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 16:21:02 +0800
From: Jason Wang <jasowang@...hat.com>
To: Heng Qi <hengqi@...ux.alibaba.com>
Cc: "Michael S. Tsirkin" <mst@...hat.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 3:35 PM Heng Qi <hengqi@...ux.alibaba.com> 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.
> >

Why do we need to care about buggy management? I think libvirt has
been teached to use 2N+2 since the introduction of the multiqueue[1].

> > 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.

Having to path sounds a burden.

>
> Thanks.
>


Thanks

[1] https://www.linux-kvm.org/page/Multiqueue

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