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

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Message-ID: <1718876302.539031-8-hengqi@linux.alibaba.com>
Date: Thu, 20 Jun 2024 17:38:22 +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 Thu, 20 Jun 2024 04:32:15 -0400, "Michael S. Tsirkin" <mst@...hat.com> wrote:
> On Thu, Jun 20, 2024 at 03:29:15PM +0800, Heng Qi wrote:
> > 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.
> 
> I don't see that in the code.
> 
> I guess we'll need more info in find vqs about what can and what can't share irqs?

I mean we should add fallback code, for example if allocating interrupt for ctrlq
fails, we should clear the callback of ctrlq.

> Sharing between ctrl vq and config irq can also be an option.
> 

Not sure if this violates the spec. In the spec, used buffer notification and
configuration change notification are clearly defined - ctrlq is a virtqueue
and used buffer notification should be used.

Thanks.

> 
> 
> 
> > > 
> > > -- 
> > > MST
> > > 
> 
>