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doc: consolidate info on manual linux kernel configs
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The Nixpkgs documentation on the linux kernel builders focused on
using and extending kernels that were already packaged, but never
mentioned that it's possible to also build a kernel almost "from
scratch".
The NixOS documentation went a bit deeper on manual linux kernel
configs, but that information wasn't particularly NixOS-specific.

This commit consolidates the information related to building the
kernel on Nixpkgs's documentation, while keeping any additional
NixOS-specific information on NixOS's documentation.

An additional README.md was created for contributor-facing
documentation.
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DanielSidhion committed Oct 27, 2023
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116 changes: 98 additions & 18 deletions doc/builders/packages/linux.section.md
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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/f9d4f5f23ce7ccb7ad91d6d2150e367baf7014f6/pkgs/os-specific/linux/kernel/generic.nix) builds a kernel with [common configuration values](hthttps://github.com/NixOS/nixpkgs/blob/f9d4f5f23ce7ccb7ad91d6d2150e367baf7014f6/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/f9d4f5f23ce7ccb7ad91d6d2150e367baf7014f6/pkgs/os-specific/linux/kernel/manual-config.nix) requires a complete configuration to be passed, and has less additional features than `pkgs.buildLinux`, such as the use of common configuration values and exposing 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]
++ 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.
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 source](https://github.com/NixOS/nixpkgs/blob/f9d4f5f23ce7ccb7ad91d6d2150e367baf7014f6/pkgs/os-specific/linux/kernel/generic.nix) for `pkgs.buildLinux`.

2. Add the new kernel to the `kernels` attribute set in `linux-kernels.nix` (e.g., create an attribute `kernel_2_6_22`).
:::{.example #ex-overriding-kernel-derivation}

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:
# Overriding the kernel derivation

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

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.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;
};
}
```

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

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

5. Copy `.config` over the new config file (e.g., `config-2.6.22-i686-smp`).
:::{.example #ex-using-linux-manual-config}

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

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.
```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;
};
}
```

:::

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

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-kernel-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
```
66 changes: 3 additions & 63 deletions nixos/doc/manual/configuration/linux-kernel.chapter.md
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## 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.
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

```nix
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
```
[Moved](https://nixos.org/nixpkgs/manual#sec-linux-kernel-developing-modules) to the Nixpkgs manual.

## ZFS {#sec-linux-zfs}

Expand All @@ -163,7 +103,7 @@ available Linux kernel. It is recommended to use the latest available LTS that's
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
{
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# 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`:

```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: `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.

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