virtio: avoid modulus operation.

Since we know vq->vring.num is a power of 2, modulus is lazy (it's asserted
in vring_new_virtqueue()).

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

1 files changed, 6 insertions, 4 deletions

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index c56bbe799241..99dc9480f3fe 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -250,8 +250,8 @@ add_head:
         vq->data[head] = data;
 
         /* Put entry in available array (but don't update avail->idx until they
-         * do sync).  FIXME: avoid modulus here? */
-        avail = (vq->vring.avail->idx + vq->num_added++) % vq->vring.num;
+         * do sync). */
+        avail = ((vq->vring.avail->idx + vq->num_added++) & (vq->vring.num-1));
         vq->vring.avail->ring[avail] = head;
 
         pr_debug("Added buffer head %i to %p\n", head, vq);
@@ -384,6 +384,7 @@ void *virtqueue_get_buf(struct virtqueue *_vq, unsigned int *len)
         struct vring_virtqueue *vq = to_vvq(_vq);
         void *ret;
         unsigned int i;
+        u16 last_used;
 
         START_USE(vq);
 
@@ -401,8 +402,9 @@ void *virtqueue_get_buf(struct virtqueue *_vq, unsigned int *len)
         /* Only get used array entries after they have been exposed by host. */
         virtio_rmb(vq);
 
-        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;
+        last_used = (vq->last_used_idx & (vq->vring.num - 1));
+        i = vq->vring.used->ring[last_used].id;
+        *len = vq->vring.used->ring[last_used].len;
 
         if (unlikely(i >= vq->vring.num)) {
                 BAD_RING(vq, "id %u out of range\n", i);