Re: Virtio-net - add timeouts to control commands

[Alvaro Karsz <alvaro.karsz@solid-run.com>]

Hi Hannes,

> a) let the device do the timeout: pass in a timeout value with the
> command, and allow the device to return an ETIMEDOUT error when the
> timeout expires. Then it's up to the device to do the necessary timeout
> handling; the server won't be involved at all (except for handling an
> ETIMEDOUT error)


This won't work if the device crashes.

>
> b) implement an 'abort' command. With that the server controls the
> timeout, and is allowed to send an 'abort' command when the timeout
> expires. That requires the device to be able to abort commands (which
> not all devices are able to), but avoids having to implement a timeout
> handling in the device.


I actually thought about this idea.
This may work, but you'll still have a few moments when the server
assumes that the command failed, and the network device assumes that
it succeeded.
So the server may still receive packets in an unexpected queue.


>
> I am very much in favour of having timeouts for virtio commands; we've
> had several massive customer escalations which could have been solved if
> we were able to set the command timeout in the VM.
> As this was for virtio-scsi/virtio-block I would advocate to have a
> generic virtio command timeout, not a protocol-specific one.

This may be difficult to implement.
Especially when multiple commands may be queued at the same time, and
the device can handle the commands in any order.
We'll need to add identifiers for every command.

I'm actually referring here to the Linux kernel implementation of
virtnet control commands, in which the server spins for a response.
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