[PATCH v5 5/5] vhost/vsock: add VHOST_RESET_OWNER ioctl

[Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>]

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.

To that end, unlike the release path, RESET_OWNER keeps the guest CID
hashed: established connections survive, and host sends issued while
the device is between owners simply stay on send_pkt_queue until the
next device start drains them.

Since the device stays reachable through the CID hash, the lockless
send/cancel paths can race with the worker teardown in
vhost_workers_free().  The previous commit ("vhost: synchronize with
RCU readers when freeing workers") makes that safe.

Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Signed-off-by: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
---
 drivers/vhost/vsock.c | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
index d5022d21120b..86f25ff80722 100644
--- a/drivers/vhost/vsock.c
+++ b/drivers/vhost/vsock.c
@@ -903,6 +903,29 @@ static int vhost_vsock_set_features(struct vhost_vsock *vsock, u64 features)
 	return -EFAULT;
 }
 
+static long 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;
+	}
+	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)
 {
@@ -946,6 +969,8 @@ 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