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 16:26:05 +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 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>

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
> > >
> >