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

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Date:   Mon, 26 Dec 2022 11:45:43 +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

On Fri, Dec 23, 2022 at 3:39 PM Alvaro Karsz <alvaro.karsz@...id-run.com> wrote:
>
> > This needs to be proposed to the virtio spec first. And actually we
> > need more than this:
> >
> > 1) we still need a way to deal with the device without this feature
> > 2) driver can't depend solely on what is advertised by the device (e.g
> > device can choose to advertise a very long timeout)
>
> I think that I wasn't clear, sorry.
> I'm not talking about a new virtio feature, I'm talking about a situation when:
> * virtio_net issues a control command.
> * the device gets the command, but somehow, completes the command
> after timeout.
> * virtio_net assumes that the command failed (timeout), and issues a
> different control command.
> * virtio_net will then call virtqueue_wait_for_used, and will
> immediately get the previous response (If I'm not wrong).
>
> So, this is not a new feature that I'm proposing, just a situation
> that may occur due to cvq timeouts.
>
> Anyhow, your solution calling BAD_RING if we reach a timeout should
> prevent this situation.

Right, that is the goal.

Thanks

>
> Thanks
>