Virtualization - Dec 2022 - [RFC PATCH 4/4] virtio-net: sleep instead of busy waiting for cvq command

Hi Alvaro:

On Thu, Dec 22, 2022 at 2:44 PM Alvaro Karsz <alvaro.karsz at
solid-run.com> wrote:
>
> Hi Jason,
>
> Adding timeout to the cvq is a great idea IMO.
>
> > -       /* Spin for a response, the kick causes an ioport write,
trapping
> > -        * into the hypervisor, so the request should be handled
immediately.
> > -        */
> > -       while (!virtqueue_get_buf(vi->cvq, &tmp) &&
> > -              !virtqueue_is_broken(vi->cvq))
> > -               cpu_relax();
> > +       virtqueue_wait_for_used(vi->cvq, &tmp);
>
> Do you think that we should continue like nothing happened in case of a
timeout?
We could, but we should not depend on a device to do this since it's
not reliable. More below.
> Shouldn't we reset the device?
We can't depend on device, there's probably another loop in reset():

E.g in vp_reset() we had:

        while (vp_modern_get_status(mdev))
                msleep(1);
> What happens if a device completes the control command after timeout?
Maybe we could have a BAD_RING() here in this case (and more check in
vq->broken in this case).

Thanks
>
> Thanks
>
> Alvaro
>

My point is that the device may complete the control command after the timeout,
so, if I'm not mistaken, next time we send a control command and call
virtqueue_wait_for_used we'll get the previous response.