vhost: return failure if stop virtqueue failed in vhost_dev_stop

This patch captures the error of vhost_virtqueue_stop() in vhost_dev_stop()
and returns the error upward.

Specifically, if QEMU is disconnected from the vhost backend, some actions
in vhost_dev_stop() will fail, such as sending vhost-user messages to the
backend (GET_VRING_BASE, SET_VRING_ENABLE) and vhost_reset_status.

Considering that both set_vring_enable and vhost_reset_status require setting
the specific virtio feature bit, we can capture vhost_virtqueue_stop()'s
error to indicate that QEMU has lost connection with the backend.

This patch is the pre patch for 'vhost-user: return failure if backend crashes
when live migration', which makes the live migration aware of the loss of
connection with the vhost-user backend and aborts the live migration.

Signed-off-by: Haoqian He <haoqian.he@smartx.com>
Message-Id: <20250416024729.3289157-3-haoqian.he@smartx.com>
Tested-by: Lei Yang <leiyang@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/virtio/vhost.c

@@ -1367,10 +1367,10 @@ fail_alloc_desc:
     return r;
 }
 
-void vhost_virtqueue_stop(struct vhost_dev *dev,
-                          struct VirtIODevice *vdev,
-                          struct vhost_virtqueue *vq,
-                          unsigned idx)
+int vhost_virtqueue_stop(struct vhost_dev *dev,
+                         struct VirtIODevice *vdev,
+                         struct vhost_virtqueue *vq,
+                         unsigned idx)
 {
     int vhost_vq_index = dev->vhost_ops->vhost_get_vq_index(dev, idx);
     struct vhost_vring_state state = {
@@ -1380,7 +1380,7 @@ void vhost_virtqueue_stop(struct vhost_dev *dev,
 
     if (virtio_queue_get_desc_addr(vdev, idx) == 0) {
         /* Don't stop the virtqueue which might have not been started */
-        return;
+        return 0;
     }
 
     r = dev->vhost_ops->vhost_get_vring_base(dev, &state);
@@ -1411,6 +1411,7 @@ void vhost_virtqueue_stop(struct vhost_dev *dev,
                        0, virtio_queue_get_avail_size(vdev, idx));
     vhost_memory_unmap(dev, vq->desc, virtio_queue_get_desc_size(vdev, idx),
                        0, virtio_queue_get_desc_size(vdev, idx));
+    return r;
 }
 
 static int vhost_virtqueue_set_busyloop_timeout(struct vhost_dev *dev,
@@ -2135,9 +2136,10 @@ fail_features:
 }
 
 /* Host notifiers must be enabled at this point. */
-void vhost_dev_stop(struct vhost_dev *hdev, VirtIODevice *vdev, bool vrings)
+int vhost_dev_stop(struct vhost_dev *hdev, VirtIODevice *vdev, bool vrings)
 {
     int i;
+    int rc = 0;
 
     /* should only be called after backend is connected */
     assert(hdev->vhost_ops);
@@ -2156,10 +2158,10 @@ void vhost_dev_stop(struct vhost_dev *hdev, VirtIODevice *vdev, bool vrings)
         vhost_dev_set_vring_enable(hdev, false);
     }
     for (i = 0; i < hdev->nvqs; ++i) {
-        vhost_virtqueue_stop(hdev,
-                             vdev,
-                             hdev->vqs + i,
-                             hdev->vq_index + i);
+        rc |= vhost_virtqueue_stop(hdev,
+                                   vdev,
+                                   hdev->vqs + i,
+                                   hdev->vq_index + i);
     }
     if (hdev->vhost_ops->vhost_reset_status) {
         hdev->vhost_ops->vhost_reset_status(hdev);
@@ -2176,6 +2178,7 @@ void vhost_dev_stop(struct vhost_dev *hdev, VirtIODevice *vdev, bool vrings)
     hdev->started = false;
     vdev->vhost_started = false;
     hdev->vdev = NULL;
+    return rc;
 }
 
 int vhost_net_set_backend(struct vhost_dev *hdev,

include/hw/virtio/vhost.h

@@ -232,8 +232,10 @@ int vhost_dev_start(struct vhost_dev *hdev, VirtIODevice *vdev, bool vrings);
  * Stop the vhost device. After the device is stopped the notifiers
  * can be disabled (@vhost_dev_disable_notifiers) and the device can
  * be torn down (@vhost_dev_cleanup).
+ *
+ * Return: 0 on success, != 0 on error when stopping dev.
  */
-void vhost_dev_stop(struct vhost_dev *hdev, VirtIODevice *vdev, bool vrings);
+int vhost_dev_stop(struct vhost_dev *hdev, VirtIODevice *vdev, bool vrings);
 
 /**
  * DOC: vhost device configuration handling
@@ -333,8 +335,8 @@ int vhost_device_iotlb_miss(struct vhost_dev *dev, uint64_t iova, int write);
 
 int vhost_virtqueue_start(struct vhost_dev *dev, struct VirtIODevice *vdev,
                           struct vhost_virtqueue *vq, unsigned idx);
-void vhost_virtqueue_stop(struct vhost_dev *dev, struct VirtIODevice *vdev,
-                          struct vhost_virtqueue *vq, unsigned idx);
+int vhost_virtqueue_stop(struct vhost_dev *dev, struct VirtIODevice *vdev,
+                         struct vhost_virtqueue *vq, unsigned idx);
 
 void vhost_dev_reset_inflight(struct vhost_inflight *inflight);
 void vhost_dev_free_inflight(struct vhost_inflight *inflight);