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

[Thomas Huth <thuth@redhat.com>]

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