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 <jhansen@vmware.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/misc/vmw_vmci/vmci_driver.c

@@ -28,6 +28,10 @@ MODULE_PARM_DESC(disable_guest,
 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.
  *
@@ -45,6 +49,69 @@ u32 vmci_get_context_id(void)
 }
 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)
 {
 	int vmci_err;

drivers/misc/vmw_vmci/vmci_driver.h

@@ -36,10 +36,12 @@ extern struct pci_dev *vmci_pdev;
 
 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

@@ -637,6 +637,8 @@ static int vmci_guest_probe_device(struct pci_dev *pdev,
 		  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

@@ -108,6 +108,11 @@ bool vmci_host_code_active(void)
 	     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.
  */
@@ -338,6 +343,8 @@ static int vmci_host_do_init_context(struct vmci_host_dev *vmci_host_dev,
 	vmci_host_dev->ct_type = VMCIOBJ_CONTEXT;
 	atomic_inc(&vmci_host_active_users);
 
+	vmci_call_vsock_callback(true);
+
 	retval = 0;
 
 out: