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>

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index c56bbe7..99dc948 100644 (file)
--- 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);