Re: [PATCH] virtio: Always reset vhost devices

[Stefan Hajnoczi]

On Wed, 10 Jul 2024 at 13:25, Hanna Czenczek <hreitz@redhat.com> 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`).
>
> Signed-off-by: Hanna Czenczek <hreitz@redhat.com>

I think an additional SET_STATUS 0 call is made to the vDPA vhost
backend after this patch, but that seems fine.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

> ---
>  hw/virtio/virtio.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> index 893a072c9d..4410d62126 100644
> --- a/hw/virtio/virtio.c
> +++ b/hw/virtio/virtio.c
> @@ -2146,8 +2146,12 @@ void virtio_reset(void *opaque)
>          vdev->device_endian = virtio_default_endian();
>      }
>
> -    if (vdev->vhost_started && k->get_vhost) {
> -        vhost_reset_device(k->get_vhost(vdev));
> +    if (k->get_vhost) {
> +        struct vhost_dev *hdev = k->get_vhost(vdev);
> +        /* Only reset when vhost back-end is connected */
> +        if (hdev && hdev->vhost_ops) {
> +            vhost_reset_device(hdev);
> +        }
>      }
>
>      if (k->reset) {
> --
> 2.45.2
>
>