Re: [PATCH] virtio: Always reset vhost devices

[Hanna Czenczek]

On 10.07.24 18:28, Stefan Hajnoczi wrote:

> 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>
>     
    
    Thanks!  I agree that double-sending SET_STATUS with the same value
    should be fine.  The virtio specification states: “The device status field starts out as 0, and is reinitialized to 0 by the device during reset.” – I interpret that to mean that (re-)setting the field to 0 is always OK.

Hanna



> > ---
> >  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
> >
> >
> >       
>