Re: [PATCH v1] vhost: cache avail index in vhost_enable_notify()

[Stefano Garzarella <sgarzare@redhat.com>]

On Mon, Jan 24, 2022 at 11:31:49AM +0000, Stefan Hajnoczi wrote:
>On Fri, Jan 14, 2022 at 10:05:08AM +0100, Stefano Garzarella wrote:
>> In vhost_enable_notify() we enable the notifications and we read
>> the avail index to check if new buffers have become available in
>> the meantime.
>>
>> We are not caching the avail index, so when the device will call
>> vhost_get_vq_desc(), it will find the old value in the cache and
>> it will read the avail index again.
>
>I think this wording is clearer because we do keep a cached the avail
>index value, but the issue is we don't update it:
>s/We are not caching the avail index/We do not update the cached avail
>index value/

I'll fix in v3.
It seems I forgot to CC you on v2: 
https://lore.kernel.org/virtualization/20220121153108.187291-1-sgarzare@redhat.com/

>
>>
>> It would be better to refresh the cache every time we read avail
>> index, so let's change vhost_enable_notify() caching the value in
>> `avail_idx` and compare it with `last_avail_idx` to check if there
>> are new buffers available.
>>
>> Anyway, we don't expect a significant performance boost because
>> the above path is not very common, indeed vhost_enable_notify()
>> is often called with unlikely(), expecting that avail index has
>> not been updated.
>>
>> Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
>> ---
>> v1:
>> - improved the commit description [MST, Jason]
>> ---
>>  drivers/vhost/vhost.c | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
>> index 59edb5a1ffe2..07363dff559e 100644
>> --- a/drivers/vhost/vhost.c
>> +++ b/drivers/vhost/vhost.c
>> @@ -2543,8 +2543,9 @@ bool vhost_enable_notify(struct vhost_dev *dev, 
>> struct vhost_virtqueue *vq)
>>  		       &vq->avail->idx, r);
>>  		return false;
>>  	}
>> +	vq->avail_idx = vhost16_to_cpu(vq, avail_idx);
>>
>> -	return vhost16_to_cpu(vq, avail_idx) != vq->avail_idx;
>> +	return vq->avail_idx != vq->last_avail_idx;
>
>vhost_vq_avail_empty() has a fast path that's missing in
>vhost_enable_notify():
>
>  if (vq->avail_idx != vq->last_avail_idx)
>      return false;

Yep, I thought about that, but devices usually call 
vhost_enable_notify() right when vq->avail_idx == vq->last_avail_idx, so 
I don't know if it's an extra check for a branch that will never be 
taken.

Do you think it is better to add that check? (maybe with unlikely())

Thanks,
Stefano