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

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

[Michael S. Tsirkin]

We know vring num is a power of 2, so use &
to mask the high bits.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---

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);
-- 
MST

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

[Christian Borntraeger]

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 <borntraeger@de.ibm.com>

> 
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---
> 
> 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);
> 

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

[David Miller]

From: "Michael S. Tsirkin" <mst@redhat.com>
Date: Mon, 30 Nov 2015 11:15:23 +0200

> We know vring num is a power of 2, so use &
> to mask the high bits.
> 
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

Acked-by: David S. Miller <davem@davemloft.net>

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

[Venkatesh Srinivas via Virtualization]

On Mon, Nov 30, 2015 at 11:15:23AM +0200, Michael S. Tsirkin wrote:
> We know vring num is a power of 2, so use &
> to mask the high bits.
> 
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---

The generated code switches from DIV -> masking, source is clearer as well.

Tested-by: Venkatesh Srinivas <venkateshs@google.com>

-- vs;

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

[Andy Shevchenko]

On Fri, Dec 4, 2015 at 10:19 PM, Venkatesh Srinivas
<venkateshs@google.com> wrote:
> On Mon, Nov 30, 2015 at 11:15:23AM +0200, Michael S. Tsirkin wrote:
>> We know vring num is a power of 2, so use &
>> to mask the high bits.
>>
>> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
>> ---
>
> The generated code switches from DIV -> masking, source is clearer as well.

First impression was why, now it seems that compiler can't predict
this for variables.
For constants it would be optimized to the same, I suppose.

>
> Tested-by: Venkatesh Srinivas <venkateshs@google.com>
>
> -- vs;
> --
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-- 
With Best Regards,
Andy Shevchenko