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

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Jun 25, 2024 at 09:27:24AM +0800, Jason Wang wrote:
> On Thu, Jun 20, 2024 at 6:12 PM Michael S. Tsirkin <mst@redhat.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@redhat.com> wrote:
> > > > > On Thu, Jun 20, 2024 at 4:21 PM Jason Wang <jasowang@redhat.com> wrote:
> > > > > >
> > > > > > On Thu, Jun 20, 2024 at 3:35 PM Heng Qi <hengqi@linux.alibaba.com> wrote:
> > > > > > >
> > > > > > > On Wed, 19 Jun 2024 17:19:12 -0400, "Michael S. Tsirkin" <mst@redhat.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@redhat.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@redhat.com
> > > > >     Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
> > > > >     Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
> > > > >     Signed-off-by: Jason Wang <jasowang@redhat.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.

sharing queues is exactly "bothering".

> Any reason to bother now?
> 
> Thanks

This patch can make datapath slower for such configs by switching to
sharing msix for a control path benefit. Not a tradeoff many users want
to make.


> > >  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
> > > > > > > >
> > > > > > >
> > > > >
> >