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

[Thomas Huth]

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 <th...@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