Re: [PATCH] virtio: Always reset vhost devices

[Matias Ezequiel Vara Larsen <mvaralar@redhat.com>]

Hello Hanna,

On Wed, Jul 10, 2024 at 01:23:10PM +0200, Hanna Czenczek wrote:
> Requiring `vhost_started` to be true for resetting vhost devices in
> `virtio_reset()` seems like the wrong condition: Most importantly, the
> preceding `virtio_set_status(vdev, 0)` call will (for vhost devices) end
> up in `vhost_dev_stop()` (through vhost devices' `.set_status`
> implementations), setting `vdev->vhost_started = false`.  Therefore, the
> gated `vhost_reset_device()` call is unreachable.
> 
> `vhost_started` is not documented, so it is hard to say what exactly it
> is supposed to mean, but judging from the fact that `vhost_dev_start()`
> sets it and `vhost_dev_stop()` clears it, it seems like it indicates
> whether there is a vhost back-end, and whether that back-end is
> currently running and processing virtio requests.
> 
> Making a reset conditional on whether the vhost back-end is processing
> virtio requests seems wrong; in fact, it is probably better to reset it
> only when it is not currently processing requests, which is exactly the
> current order of operations in `virtio_reset()`: First, the back-end is
> stopped through `virtio_set_status(vdev, 0)`, then we want to send a
> reset.
> 
> Therefore, we should drop the `vhost_started` condition, but in its
> stead we then have to verify that we can indeed send a reset to this
> vhost device, by not just checking `k->get_vhost != NULL` (introduced by
> commit 95e1019a4a9), but also that the vhost back-end is connected
> (`hdev = k->get_vhost(); hdev != NULL && hdev->vhost_ops != NULL`).
> 

I am not a native speaker but I think the commit message could be
rephrased to make it clear. This is a minor comment so feel free to
ignore it:

Before `virtio_reset()` is invoked, `virtio_set_status(vdev, 0)` for
vhost devices ends up setting `vhost_stared` to false thus resulting in
`vhost_reset_device()` being never reached during a reset.

`vhost_started` is not documented, however, from `vhost_dev_start()` and
`vhost_dev_stop()`, it seems indicating that there is a vhost back-end
and that the back-end is running and processing virtio requests.
Resetting should not rely on whether the vhost back-end is processing
virtio requests, instead, reset must happen if there is no processing
request. This is what `virtio_reset()` does, it first stops the backend
and then sends the reset.

To trigger a reset, this commit drops the `vhost_started` condition and
checks that the device is running (introduced by 95e1019a4a9) but also
that the vhost back-end is connected so it is possible to send a reset
to the vhost device.

Matias