On Wed, 11 Jan 2012 11:07:47 +0400
Michael Tokarev [off-list ref] wrote:

On 11.01.2012 08:54, Stephen Hemminger wrote:

quoted

By adding the a module alias, programs (or users) won't have to explicitly
call modprobe. Vhost-net will always be available if built into the kernel.
It does require assigning a permanent minor number for depmod to work.
Choose one next to TUN since this driver is related to it.

Why do you think a statically-allocated device number will do any good
at all?  Static /dev is gone almost completely, at least on the systems
where whole virt stuff makes any sense, so you don't have pre-created
vhost-net device anymore, and hence this allocation makes no sense.
Just IMHO anyway.

The statically allocated device number is required for the udev/module
autoloading to work. Probably the udev infrastructure needs a consistent
number to hang off of.

It looks like:
  * driver adds MODULE_ALIAS() for devname and character device
  * depmod scans modules and creates modules.devname (in /lib/modules)
  * udev uses modules.devname to autoload the module

$ /sbin/modinfo vhost_net
filename:       /lib/modules/3.2.0-net+/kernel/drivers/vhost/vhost_net.ko
alias:          devname:vhost-net
alias:          char-major-10-201
description:    Host kernel accelerator for virtio net
...

See also: https://lkml.org/lkml/2010/5/21/134