depot/third_party/nixpkgs/doc/packages/linux.section.md

Linux kernel

The Nix expressions to build the Linux kernel are in pkgs/os-specific/linux/kernel.

The function that builds the kernel has an argument kernelPatches which should be a list of {name, patch, extraConfig} attribute sets, where name is the name of the patch (which is included in the kernel’s meta.description attribute), patch is the patch itself (possibly compressed), and extraConfig (optional) is a string specifying extra options to be concatenated to the kernel configuration file (.config).

The kernel derivation exports an attribute features specifying whether optional functionality is or isn’t enabled. This is used in NixOS to implement kernel-specific behaviour. For instance, if the kernel has the iwlwifi feature (i.e., has built-in support for Intel wireless chipsets), then NixOS doesn’t have to build the external iwlwifi package:

modulesTree = [kernel]
  ++ pkgs.lib.optional (!kernel.features ? iwlwifi) kernelPackages.iwlwifi
  ++ ...;

How to add a new (major) version of the Linux kernel to Nixpkgs:

1. Copy the old Nix expression (e.g., linux-2.6.21.nix) to the new one (e.g., linux-2.6.22.nix) and update it.

2. Add the new kernel to the kernels attribute set in linux-kernels.nix (e.g., create an attribute kernel_2_6_22).

3. Now we’re going to update the kernel configuration. First unpack the kernel. Then for each supported platform (i686, x86_64, uml) do the following:

   1. Make a copy from the old config (e.g., config-2.6.21-i686-smp) to the new one (e.g., config-2.6.22-i686-smp).

   2. Copy the config file for this platform (e.g., config-2.6.22-i686-smp) to .config in the kernel source tree.

   3. Run make oldconfig ARCH={i386,x86_64,um} and answer all questions. (For the uml configuration, also add SHELL=bash.) Make sure to keep the configuration consistent between platforms (i.e., don’t enable some feature on i686 and disable it on x86_64).

   4. If needed, you can also run make menuconfig:

      $ nix-env -f "<nixpkgs>" -iA ncurses
      $ export NIX_CFLAGS_LINK=-lncurses
      $ make menuconfig ARCH=arch
      

   5. Copy .config over the new config file (e.g., config-2.6.22-i686-smp).

4. Test building the kernel: nix-build -A linuxKernel.kernels.kernel_2_6_22. If it compiles, ship it! For extra credit, try booting NixOS with it.

5. It may be that the new kernel requires updating the external kernel modules and kernel-dependent packages listed in the linuxPackagesFor function in linux-kernels.nix (such as the NVIDIA drivers, AUFS, etc.). If the updated packages aren’t backwards compatible with older kernels, you may need to keep the older versions around.