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>

drivers/virtio/virtio_ring.c

@@ -250,8 +250,8 @@ int virtqueue_add_buf(struct virtqueue *_vq,
 	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);