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Commit 3b720b8c authored by Rusty Russell's avatar Rusty Russell
Browse files

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: default avatarRusty Russell <rusty@rustcorp.com.au>
parent 41f0377f

drivers/virtio/virtio_ring.c

+6 −4
Original line number Diff line number Diff line
@@ -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);