<chapter xmlns="http://docbook.org/ns/docbook"
         xmlns:xlink="http://www.w3.org/1999/xlink"
         xmlns:xi="http://www.w3.org/2001/XInclude"
         version="5.0"
         xml:id="sec-kernel-config">
 <title>Linux Kernel</title>
 <para>
  You can override the Linux kernel and associated packages using the option
  <option>boot.kernelPackages</option>. For instance, this selects the Linux
  3.10 kernel:
<programlisting>
<xref linkend="opt-boot.kernelPackages"/> = pkgs.linuxPackages_3_10;
</programlisting>
  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>
  The default Linux kernel configuration should be fine for most users. You can
  see the configuration of your current kernel with the following command:
<programlisting>
zcat /proc/config.gz
</programlisting>
  If you want to change the kernel configuration, you can use the
  <option>packageOverrides</option> feature (see
  <xref
linkend="sec-customising-packages" />). For instance, to enable support
  for the kernel debugger KGDB:
<programlisting>
nixpkgs.config.packageOverrides = pkgs:
  { linux_3_4 = pkgs.linux_3_4.override {
      extraConfig =
        ''
          KGDB y
        '';
    };
  };
</programlisting>
  <varname>extraConfig</varname> 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
  <command>udev</command>. You can force a module to be loaded via
  <xref linkend="opt-boot.kernelModules"/>, e.g.
<programlisting>
<xref linkend="opt-boot.kernelModules"/> = [ "fuse" "kvm-intel" "coretemp" ];
</programlisting>
  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"/>:
<programlisting>
<xref linkend="opt-boot.initrd.kernelModules"/> = [ "cifs" ];
</programlisting>
  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.
<programlisting>
<xref linkend="opt-boot.kernel.sysctl"/>."net.ipv4.tcp_keepalive_time" = 120;
</programlisting>
  sets the kernel’s TCP keepalive time to 120 seconds. To see the available
  parameters, run <command>sysctl -a</command>.
 </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>linuxManualConfig</literal>:
<screen><![CDATA[
  custom-kernel = super.linuxManualConfig {
    inherit (super) stdenv hostPlatform;
    inherit (linux_4_9) src;
    version = "${linux_4_9.version}-custom";

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

  mptcp93.override ({
      name="mptcp-local";

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

      enableParallelBuilding = true;

      extraConfig = ''
        DEBUG_KERNEL y
        FRAME_POINTER y
        KGDB y
        KGDB_SERIAL_CONSOLE y
        DEBUG_INFO y
      '';
    });
  ]]></screen>
  </para>
 </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>

<screen>
<prompt>$ </prompt>nix-build '&lt;nixpkgs>' -A linuxPackages.kernel.dev
<prompt>$ </prompt>nix-shell '&lt;nixpkgs>' -A linuxPackages.kernel
<prompt>$ </prompt>unpackPhase
<prompt>$ </prompt>cd linux-*
<prompt>$ </prompt>make -C $dev/lib/modules/*/build M=$(pwd)/drivers/net/ethernet/mellanox modules
<prompt># </prompt>insmod ./drivers/net/ethernet/mellanox/mlx5/core/mlx5_core.ko
</screen>
 </section>
</chapter>
