vhost/vsock: use static minor number

Vhost-vsock is a software device so there is no probe call that causes
the driver to register its misc char device node.  This creates a
chicken and egg problem: userspace applications must open
/dev/vhost-vsock to use the driver but the file doesn't exist until the
kernel module has been loaded.

Use the devname modalias mechanism so that /dev/vhost-vsock is created
at boot.  The vhost_vsock kernel module is automatically loaded when the
first application opens /dev/host-vsock.

Note that the "reserved for local use" range in
Documentation/admin-guide/devices.txt is incorrect.  The userio driver
already occupies part of that range.  I've updated the documentation
accordingly.

Cc: [email protected]
Signed-off-by: Stefan Hajnoczi <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Documentation/admin-guide/devices.txt

@@ -369,8 +369,10 @@
 		237 = /dev/loop-control Loopback control device
 		238 = /dev/vhost-net	Host kernel accelerator for virtio net
 		239 = /dev/uhid		User-space I/O driver support for HID subsystem
+		240 = /dev/userio	Serio driver testing device
+		241 = /dev/vhost-vsock	Host kernel driver for virtio vsock
 
-		240-254			Reserved for local use
+		242-254			Reserved for local use
 		255			Reserved for MISC_DYNAMIC_MINOR
 
   11 char	Raw keyboard device	(Linux/SPARC only)

drivers/vhost/vsock.c

@@ -706,7 +706,7 @@ static const struct file_operations vhost_vsock_fops = {
 };
 
 static struct miscdevice vhost_vsock_misc = {
-	.minor = MISC_DYNAMIC_MINOR,
+	.minor = VHOST_VSOCK_MINOR,
 	.name = "vhost-vsock",
 	.fops = &vhost_vsock_fops,
 };
@@ -778,3 +778,5 @@ module_exit(vhost_vsock_exit);
 MODULE_LICENSE("GPL v2");
 MODULE_AUTHOR("Asias He");
 MODULE_DESCRIPTION("vhost transport for vsock ");
+MODULE_ALIAS_MISCDEV(VHOST_VSOCK_MINOR);
+MODULE_ALIAS("devname:vhost-vsock");

include/linux/miscdevice.h

@@ -54,6 +54,7 @@
 #define VHOST_NET_MINOR		238
 #define UHID_MINOR		239
 #define USERIO_MINOR		240
+#define VHOST_VSOCK_MINOR	241
 #define MISC_DYNAMIC_MINOR	255
 
 struct device;