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

[Stefano Garzarella]

On Fri, Jun 12, 2026 at 07:57:16PM +0300, Andrey Drobyshev wrote:
> From: Pavel Tikhomirov <ptikhomirov@xxxxxxxxxxxxx>
>
> 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@xxxxxxxxxxxxx>
> ---
> 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