[RDY] kernel doc: explain how to configure custom kernel

[teto]

Motivation for this change

compiling kernel is currently hard, hopefully it will improve overtime. (for instance:

 nix-build <nixpkgs> --arg crossSystem '(import <nixpkgs/lib>).systems.examples.aarch64-multiplatform' -iA lin

$ nix-build -A linux_mptcp --arg 'localSystem' 'let lib = (import <nixpkgs/lib>); in lib.recursiveUpdate (lib.systems.elaborate { system = builtins.currentSystem; }) { platform = test-platform; }' '<nixpkgs>' --show-trace

)
Until then here is some doc.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • Linux
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

---

[teto]

My intent is to document the current way of customizing kernels but I would rather improve the situation.

@Ericson2314 I am very eager to ease kernel customization. My main question that remains unanswered (#2296): nixos generates first the config and then restarts the process (unpacking/patching) for the kernel compilation. Would it be ok to generate the config on the go instead ? Or does that break some workflow ?

[Ericson2314]

@teto I haven't really looked at it, but I'm rather found of the idea of configuring in a separate step. Configuring is rarely as deterministic as we want it to be, and this silos it off from the rest. That would especially be good in a future with the intensional store.

But do feel free to improve before documenting :)

[teto]

I prepare a patch that makes kernel overrides easier (i.e., moves fields out of platform), yet I lose my way when it comes to override hostPlatform from an overlay

  test-platform =  {
    name="zizou";
    kernelAutoModules = false;
    kernelExtraConfig = mptcpKernelExtraConfig ;
    ignoreConfigErrors = true;
    kernelPreferBuiltin = true;
  };

  # builtins.currentSystem returns "x86_64-linux"
  test-localSystem = let system = super.lib.systems.elaborate { system = builtins.currentSystem; };
   in super.lib.recursiveUpdate (system) { platform = system.platform // test-platform; };

  my_kernel = linux_mptcp.override ({
      modDirVersion="4.9.60-00010-g5a1ca10181c6";
      name="mptcp-local";
     # it doesn't seem used
      hostPlatform=test-localSystem;

In nix-repl:
my_kernel.configfile.platformName still prints "pc" while I expect "zizou".
I am trying to investigate what I do wrong but feel free to bump in if you have a solution :)

[Ericson2314]

@teto don't override hostPlatform on a derivation. That's not good. Just go straight to moving the config out of hostPlatform altogether, like you want it to be.

[teto]

@Ericson2314 I pushed an update that I hope to be ok. I dropped the references to hostPlaform overriding since there are not necessary anymore. I am not that familiar with docbook so I might have used wrong tags.

[Ericson2314]

Look good to me. @dezgeg or @vcunat you know more about this. What do you think?

[dezgeg]

I would title this Building a customized kernel or something like that.

[teto]

is Customize your kernel ok ?

[dezgeg]

<filename>pkgs/os-specific/linux/kernel/generic.nix</filename>

[dezgeg]

There are various places missing spaces around the = in this example.

[teto]

should be fixed

[dezgeg]

Seems good overall. I gave some minor comments.

[teto]

i pushed an update that should address the comments.

[teto]

Does that seem ok :> ?

[dezgeg]

Yes, looks good. Sorry this fell through the cracks.

[teto]

sure no problem. If I can get around to complete/merge #41393 I'll update the doc accordingly.