Virtualization - Jan 2022 - [RFC PATCH] vhost: cache avail index in vhost_enable_notify()

On Thu, Jan 13, 2022 at 03:56:42PM +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. In this case, the device would go to re-read avail
> index to access the descriptor.
> 
> As we already do in other place, we can cache the value in `avail_idx`
> and compare it with `last_avail_idx` to check if there are new
> buffers available.
> 
> Signed-off-by: Stefano Garzarella <sgarzare at redhat.com>
I guess we can ... but what's the point?
> ---
>  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;
>  }
>  EXPORT_SYMBOL_GPL(vhost_enable_notify);
>  
> -- 
> 2.31.1

On Thu, Jan 13, 2022 at 10:19:46AM -0500, Michael S. Tsirkin
wrote:
>On Thu, Jan 13, 2022 at 03:56:42PM +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. In this case, the device would go to re-read avail
>> index to access the descriptor.
>>
>> As we already do in other place, we can cache the value in `avail_idx`
>> and compare it with `last_avail_idx` to check if there are new
>> buffers available.
>>
>> Signed-off-by: Stefano Garzarella <sgarzare at redhat.com>
>
>I guess we can ... but what's the point?
>
That without this patch if avail index is new, then device when will 
call vhost_get_vq_desc() will find old value in cache and will read it 
again.

With this patch we also do the same path and update the cache every time 
we read avail index.

I marked it RFC because I don't know if it's worth it :-)

Stefano