	vq = kmalloc(sizeof(*vq) + sizeof(void *)*num, GFP_KERNEL);
 	if (!vq)
 		return NULL;
 
-	vring_init(&vq->vring, num, pages);
+	vring_init(&vq->vring, num, pages, PAGE_SIZE);
 	vq->vq.callback = callback;
 	vq->vq.vdev = vdev;
 	vq->vq.vq_ops = &vring_vq_ops;