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>

1 files changed, 2 insertions, 1 deletions

diff --git a/drivers/lguest/lguest_device.c b/drivers/lguest/lguest_device.c
index 8904f72f97c..66f38722253 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;