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

[Andy Shevchenko]

On Fri, Dec 4, 2015 at 10:19 PM, Venkatesh Srinivas
 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 
>> ---
>
> 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 
>
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Andy Shevchenko
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Re: [PATCH v2] vhost: replace % with & on data path

[Venkatesh Srinivas]

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 
> ---

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

Tested-by: Venkatesh Srinivas 

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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 

> 
> Signed-off-by: Michael S. Tsirkin 
> ---
> 
> 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,
> - >avail->ring[last_avail_idx % vq->num]))) {
> + >avail->ring[last_avail_idx & (vq->num - 
> 1)]))) {
>   vq_err(vq, "Failed to read head: idx %d address %p\n",
>  last_avail_idx,
>  >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, >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);
> 

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

[David Miller]

From: "Michael S. Tsirkin" 
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 

Acked-by: David S. Miller 
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[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 
---

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,
-   >avail->ring[last_avail_idx % vq->num]))) {
+   >avail->ring[last_avail_idx & (vq->num - 
1)]))) {
vq_err(vq, "Failed to read head: idx %d address %p\n",
   last_avail_idx,
   >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, >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
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