Merge pull request #263056 from DanielSidhion/kernel-docs

The Nix expressions to build the Linux kernel are in [`pkgs/os-specific/linux/kernel`](https://github.com/NixOS/nixpkgs/blob/master/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 function [`pkgs.buildLinux`](https://github.com/NixOS/nixpkgs/blob/d77bda728d5041c1294a68fb25c79e2d161f62b9/pkgs/os-specific/linux/kernel/generic.nix) builds a kernel with [common configuration values](https://github.com/NixOS/nixpkgs/blob/d77bda728d5041c1294a68fb25c79e2d161f62b9/pkgs/os-specific/linux/kernel/common-config.nix).

This is the preferred option unless you have a very specific use case.

Most kernels packaged in Nixpkgs are built that way, and it will also generate kernels suitable for NixOS.

[`pkgs.linuxManualConfig`](https://github.com/NixOS/nixpkgs/blob/d77bda728d5041c1294a68fb25c79e2d161f62b9/pkgs/os-specific/linux/kernel/manual-config.nix) requires a complete configuration to be passed.

It has fewer additional features than `pkgs.buildLinux`, which provides common configuration values and exposes the `features` attribute, as explained below.

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:

Both functions have 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 created with `pkgs.buildLinux` exports an attribute `features` specifying whether optional functionality is or isn’t enabled. This is used in NixOS to implement kernel-specific behaviour.

:::{.example #ex-skip-package-from-kernel-feature}

# Skipping an external package because of a kernel feature

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:

```nix

modulesTree = [kernel]

  ++ ...;

```

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

:::

If you are using a kernel packaged in Nixpkgs, you can customize it by overriding its arguments. For details on how each argument affects the generated kernel, refer to [the `pkgs.buildLinux` source code](https://github.com/NixOS/nixpkgs/blob/d77bda728d5041c1294a68fb25c79e2d161f62b9/pkgs/os-specific/linux/kernel/generic.nix).

:::{.example #ex-overriding-kernel-derivation}

# Overriding the kernel derivation

Assuming you are using the kernel from `pkgs.linux_latest`:

```nix

pkgs.linux_latest.override {

  ignoreConfigErrors = true;

  autoModules = false;

  kernelPreferBuiltin = true;

  extraStructuredConfig = with lib.kernel; {

    DEBUG_KERNEL = yes;

    FRAME_POINTER = yes;

    KGDB = yes;

    KGDB_SERIAL_CONSOLE = yes;

    DEBUG_INFO = yes;

  };

}

```

:::

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.

## Manual kernel configuration {#sec-manual-kernel-configuration}

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

Sometimes it may not be desirable to use kernels built with `pkgs.buildLinux`, especially if most of the common configuration has to be altered or disabled to achieve a kernel as expected by the target use case.

An example of this is building a kernel for use in a VM or micro VM. You can use `pkgs.linuxManualConfig` in these cases. It requires the `src`, `version`, and `configfile` attributes to be specified.

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:

:::{.example #ex-using-linux-manual-config}

    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`).

# Using `pkgs.linuxManualConfig` with a specific source, version, and config file

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

```nix

{ pkgs, ... }: {

  version = "6.1.55";

  src = pkgs.fetchurl {

    url = "https://cdn.kernel.org/pub/linux/kernel/v6.x/linux-${version}.tar.xz";

    hash = "sha256:1h0mzx52q9pvdv7rhnvb8g68i7bnlc9rf8gy9qn4alsxq4g28zm8";

  };

  configfile = ./path_to_config_file;

  linux = pkgs.linuxManualConfig {

    inherit version src configfile;

    allowImportFromDerivation = true;

  };

}

```

If necessary, the version string can be slightly modified to explicitly mark it as a custom version. If you do so, ensure the `modDirVersion` attribute matches the source's version, otherwise the build will fail.

```nix

{ pkgs, ... }: {

  version = "6.1.55-custom";

  modDirVersion = "6.1.55";

  src = pkgs.fetchurl {

    url = "https://cdn.kernel.org/pub/linux/kernel/v6.x/linux-${modDirVersion}.tar.xz";

    hash = "sha256:1h0mzx52q9pvdv7rhnvb8g68i7bnlc9rf8gy9qn4alsxq4g28zm8";

  };

  configfile = ./path_to_config_file;

  linux = pkgs.linuxManualConfig {

    inherit version modDirVersion src configfile;

    allowImportFromDerivation = true;

  };

}

```

    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`:

Additional attributes can be used with `linuxManualConfig` for further customisation. You're encouraged to read [the `pkgs.linuxManualConfig` source code](https://github.com/NixOS/nixpkgs/blob/d77bda728d5041c1294a68fb25c79e2d161f62b9/pkgs/os-specific/linux/kernel/manual-config.nix) to understand how to use them.

To edit the `.config` file for Linux X.Y from within Nix, proceed as follows:

```ShellSession

        $ nix-env -f "<nixpkgs>" -iA ncurses

        $ export NIX_CFLAGS_LINK=-lncurses

        $ make menuconfig ARCH=arch

$ nix-shell '<nixpkgs>' -A linuxKernel.kernels.linux_X_Y.configEnv

$ unpackPhase

$ cd linux-*

$ make nconfig

```

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

## Developing kernel modules {#sec-linux-kernel-developing-modules}

When developing kernel modules it's often convenient to run the edit-compile-run loop as quickly as possible.

See the snippet below as an example.

:::{.example #ex-edit-compile-run-kernel-modules}

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.

# Edit-compile-run loop when developing `mellanox` drivers

```ShellSession

$ nix-build '<nixpkgs>' -A linuxPackages.kernel.dev

$ nix-shell '<nixpkgs>' -A linuxPackages.kernel

$ unpackPhase

$ cd linux-*

$ make -C $dev/lib/modules/*/build M=$(pwd)/drivers/net/ethernet/mellanox modules

# insmod ./drivers/net/ethernet/mellanox/mlx5/core/mlx5_core.ko

```

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.

:::

## Building a custom kernel {#sec-linux-config-customizing}

You can customize the default kernel configuration by overriding the arguments for your kernel package:

```nix

pkgs.linux_latest.override {

  ignoreConfigErrors = true;

  autoModules = false;

  kernelPreferBuiltin = true;

  extraStructuredConfig = with lib.kernel; {

    DEBUG_KERNEL = yes;

    FRAME_POINTER = yes;

    KGDB = yes;

    KGDB_SERIAL_CONSOLE = yes;

    DEBUG_INFO = yes;

  };

}

```

See `pkgs/os-specific/linux/kernel/generic.nix` for details on how these arguments

affect the generated configuration. You can also build a custom version of Linux by calling

`pkgs.buildLinux` directly, which requires the `src` and `version` arguments to be specified.

Please refer to the Nixpkgs manual for the various ways of [building a custom kernel](https://nixos.org/nixpkgs/manual#sec-linux-kernel).

To use your custom kernel package in your NixOS configuration, set

boot.kernelPackages = pkgs.linuxPackagesFor yourCustomKernel;

```

Note that this method will use the common configuration defined in `pkgs/os-specific/linux/kernel/common-config.nix`,

which is suitable for a NixOS system.

If you already have a generated configuration file, you can build a kernel that uses it with `pkgs.linuxManualConfig`:

```nix

let

  baseKernel = pkgs.linux_latest;

in pkgs.linuxManualConfig {

  inherit (baseKernel) src modDirVersion;

  version = "${baseKernel.version}-custom";

  configfile = ./my_kernel_config;

  allowImportFromDerivation = true;

}

```

::: {.note}

The build will fail if `modDirVersion` does not match the source's `kernel.release` file,

so `modDirVersion` should remain tied to `src`.

:::

To edit the `.config` file for Linux X.Y, proceed as follows:

```ShellSession

$ nix-shell '<nixpkgs>' -A linuxKernel.kernels.linux_X_Y.configEnv

$ unpackPhase

$ cd linux-*

$ make nconfig

```

## Developing kernel modules {#sec-linux-config-developing-modules}

When developing kernel modules it's often convenient to run

edit-compile-run loop as quickly as possible. See below snippet as an

example of developing `mellanox` drivers.

```ShellSession

$ nix-build '<nixpkgs>' -A linuxPackages.kernel.dev

$ nix-shell '<nixpkgs>' -A linuxPackages.kernel

$ unpackPhase

$ cd linux-*

$ make -C $dev/lib/modules/*/build M=$(pwd)/drivers/net/ethernet/mellanox modules

# insmod ./drivers/net/ethernet/mellanox/mlx5/core/mlx5_core.ko

```

This section was moved to the [Nixpkgs manual](https://nixos.org/nixpkgs/manual#sec-linux-kernel-developing-modules).

## ZFS {#sec-linux-zfs}

with ZFS. Usually this is the default kernel provided by nixpkgs (i.e. `pkgs.linuxPackages`).

Alternatively, it's possible to pin the system to the latest available kernel

version *that is supported by ZFS* like this:

version _that is supported by ZFS_ like this:

```nix

{

# 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`](./linux-kernels.nix) (e.g., create an attribute `kernel_2_6_22`).

3.  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`:

        ```ShellSession

        $ 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:

```ShellSession

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.