On Thu, 20 Jun 2024 16:26:05 +0800, Jason Wang <jasowang@xxxxxxxxxx> wrote:
> On Thu, Jun 20, 2024 at 4:21 PM Jason Wang <jasowang@xxxxxxxxxx> wrote:
> >
> > On Thu, Jun 20, 2024 at 3:35 PM Heng Qi <hengqi@xxxxxxxxxxxxxxxxx> wrote:
> > >
> > > On Wed, 19 Jun 2024 17:19:12 -0400, "Michael S. Tsirkin" <mst@xxxxxxxxxx> 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@xxxxxxxxxx>
> 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@xxxxxxxxxx
> Reviewed-by: Stefano Garzarella <sgarzare@xxxxxxxxxx>
> Reviewed-by: Stefan Hajnoczi <stefanha@xxxxxxxxxx>
> Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
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
> > > >
> > >
>
