[bug#37868] [PATCH] guix: Allow multiple packages to provide Linux modules in the system profile.

[Danny Milosavljevic <dannym@scratchpost.org>]

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Hi,

any comments about this patch?

I don't want to just push this to guix master without any discussion since it
establishes an interface that has to keep working for a long time.

Rationale of the patch:

* Make Linux more modular, allowing the user to specify a union of Guix packages
to use as "the kernel" (especially kernel modules).

Summary of the patch:

* Add a profile hook "linux-module-database" which creates the union of all
system packages that have a subdirectory "lib/modules" in their derivation,
then invokes depmod on that union and then provides the result in the system
profile.

* Adapt modprobe to check "lib/modules" inside the system profile, if available.
Fall back to "/run/booted-system/kernel/lib/modules" otherwise.

For the case where a person has just reconfigured Guix but doesn't want to reboot,
modprobe will still work, taking the modules of the old generation (which doesn't
necessarily have Linux kernel modules inside the profile yet--because it doesn't
necessarily have this patch yet.  But maybe it does).

* Adapt operating-system-profile to automatically add the Kernel's modules to
the system profile (since the system profile would be the only place searched,
not doing so would be very bad).

* Adapt linux-build-system not to invoke depmod again.  Also, its worldview
would be incomplete anyway because it wouldn't have the entire system profile.

Open questions:

* Why doesn't operating-system-profile successfully add linux-libre ?
It should.  I don't think Guix ever gets there in the first place. (adding
linux-libre to operating-system's "packages" field manually does work)

* Do we want to have this stuff in the system profile or do we want to have
a "kernel profile" instead or something?  I don't think the latter would help
us much, but if we want it, better do it now.

* Do we want to be able to add kernel modules in this fashion without requiring
a reboot?  If so, that would make the situation a lot more complicated and I
don't see a safe way to do that.

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