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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 26 Jun 2024 05:58:08 -0400

On Wed, Jun 26, 2024 at 10:43:11AM +0200, Jiri Pirko wrote:
> Wed, Jun 26, 2024 at 10:08:14AM CEST, [email-masked] wrote:
> >On Wed, Jun 26, 2024 at 09:52:58AM +0200, Jiri Pirko wrote:
> >> Thu, Jun 20, 2024 at 12:31:34PM CEST, [email-masked] wrote:
> >> >On Thu, 20 Jun 2024 06:11:40 -0400, "Michael S. Tsirkin" <[email-masked]> 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 <[email-masked]> wrote:
> >> >> > > > On Thu, Jun 20, 2024 at 4:21 PM Jason Wang <[email-masked]> wrote:
> >> >> > > > >
> >> >> > > > > On Thu, Jun 20, 2024 at 3:35 PM Heng Qi <[email-masked]> wrote:
> >> >> > > > > >
> >> >> > > > > > On Wed, 19 Jun 2024 17:19:12 -0400, "Michael S. Tsirkin" <[email-masked]> 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 <[email-masked]>
> >> >> > > > 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 <[email-masked]
> >> >> > > >     Reviewed-by: Stefano Garzarella <[email-masked]>
> >> >> > > >     Reviewed-by: Stefan Hajnoczi <[email-masked]>
> >> >> > > >     Signed-off-by: Jason Wang <[email-masked]>
> >> >> > > 
> >> >> > > 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 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.
> >> >
> >> >Yes, it works well - config change irq is used less before - and will not fail.
> >> 
> >> Please note I'm working on such fallback for admin queue. I would Like
> >> to send the patchset by the end of this week. You can then use it easily
> >> for cvq.
> >> 
> >> Something like:
> >> /* the config->find_vqs() implementation */
> >> int vp_find_vqs(struct virtio_device *vdev, unsigned int nvqs,
> >>                 struct virtqueue *vqs[], vq_callback_t *callbacks[],
> >>                 const char * const names[], const bool *ctx,
> >>                 struct irq_affinity *desc)
> >> {
> >>         int err;
> >> 
> >>         /* Try MSI-X with one vector per queue. */
> >>         err = vp_find_vqs_msix(vdev, nvqs, vqs, callbacks, names,
> >>                                VP_VQ_VECTOR_POLICY_EACH, ctx, desc);
> >>         if (!err)
> >>                 return 0;
> >>         /* Fallback: MSI-X with one shared vector for config and
> >>          * slow path queues, one vector per queue for the rest. */
> >>         err = vp_find_vqs_msix(vdev, nvqs, vqs, callbacks, names,
> >>                                VP_VQ_VECTOR_POLICY_SHARED_SLOW, ctx, desc);
> >>         if (!err)
> >>                 return 0;
> >>         /* Fallback: MSI-X with one vector for config, one shared for queues. */
> >>         err = vp_find_vqs_msix(vdev, nvqs, vqs, callbacks, names,
> >>                                VP_VQ_VECTOR_POLICY_SHARED, ctx, desc);
> >>         if (!err)
> >>                 return 0;
> >>         /* Is there an interrupt? If not give up. */
> >>         if (!(to_vp_device(vdev)->pci_dev->irq))
> >>                 return err;
> >>         /* Finally fall back to regular interrupts. */
> >>         return vp_find_vqs_intx(vdev, nvqs, vqs, callbacks, names, ctx);
> >> }
> >> 
> >> 
> >
> >
> >Well for cvq, we'll need to adjust the API so core
> >knows cvq interrupts are be shared with config not
> >datapath.
> 
> Agreed. I was thinking about introducing some info struct and pass array
> of it instead of callbacks[] and names[]. Then the struct can contain
> flag indication. Something like:
> 
> struct vq_info {
> 	vq_callback_t *callback;
> 	const char *name;
> 	bool slow_path;
> };
> 

Yes. Add ctx too? There were attempts at it already btw.

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