Kernel module configuration service

[Brice Waegeneire <brice@waegenei.re>]

Hello Guix,

I'm working on the future 'kernel-module-configuration-service-type'[0]
(KMCS) and I need some guidance. This service will be a one stop shop 
for
all kernel module related configuration by doing the following 4 things:
1. Generate a config directory for modrope to use
2. Load loadable kernel module by extending 
'kernel-module-loader-service'
3. Configure built-in module by being a new source for
    'operating-system-kernel-arguments'
4. Add loadable kernel modules to the kernel profile by being a source 
for
    'operating-system-kernel-loadable-modules'

ATM I need help with point number 1 in regard to putting in place the
config directory for modprobe. To do so I need to change
'%modprobe-wrapper' into a procedure to take the configuration directory 
as
an argument, since the directory will now come from '/gnu/store/' 
instead
of '/etc/'. The problem is that the wrapper is currently put in place by
'%linux-bare-metal-service' which is an essential service, so it seems 
that
KMCS will have to become one too. Looking around at some essential 
service
most of them are straight forward and don't extend services like
'kernel-module-loader-service-type' (which itself extend
'shepherd-root-service-type').

What are the guidelines to add a service to the essential-services? Can
KMCS become an essential-service? Any other remarks/idea?

[0]: https://issues.guix.info/40274#18

Have a good day,
- Brice