Re: [PATCH net-next v4 2/5] virtio_net: enable irq for the control vq

[Heng Qi <hengqi@linux.alibaba.com>]

On Wed, 19 Jun 2024 17:19:12 -0400, "Michael S. Tsirkin" <mst@redhat.com> wrote:
> On Thu, Jun 20, 2024 at 12:19:05AM +0800, Heng Qi wrote:
> > @@ -5312,7 +5315,7 @@ static int virtnet_find_vqs(struct virtnet_info *vi)
> >  
> >  	/* Parameters for control virtqueue, if any */
> >  	if (vi->has_cvq) {
> > -		callbacks[total_vqs - 1] = NULL;
> > +		callbacks[total_vqs - 1] = virtnet_cvq_done;
> >  		names[total_vqs - 1] = "control";
> >  	}
> >  
> 
> If the # of MSIX vectors is exactly for data path VQs,
> this will cause irq sharing between VQs which will degrade
> performance significantly.
> 
> So no, you can not just do it unconditionally.
> 
> The correct fix probably requires virtio core/API extensions.

If the introduction of cvq irq causes interrupts to become shared, then
ctrlq need to fall back to polling mode and keep the status quo.

Thanks.

> 
> -- 
> MST
>