vsock/vmci: register vmci_transport only when VMCI guest/host are active

To allow other transports to be loaded with vmci_transport,
we register the vmci_transport as G2H or H2G only when a VMCI guest
or host is active.

To do that, this patch adds a callback registered in the vmci driver
that will be called when the host or guest becomes active.
This callback will register the vmci_transport in the VSOCK core.

Cc: Jorgen Hansen <[e-mail]>
Signed-off-by: Stefano Garzarella <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/misc/vmw_vmci/vmci_driver.c

···
 static bool vmci_guest_personality_initialized;
 static bool vmci_host_personality_initialized;
 
+static DEFINE_MUTEX(vmci_vsock_mutex); /* protects vmci_vsock_transport_cb */
+static vmci_vsock_cb vmci_vsock_transport_cb;
+bool vmci_vsock_cb_host_called;
+
 /*
  * vmci_get_context_id() - Gets the current context ID.
  *
···
 	return VMCI_INVALID_ID;
 }
 EXPORT_SYMBOL_GPL(vmci_get_context_id);
+
+/*
+ * vmci_register_vsock_callback() - Register the VSOCK vmci_transport callback.
+ *
+ * The callback will be called when the first host or guest becomes active,
+ * or if they are already active when this function is called.
+ * To unregister the callback, call this function with NULL parameter.
+ *
+ * Returns 0 on success. -EBUSY if a callback is already registered.
+ */
+int vmci_register_vsock_callback(vmci_vsock_cb callback)
+{
+	int err = 0;
+
+	mutex_lock(&vmci_vsock_mutex);
+
+	if (vmci_vsock_transport_cb && callback) {
+		err = -EBUSY;
+		goto out;
+	}
+
+	vmci_vsock_transport_cb = callback;
+
+	if (!vmci_vsock_transport_cb) {
+		vmci_vsock_cb_host_called = false;
+		goto out;
+	}
+
+	if (vmci_guest_code_active())
+		vmci_vsock_transport_cb(false);
+
+	if (vmci_host_users() > 0) {
+		vmci_vsock_cb_host_called = true;
+		vmci_vsock_transport_cb(true);
+	}
+
+out:
+	mutex_unlock(&vmci_vsock_mutex);
+	return err;
+}
+EXPORT_SYMBOL_GPL(vmci_register_vsock_callback);
+
+void vmci_call_vsock_callback(bool is_host)
+{
+	mutex_lock(&vmci_vsock_mutex);
+
+	if (!vmci_vsock_transport_cb)
+		goto out;
+
+	/* In the host, this function could be called multiple times,
+	 * but we want to register it only once.
+	 */
+	if (is_host) {
+		if (vmci_vsock_cb_host_called)
+			goto out;
+
+		vmci_vsock_cb_host_called = true;
+	}
+
+	vmci_vsock_transport_cb(is_host);
+out:
+	mutex_unlock(&vmci_vsock_mutex);
+}
 
 static int __init vmci_drv_init(void)
 {

drivers/misc/vmw_vmci/vmci_driver.h

···
 
 u32 vmci_get_context_id(void);
 int vmci_send_datagram(struct vmci_datagram *dg);
+void vmci_call_vsock_callback(bool is_host);
 
 int vmci_host_init(void);
 void vmci_host_exit(void);
 bool vmci_host_code_active(void);
+int vmci_host_users(void);
 
 int vmci_guest_init(void);
 void vmci_guest_exit(void);

drivers/misc/vmw_vmci/vmci_guest.c

···
 		  vmci_dev->iobase + VMCI_CONTROL_ADDR);
 
 	pci_set_drvdata(pdev, vmci_dev);
+
+	vmci_call_vsock_callback(false);
 	return 0;
 
 err_free_irq:

drivers/misc/vmw_vmci/vmci_host.c

···
 	     atomic_read(&vmci_host_active_users) > 0);
 }
 
+int vmci_host_users(void)
+{
+	return atomic_read(&vmci_host_active_users);
+}
+
 /*
  * Called on open of /dev/vmci.
  */
···
 
 	vmci_host_dev->ct_type = VMCIOBJ_CONTEXT;
 	atomic_inc(&vmci_host_active_users);
+
+	vmci_call_vsock_callback(true);
 
 	retval = 0;
 

include/linux/vmw_vmci_api.h

···
 struct msghdr;
 typedef void (vmci_device_shutdown_fn) (void *device_registration,
 					void *user_data);
+typedef void (*vmci_vsock_cb) (bool is_host);
 
 int vmci_datagram_create_handle(u32 resource_id, u32 flags,
 				vmci_datagram_recv_cb recv_cb,
···
 int vmci_doorbell_notify(struct vmci_handle handle, u32 priv_flags);
 u32 vmci_get_context_id(void);
 bool vmci_is_context_owner(u32 context_id, kuid_t uid);
+int vmci_register_vsock_callback(vmci_vsock_cb callback);
 
 int vmci_event_subscribe(u32 event,
 			 vmci_event_cb callback, void *callback_data,

net/vmw_vsock/vmci_transport.c

···
 	return vsk->transport == &vmci_transport;
 }
 
+void vmci_vsock_transport_cb(bool is_host)
+{
+	int features;
+
+	if (is_host)
+		features = VSOCK_TRANSPORT_F_H2G;
+	else
+		features = VSOCK_TRANSPORT_F_G2H;
+
+	vsock_core_register(&vmci_transport, features);
+}
+
 static int __init vmci_transport_init(void)
 {
-	int features = VSOCK_TRANSPORT_F_DGRAM | VSOCK_TRANSPORT_F_H2G;
-	int cid;
 	int err;
-
-	cid = vmci_get_context_id();
-
-	if (cid == VMCI_INVALID_ID)
-		return -EINVAL;
-
-	if (cid != VMCI_HOST_CONTEXT_ID)
-		features |= VSOCK_TRANSPORT_F_G2H;
 
 	/* Create the datagram handle that we will use to send and receive all
 	 * VSocket control messages for this context.
···
 		pr_err("Unable to create datagram handle. (%d)\n", err);
 		return vmci_transport_error_to_vsock_error(err);
 	}
-
 	err = vmci_event_subscribe(VMCI_EVENT_QP_RESUMED,
 				   vmci_transport_qp_resumed_cb,
 				   NULL, &vmci_transport_qp_resumed_sub_id);
···
 		goto err_destroy_stream_handle;
 	}
 
-	err = vsock_core_register(&vmci_transport, features);
+	/* Register only with dgram feature, other features (H2G, G2H) will be
+	 * registered when the first host or guest becomes active.
+	 */
+	err = vsock_core_register(&vmci_transport, VSOCK_TRANSPORT_F_DGRAM);
 	if (err < 0)
 		goto err_unsubscribe;
 
+	err = vmci_register_vsock_callback(vmci_vsock_transport_cb);
+	if (err < 0)
+		goto err_unregister;
+
 	return 0;
 
+err_unregister:
+	vsock_core_unregister(&vmci_transport);
 err_unsubscribe:
 	vmci_event_unsubscribe(vmci_transport_qp_resumed_sub_id);
 err_destroy_stream_handle:
···
 		vmci_transport_qp_resumed_sub_id = VMCI_INVALID_ID;
 	}
 
+	vmci_register_vsock_callback(NULL);
 	vsock_core_unregister(&vmci_transport);
 }
 module_exit(vmci_transport_exit);