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

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Message-ID: <CAJs=3_BqDqEoXGiU9zCNfGNqEjd1eijqkE_8_mb3nC=GwQoxHA@mail.gmail.com>
Date:   Fri, 23 Dec 2022 09:38:37 +0200
From:   Alvaro Karsz <alvaro.karsz@...id-run.com>
To:     Jason Wang <jasowang@...hat.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

> 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.

Thanks