linux-kernel - [PATCH v2] vhost/vsock: use static minor number

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Date:   Wed, 10 May 2017 10:19:18 -0400
From:   Stefan Hajnoczi <stefanha@...hat.com>
To:     kvm@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>, device@...ana.org,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Arnd Bergmann <arnd@...db.de>, linux-kernel@...r.kernel.org,
        Stefan Hajnoczi <stefanha@...hat.com>
Subject: [PATCH v2] 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: device@...ana.org
Signed-off-by: Stefan Hajnoczi <stefanha@...hat.com>
---
v2:
 * Document userio in Documentation/admin-guide/devices.txt [Arnd]

 Documentation/admin-guide/devices.txt | 4 +++-
 include/linux/miscdevice.h            | 1 +
 drivers/vhost/vsock.c                 | 4 +++-
 3 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/Documentation/admin-guide/devices.txt b/Documentation/admin-guide/devices.txt
index c9cea2e..6b71852 100644
--- a/Documentation/admin-guide/devices.txt
+++ b/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)
diff --git a/include/linux/miscdevice.h b/include/linux/miscdevice.h
index 762b5fe..58751ea 100644
--- a/include/linux/miscdevice.h
+++ b/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;
diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
index 44eed8e..53e3e95 100644
--- a/drivers/vhost/vsock.c
+++ b/drivers/vhost/vsock.c
@@ -701,7 +701,7 @@ 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,
 };
@@ -773,3 +773,5 @@ 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");
-- 
2.9.3