Subject: Re: [PATCH v2] vhost: replace % with & on data path

From: Christian Borntraeger <[email-masked]>
Date: Mon, 30 Nov 2015 10:21:48 +0100

On 11/30/2015 10:15 AM, Michael S. Tsirkin wrote:
> We know vring num is a power of 2, so use &
> to mask the high bits.

Makes a lot of sense and virtio_ring.c seems to use the same logic.

Acked-by: Christian Borntraeger <[email-masked]>

> 
> Signed-off-by: Michael S. Tsirkin <[email-masked]>
> ---
> 
> Changes from v1: drop an unrelated chunk
> 
>  drivers/vhost/vhost.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 080422f..ad2146a 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -1369,7 +1369,7 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq,
>  	/* Grab the next descriptor number they're advertising, and increment
>  	 * the index we've seen. */
>  	if (unlikely(__get_user(ring_head,
> -				&vq->avail->ring[last_avail_idx % vq->num]))) {
> +				&vq->avail->ring[last_avail_idx & (vq->num - 1)]))) {
>  		vq_err(vq, "Failed to read head: idx %d address %p\n",
>  		       last_avail_idx,
>  		       &vq->avail->ring[last_avail_idx % vq->num]);
> @@ -1489,7 +1489,7 @@ static int __vhost_add_used_n(struct vhost_virtqueue *vq,
>  	u16 old, new;
>  	int start;
> 
> -	start = vq->last_used_idx % vq->num;
> +	start = vq->last_used_idx & (vq->num - 1);
>  	used = vq->used->ring + start;
>  	if (count == 1) {
>  		if (__put_user(heads[0].id, &used->id)) {
> @@ -1531,7 +1531,7 @@ int vhost_add_used_n(struct vhost_virtqueue *vq, struct vring_used_elem *heads,
>  {
>  	int start, n, r;
> 
> -	start = vq->last_used_idx % vq->num;
> +	start = vq->last_used_idx & (vq->num - 1);
>  	n = vq->num - start;
>  	if (n < count) {
>  		r = __vhost_add_used_n(vq, heads, n);
>