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

[Thomas Huth <thuth@redhat.com>]

On 01/07/2024 17.06, Michael S. Tsirkin wrote:
> 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 ...

Ah, perfect, please include it in your PR then!

  Thanks,
   Thomas