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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>
This commit is contained in:
Rusty Russell 2012-01-12 15:44:43 +10:30
parent 41f0377f73
commit 3b720b8c86
1 changed files with 6 additions and 4 deletions
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10
drivers/virtio/virtio_ring.c
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@ -250,8 +250,8 @@ add_head:
vq->data[head] = data; vq->data[head] = data;
/* Put entry in available array (but don't update avail->idx until they /* Put entry in available array (but don't update avail->idx until they
* do sync). FIXME: avoid modulus here? */ * do sync). */
avail = (vq->vring.avail->idx + vq->num_added++) % vq->vring.num; avail = ((vq->vring.avail->idx + vq->num_added++) & (vq->vring.num-1));
vq->vring.avail->ring[avail] = head; vq->vring.avail->ring[avail] = head;
pr_debug("Added buffer head %i to %p\n", head, vq); 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); struct vring_virtqueue *vq = to_vvq(_vq);
void *ret; void *ret;
unsigned int i; unsigned int i;
u16 last_used;
START_USE(vq); 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. */ /* Only get used array entries after they have been exposed by host. */
virtio_rmb(vq); virtio_rmb(vq);
i = vq->vring.used->ring[vq->last_used_idx%vq->vring.num].id; last_used = (vq->last_used_idx & (vq->vring.num - 1));
*len = vq->vring.used->ring[vq->last_used_idx%vq->vring.num].len; i = vq->vring.used->ring[last_used].id;
*len = vq->vring.used->ring[last_used].len;
if (unlikely(i >= vq->vring.num)) { if (unlikely(i >= vq->vring.num)) {
BAD_RING(vq, "id %u out of range\n", i); BAD_RING(vq, "id %u out of range\n", i);