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

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Date: Tue, 25 Jun 2024 09:27:24 +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 6:12 PM Michael S. Tsirkin <mst@...hat.com> wrote:
>
> On Thu, Jun 20, 2024 at 06:10:51AM -0400, Michael S. Tsirkin wrote:
> > On Thu, Jun 20, 2024 at 05:53:15PM +0800, Heng Qi wrote:
> > > 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.
> >
> > These aren't buggy, the spec allows this.

So it doesn't differ from the case when we are lacking sufficient msix
vectors in the case of multiqueue. In that case we just fallback to
share one msix for all queues and another for config and we don't
bother at that time.

Any reason to bother now?

Thanks

> >  So don't fail, but
> > I'm fine with using polling if not enough vectors.
>
> sharing with config interrupt is easier code-wise though, FWIW -
> we don't need to maintain two code-paths.
>
> > > >
> > > > 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
> > > > > > >
> > > > > >
> > > >
>