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 17:53:15 +0800
From: Heng Qi <hengqi@...ux.alibaba.com>
To: Jason Wang <jasowang@...hat.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, 20 Jun 2024 16:26:05 +0800, Jason Wang <jasowang@...hat.com> wrote:
> On Thu, Jun 20, 2024 at 4:21 PM Jason Wang <jasowang@...hat.com> wrote:
> >
> > 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].
> 
> And Qemu can calculate it correctly automatically since:
> 
> commit 51a81a2118df0c70988f00d61647da9e298483a4
> Author: Jason Wang <jasowang@...hat.com>
> Date:   Mon Mar 8 12:49:19 2021 +0800
> 
>     virtio-net: calculating proper msix vectors on init
> 
>     Currently, the default msix vectors for virtio-net-pci is 3 which is
>     obvious not suitable for multiqueue guest, so we depends on the user
>     or management tools to pass a correct vectors parameter. In fact, we
>     can simplifying this by calculating the number of vectors on realize.
> 
>     Consider we have N queues, the number of vectors needed is 2*N + 2
>     (#queue pairs + plus one config interrupt and control vq). We didn't
>     check whether or not host support control vq because it was added
>     unconditionally by qemu to avoid breaking legacy guests such as Minix.
> 
>     Reviewed-by: Philippe Mathieu-Daudé <philmd@...hat.com
>     Reviewed-by: Stefano Garzarella <sgarzare@...hat.com>
>     Reviewed-by: Stefan Hajnoczi <stefanha@...hat.com>
>     Signed-off-by: Jason Wang <jasowang@...hat.com>

Yes, devices designed according to the spec need to reserve an interrupt
vector for ctrlq. So, Michael, do we want to be compatible with buggy devices?

Thanks.

> 
> Thanks
> 
> >
> > > > 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
> > > >
> > >
>