netdev - Re: [RFC PATCH 4/4] virtio-net: sleep instead of busy waiting for cvq command

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Date:   Thu, 22 Dec 2022 16:43:09 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Alvaro Karsz <alvaro.karsz@...id-run.com>
Cc:     mst@...hat.com, davem@...emloft.net, edumazet@...gle.com,
        kuba@...nel.org, pabeni@...hat.com,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, maxime.coquelin@...hat.com,
        eperezma@...hat.com
Subject: Re: [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@...id-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
>