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

[Jason Wang]

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.

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
> >>
>