Re: [PATCH 2/4] vhost/vsock: add VHOST_RESET_OWNER ioctl

[Stefano Garzarella <sgarzare@redhat.com>]

On Fri, Jun 12, 2026 at 07:57:16PM +0300, Andrey Drobyshev wrote:
>From: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
>
>This ioctl is needed for QEMU's CPR (checkpoint-restore) migration of
>the guest with vhost-vsock device.  For this to work, we need to reset
>the device ownership on the source side by calling RESET_OWNER, and then
>claim it on the dest side by calling SET_OWNER.  We expect not to lose any
>AF_VSOCK connection while this happens.
>
>Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
>---
> drivers/vhost/vsock.c | 28 ++++++++++++++++++++++++++++
> 1 file changed, 28 insertions(+)
>
>diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
>index b12221ce6faf..e629886e5cf8 100644
>--- a/drivers/vhost/vsock.c
>+++ b/drivers/vhost/vsock.c
>@@ -894,6 +894,32 @@ static int vhost_vsock_set_features(struct vhost_vsock *vsock, u64 features)
> 	return -EFAULT;
> }
>
>+static int vhost_vsock_reset_owner(struct vhost_vsock *vsock)
>+{
>+	struct vhost_iotlb *umem;
>+	long err;
>+
>+	mutex_lock(&vsock->dev.mutex);
>+	err = vhost_dev_check_owner(&vsock->dev);
>+	if (err)
>+		goto done;
>+	umem = vhost_dev_reset_owner_prepare();
>+	if (!umem) {
>+		err = -ENOMEM;
>+		goto done;
>+	}
>+	/* Follows vhost_vsock_dev_release closely except for guest_cid drop */
>+	vsock_for_each_connected_socket(&vhost_transport.transport,
>+					vhost_vsock_reset_orphans);

In vhost_vsock_reset_orphans() we have:

	rcu_read_lock();

	/* If the peer is still valid, no need to reset connection */
	if (vhost_vsock_get(vsk->remote_addr.svm_cid, sock_net(sk))) {
		rcu_read_unlock();
		return;
	}

IIUC we are not removing the guest cid from the hash table, so this 
check will be always true, and nothing is done.

So, is this call really useful?

>+	vhost_vsock_drop_backends(vsock);
>+	vhost_vsock_flush(vsock);
>+	vhost_dev_stop(&vsock->dev);
>+	vhost_dev_reset_owner(&vsock->dev, umem);
>+done:
>+	mutex_unlock(&vsock->dev.mutex);
>+	return err;
>+}
>+
> static long vhost_vsock_dev_ioctl(struct file *f, unsigned int ioctl,
> 				  unsigned long arg)
> {
>@@ -937,6 +963,8 @@ static long vhost_vsock_dev_ioctl(struct file *f, unsigned int ioctl,
> 			return -EOPNOTSUPP;
> 		vhost_set_backend_features(&vsock->dev, features);
> 		return 0;
>+	case VHOST_RESET_OWNER:
>+		return vhost_vsock_reset_owner(vsock);
> 	default:
> 		mutex_lock(&vsock->dev.mutex);
> 		r = vhost_dev_ioctl(&vsock->dev, ioctl, argp);
>-- 
>2.47.1
>