Re: [PATCH] hw/virtio: Fix the de-initialization of vhost-user devices

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Jul 01, 2024 at 04:07:56PM +0200, Thomas Huth wrote:
> On 18/06/2024 14.19, Thomas Huth wrote:
> > The unrealize functions of the various vhost-user devices are
> > calling the corresponding vhost_*_set_status() functions with a
> > status of 0 to shut down the device correctly.
> > 
> > Now these vhost_*_set_status() functions all follow this scheme:
> > 
> >      bool should_start = virtio_device_should_start(vdev, status);
> > 
> >      if (vhost_dev_is_started(&vvc->vhost_dev) == should_start) {
> >          return;
> >      }
> > 
> >      if (should_start) {
> >          /* ... do the initialization stuff ... */
> >      } else {
> >          /* ... do the cleanup stuff ... */
> >      }
> > 
> > The problem here is virtio_device_should_start(vdev, 0) currently
> > always returns "true" since it internally only looks at vdev->started
> > instead of looking at the "status" parameter. Thus once the device
> > got started once, virtio_device_should_start() always returns true
> > and thus the vhost_*_set_status() functions return early, without
> > ever doing any clean-up when being called with status == 0. This
> > causes e.g. problems when trying to hot-plug and hot-unplug a vhost
> > user devices multiple times since the de-initialization step is
> > completely skipped during the unplug operation.
> > 
> > This bug has been introduced in commit 9f6bcfd99f ("hw/virtio: move
> > vm_running check to virtio_device_started") which replaced
> > 
> >   should_start = status & VIRTIO_CONFIG_S_DRIVER_OK;
> > 
> > with
> > 
> >   should_start = virtio_device_started(vdev, status);
> > 
> > which later got replaced by virtio_device_should_start(). This blocked
> > the possibility to set should_start to false in case the status flag
> > VIRTIO_CONFIG_S_DRIVER_OK was not set.
> > 
> > Fix it by adjusting the virtio_device_should_start() function to
> > only consider the status flag instead of vdev->started. Since this
> > function is only used in the various vhost_*_set_status() functions
> > for exactly the same purpose, it should be fine to fix it in this
> > central place there without any risk to change the behavior of other
> > code.
> > 
> > Fixes: 9f6bcfd99f ("hw/virtio: move vm_running check to virtio_device_started")
> > Buglink: https://issues.redhat.com/browse/RHEL-40708
> > Signed-off-by: Thomas Huth <thuth@redhat.com>
> > ---
> >   include/hw/virtio/virtio.h | 8 ++++----
> >   1 file changed, 4 insertions(+), 4 deletions(-)
> > 
> > diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
> > index 7d5ffdc145..2eafad17b8 100644
> > --- a/include/hw/virtio/virtio.h
> > +++ b/include/hw/virtio/virtio.h
> > @@ -470,9 +470,9 @@ static inline bool virtio_device_started(VirtIODevice *vdev, uint8_t status)
> >    * @vdev - the VirtIO device
> >    * @status - the devices status bits
> >    *
> > - * This is similar to virtio_device_started() but also encapsulates a
> > - * check on the VM status which would prevent a device starting
> > - * anyway.
> > + * This is similar to virtio_device_started() but ignores vdev->started
> > + * and also encapsulates a check on the VM status which would prevent a
> > + * device from starting anyway.
> >    */
> >   static inline bool virtio_device_should_start(VirtIODevice *vdev, uint8_t status)
> >   {
> > @@ -480,7 +480,7 @@ static inline bool virtio_device_should_start(VirtIODevice *vdev, uint8_t status
> >           return false;
> >       }
> > -    return virtio_device_started(vdev, status);
> > +    return status & VIRTIO_CONFIG_S_DRIVER_OK;
> >   }
> 
> Michael, any concerns or comments about this patch?
> 
> If not, I could also take it via my s390x tree since this fixes vhost-ccw
> devices on s390x.
> 
>  Thomas

I'm working on a pull request with this today.
I can drop it if you prefer ...