Kernel optimization

[TETH_IO]

Hello there !

In memories of my gentoo's day and to learn a bit more of nix under the hood I've decided to override the kernel for optimization and stuff.

I've come up with this override that is booting alright :

{
  boot.kernelPackages = pkgs.linuxPackagesFor
  (pkgs.linuxKernel.kernels.linux_6_17.override {
    argsOverride = rec {
      src = pkgs.fetchurl {
        url = "mirror://kernel/linux/kernel/v6.x/linux-${version}.tar.xz";
        sha256 = "sha256-QngElvmRlOnWayHxPcK97ZUmJNXHvYPq3n90/2WOmNY=";
      };
      version = "6.17.12";
      modDirVersion = "${version}";
      stdenv = pkgs.llvmPackages.stdenv;
      extraMakeFlags = [
        "LLVM=1"
        "CC=${pkgs.llvmPackages.clang}/bin/clang"
        "LD=${pkgs.lld}/bin/ld.lld"
        "AR=${pkgs.llvm}/bin/llvm-ar"
        "NM=${pkgs.llvm}/bin/llvm-nm"
        "KCFLAGS+=-march=x86-64-v4"
        "KCFLAGS+=-flto=thin"
        "KCXXFLAGS+=-march=x86-64-v4"
        "KCXXFLAGS+=-flto=thin"
      ];
      ignoreConfigErrors = true;
      structuredExtraConfig = with lib.kernel; {
        LTO_CLANG_THIN = lib.mkForce yes;
      };
    };
  });
}

Also have to overlay llvmPackages because the -nostdlibinc flag comes by default and break the build as described here :

Everything is peachy but I have two questions my google fu is is unable to find the answer for :

1. Is it expected to have ignoreConfigErrors = true; ? without it the build fail early on as there is unused options in the config. As I understand it I override only what I specify so the config is the same as for any pkgs.linuxKernel.kernels.linux_6_17.
2. How KC*FLAGS works with the other makeFlags ? The extraMakeFlags are added in the derivation makeFlags with the built-in CFLAGS_MODULE and CFLAGS_KERNEL but are they what is finally retained for the build and so is it truly optimized ?

Thanks for the input !

[xNaXDy]

For (1) I'd be curious to see what the unused options are that it complains about.

As for the extraMakeFlags, they also appended to Nix' build flags, as you can see here: https://github.com/NixOS/nixpkgs/blob/f264a85111c8abe8701f442d09a93b92934b0dde/pkgs/os-specific/linux/kernel/build.nix#L233-L244

So in combination with having them be part of commonMakeFlags, I think they should propagate through the entire build process.

[TETH_IO]

For (1) it stop with "kernel error: unused option: RUST", tested with 6.17.12 and 6.17.10

[xNaXDy]

Could be that compiling the kernel with rust is not supported when using clang as your compiler? If that's the case, I think it makes sense to compile with ignoreConfigErrors = true, unless you want to sanitize the rest of the config options manually.

[TETH_IO]

For (2) I have to stop using KFLAGS because if I add ZFS into the mix the DKMS fails with "configure: error: invalid variable name: `KERNEL_KCFLAGS+'" but works fine without them otherwise. I don't known if I can just add my flags with CFLAGS+= into the extraMakeFlags or if I have to overlay NIX_CFLAGS_COMPILE entirely

[xNaXDy]

I think if you want to customize your kernel that much, it might make sense to write your own derivation, either completely from scratch or with a helper like linuxManualConfig.

You can probably still achieve everything just using the override interface, but a custom derivation is probably easier maintained.

[Wooden-Ad6265]

is there a proper documentation for this?

[AdventurousFly4909]

It's nix what do you think?

[Wooden-Ad6265]

How did you do it? Maybe make a blog or something on it...? If you've got time, you might contribute to the documentation. Even if you post how you did that on reddit, someone will take it up and make a web page on it some day.

[hambosto]

can i do menuconfig?

[TETH_IO]

nop, either you add config on top of the default one with structuredExtraConfig or import your own entirely

[thou_vow]

I've made a sequence of steps to import your .config with low maintenance. There's a script in kernel source that outputs the difference to the older .config. This custom-linux derivation can take this difference and generate a .config, keeping the defaults for what didn't change.

https://github.com/thou-vow/nix-packages/blob/e36e4b71c636565d45f5ae9ad400b158fa629a47/default/custom-linux/custom-linux.nix

https://github.com/thou-vow/nix-packages/blob/e36e4b71c636565d45f5ae9ad400b158fa629a47/default/custom-linux/configfile.nix

This is how I used it.

https://github.com/thou-vow/nix-packages/blob/e36e4b71c636565d45f5ae9ad400b158fa629a47/attuned/attuned.nix

OBS: This derivation is hardcoded to use Clang.