Re: [RFC v2 12/13] vdpa: preemptive kick at enable

[Eugenio Perez Martin]

On Fri, Jan 13, 2023 at 4:39 AM Jason Wang <jasowang@redhat.com> wrote:
>
> On Fri, Jan 13, 2023 at 11:25 AM Zhu, Lingshan <lingshan.zhu@intel.com> wrote:
> >
> >
> >
> > On 1/13/2023 10:31 AM, Jason Wang wrote:
> > > On Fri, Jan 13, 2023 at 1:27 AM Eugenio Pérez <eperezma@redhat.com> wrote:
> > >> Spuriously kick the destination device's queue so it knows in case there
> > >> are new descriptors.
> > >>
> > >> RFC: This is somehow a gray area. The guest may have placed descriptors
> > >> in a virtqueue but not kicked it, so it might be surprised if the device
> > >> starts processing it.
> > > So I think this is kind of the work of the vDPA parent. For the parent
> > > that needs this trick, we should do it in the parent driver.
> > Agree, it looks easier implementing this in parent driver,
> > I can implement it in ifcvf set_vq_ready right now
>
> Great, but please check whether or not it is really needed.
>
> Some device implementation could check the available descriptions
> after DRIVER_OK without waiting for a kick.
>

So IIUC we can entirely drop this from the series (and I hope we can).
But then, what with the devices that does *not* check for them?

If we drop it it seems to me we must mandate devices to check for
descriptors at queue_enable. The queue could stall if not, isn't it?

Thanks!

> Thanks
>
> >
> > Thanks
> > Zhu Lingshan
> > >
> > > Thanks
> > >
> > >> However, that information is not in the migration stream and it should
> > >> be an edge case anyhow, being resilient to parallel notifications from
> > >> the guest.
> > >>
> > >> Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> > >> ---
> > >>   hw/virtio/vhost-vdpa.c | 5 +++++
> > >>   1 file changed, 5 insertions(+)
> > >>
> > >> diff --git a/hw/virtio/vhost-vdpa.c b/hw/virtio/vhost-vdpa.c
> > >> index 40b7e8706a..dff94355dd 100644
> > >> --- a/hw/virtio/vhost-vdpa.c
> > >> +++ b/hw/virtio/vhost-vdpa.c
> > >> @@ -732,11 +732,16 @@ static int vhost_vdpa_set_vring_ready(struct 
> > >> vhost_dev *dev, int ready)
> > >>       }
> > >>       trace_vhost_vdpa_set_vring_ready(dev);
> > >>       for (i = 0; i < dev->nvqs; ++i) {
> > >> +        VirtQueue *vq;
> > >>           struct vhost_vring_state state = {
> > >>               .index = dev->vq_index + i,
> > >>               .num = 1,
> > >>           };
> > >>           vhost_vdpa_call(dev, VHOST_VDPA_SET_VRING_ENABLE, &state);
> > >> +
> > >> +        /* Preemptive kick */
> > >> +        vq = virtio_get_queue(dev->vdev, dev->vq_index + i);
> > >> +        event_notifier_set(virtio_queue_get_host_notifier(vq));
> > >>       }
> > >>       return 0;
> > >>   }
> > >> --
> > >> 2.31.1
> > >>
> >
>