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

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Message-ID: <1718868555.2701075-5-hengqi@linux.alibaba.com>
Date: Thu, 20 Jun 2024 15:29:15 +0800
From: Heng Qi <hengqi@...ux.alibaba.com>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: netdev@...r.kernel.org,
 virtualization@...ts.linux.dev,
 Jason Wang <jasowang@...hat.com>,
 Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
 Eugenio Pérez <eperezma@...hat.com>,
 "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>
Subject: Re: [PATCH net-next v4 2/5] virtio_net: enable irq for the control vq

On Wed, 19 Jun 2024 17:19:12 -0400, "Michael S. Tsirkin" <mst@...hat.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
>