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

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Date: Thu, 20 Jun 2024 16:37:35 +0800
From: Jason Wang <jasowang@...hat.com>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: Heng Qi <hengqi@...ux.alibaba.com>, netdev@...r.kernel.org, 
	virtualization@...ts.linux.dev, 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, Jun 20, 2024 at 4:32 PM 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?
> Sharing between ctrl vq and config irq can also be an option.

One way is to allow the driver to specify the group of virtqueues. So
the core can request irq per group instead of pre vq. I used to post a
series like this about 10 years ago (but fail to find it).

It might also help for the case for NAPI.

Thanks

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