<chapter xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xml:id="sec-kernel-config">
  <title>Linux Kernel</title>
  <para>
    You can override the Linux kernel and associated packages using the
    option <literal>boot.kernelPackages</literal>. For instance, this
    selects the Linux 3.10 kernel:
  </para>
  <programlisting language="bash">
boot.kernelPackages = pkgs.linuxKernel.packages.linux_3_10;
</programlisting>
  <para>
    Note that this not only replaces the kernel, but also packages that
    are specific to the kernel version, such as the NVIDIA video
    drivers. This ensures that driver packages are consistent with the
    kernel.
  </para>
  <para>
    While <literal>pkgs.linuxKernel.packages</literal> contains all
    available kernel packages, you may want to use one of the
    unversioned <literal>pkgs.linuxPackages_*</literal> aliases such as
    <literal>pkgs.linuxPackages_latest</literal>, that are kept up to
    date with new versions.
  </para>
  <para>
    Please note that the current convention in NixOS is to only keep
    actively maintained kernel versions on both unstable and the
    currently supported stable release(s) of NixOS. This means that a
    non-longterm kernel will be removed after it’s abandoned by the
    kernel developers, even on stable NixOS versions. If you pin your
    kernel onto a non-longterm version, expect your evaluation to fail
    as soon as the version is out of maintenance.
  </para>
  <para>
    Longterm versions of kernels will be removed before the next stable
    NixOS that will exceed the maintenance period of the kernel version.
  </para>
  <para>
    The default Linux kernel configuration should be fine for most
    users. You can see the configuration of your current kernel with the
    following command:
  </para>
  <programlisting>
zcat /proc/config.gz
</programlisting>
  <para>
    If you want to change the kernel configuration, you can use the
    <literal>packageOverrides</literal> feature (see
    <xref linkend="sec-customising-packages" />). For instance, to
    enable support for the kernel debugger KGDB:
  </para>
  <programlisting language="bash">
nixpkgs.config.packageOverrides = pkgs: pkgs.lib.recursiveUpdate pkgs {
  linuxKernel.kernels.linux_5_10 = pkgs.linuxKernel.kernels.linux_5_10.override {
    extraConfig = ''
      KGDB y
    '';
  };
};
</programlisting>
  <para>
    <literal>extraConfig</literal> takes a list of Linux kernel
    configuration options, one per line. The name of the option should
    not include the prefix <literal>CONFIG_</literal>. The option value
    is typically <literal>y</literal>, <literal>n</literal> or
    <literal>m</literal> (to build something as a kernel module).
  </para>
  <para>
    Kernel modules for hardware devices are generally loaded
    automatically by <literal>udev</literal>. You can force a module to
    be loaded via <xref linkend="opt-boot.kernelModules" />, e.g.
  </para>
  <programlisting language="bash">
boot.kernelModules = [ &quot;fuse&quot; &quot;kvm-intel&quot; &quot;coretemp&quot; ];
</programlisting>
  <para>
    If the module is required early during the boot (e.g. to mount the
    root file system), you can use
    <xref linkend="opt-boot.initrd.kernelModules" />:
  </para>
  <programlisting language="bash">
boot.initrd.kernelModules = [ &quot;cifs&quot; ];
</programlisting>
  <para>
    This causes the specified modules and their dependencies to be added
    to the initial ramdisk.
  </para>
  <para>
    Kernel runtime parameters can be set through
    <xref linkend="opt-boot.kernel.sysctl" />, e.g.
  </para>
  <programlisting language="bash">
boot.kernel.sysctl.&quot;net.ipv4.tcp_keepalive_time&quot; = 120;
</programlisting>
  <para>
    sets the kernel’s TCP keepalive time to 120 seconds. To see the
    available parameters, run <literal>sysctl -a</literal>.
  </para>
  <section xml:id="sec-linux-config-customizing">
    <title>Customize your kernel</title>
    <para>
      The first step before compiling the kernel is to generate an
      appropriate <literal>.config</literal> configuration. Either you
      pass your own config via the <literal>configfile</literal> setting
      of <literal>linuxKernel.manualConfig</literal>:
    </para>
    <programlisting language="bash">
custom-kernel = let base_kernel = linuxKernel.kernels.linux_4_9;
  in super.linuxKernel.manualConfig {
    inherit (super) stdenv hostPlatform;
    inherit (base_kernel) src;
    version = &quot;${base_kernel.version}-custom&quot;;

    configfile = /home/me/my_kernel_config;
    allowImportFromDerivation = true;
};
</programlisting>
    <para>
      You can edit the config with this snippet (by default
      <literal>make menuconfig</literal> won't work out of the box on
      nixos):
    </para>
    <programlisting>
nix-shell -E 'with import &lt;nixpkgs&gt; {}; kernelToOverride.overrideAttrs (o: {nativeBuildInputs=o.nativeBuildInputs ++ [ pkg-config ncurses ];})'
</programlisting>
    <para>
      or you can let nixpkgs generate the configuration. Nixpkgs
      generates it via answering the interactive kernel utility
      <literal>make config</literal>. The answers depend on parameters
      passed to
      <literal>pkgs/os-specific/linux/kernel/generic.nix</literal>
      (which you can influence by overriding
      <literal>extraConfig, autoModules, modDirVersion, preferBuiltin, extraConfig</literal>).
    </para>
    <programlisting language="bash">
mptcp93.override ({
  name=&quot;mptcp-local&quot;;

  ignoreConfigErrors = true;
  autoModules = false;
  kernelPreferBuiltin = true;

  enableParallelBuilding = true;

  extraConfig = ''
    DEBUG_KERNEL y
    FRAME_POINTER y
    KGDB y
    KGDB_SERIAL_CONSOLE y
    DEBUG_INFO y
  '';
});
</programlisting>
  </section>
  <section xml:id="sec-linux-config-developing-modules">
    <title>Developing kernel modules</title>
    <para>
      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 <literal>mellanox</literal> drivers.
    </para>
    <programlisting>
$ nix-build '&lt;nixpkgs&gt;' -A linuxPackages.kernel.dev
$ nix-shell '&lt;nixpkgs&gt;' -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
</programlisting>
  </section>
  <section xml:id="sec-linux-zfs">
    <title>ZFS</title>
    <para>
      It’s a common issue that the latest stable version of ZFS doesn’t
      support the latest available Linux kernel. It is recommended to
      use the latest available LTS that’s compatible with ZFS. Usually
      this is the default kernel provided by nixpkgs (i.e.
      <literal>pkgs.linuxPackages</literal>).
    </para>
    <para>
      Alternatively, it’s possible to pin the system to the latest
      available kernel version <emphasis>that is supported by
      ZFS</emphasis> like this:
    </para>
    <programlisting language="bash">
{
  boot.kernelPackages = pkgs.zfs.latestCompatibleLinuxPackages;
}
</programlisting>
    <para>
      Please note that the version this attribute points to isn’t
      monotonic because the latest kernel version only refers to kernel
      versions supported by the Linux developers. In other words, the
      latest kernel version that ZFS is compatible with may decrease
      over time.
    </para>
    <para>
      An example: the latest version ZFS is compatible with is 5.19
      which is a non-longterm version. When 5.19 is out of maintenance,
      the latest supported kernel version is 5.15 because it’s longterm
      and the versions 5.16, 5.17 and 5.18 are already out of
      maintenance because they’re non-longterm.
    </para>
  </section>
</chapter>
