Loading modules built using linux-module-build-system

[Jelle Licht]

Hello Guix,

Not too long ago, the linux-module-build-system was introduced. I ran
into some code in the wild written by Alex Griffin that defines a
shepherd service that does the following for a given kernel-module
package:

- set the LINUX_MODULE_DIRECTORY environment variable to
  <out>/lib/modules
- call modprobe on the .ko file (without .ko)

I have verified this way of loading modules to work, but was wondering
whether we should rather provide a `out-of-tree-kernel-module' service
of sorts to do this.

To resolve out-of-tree kernel module dependencies, I guess we would need
to construct a union of all outputs so we can pass along one value for
LINUX_MODULE_DIRECTORY that contains all out-of-tree modules that might
be needed for one invocation of modprobe.

Another issue is working with custom kernels; Alex' approach allows one
to override the module package by providing an expression that resolves
to a package object, which can use guile's `(inherit my-module)'
approach in combination with `substitute-keyword-arguments' to do
override the `#:linux' argument. This works and has the benefit of being
pretty explicit in defining what we want to happen. The drawback is that
one could possibly try to load a module that was built against a
different kernel version than the one in your operating system
expression.

Is there a way by which a service can refer to the
e.g. `operating-system-kernel' of the operating-system it is embedded
in?

- Jelle