<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>
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 <option>boot.kernelModules</option>, e.g.
<programlisting>
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
<option>boot.initrd.extraKernelModules</option>:
<programlisting>
boot.initrd.extraKernelModules = [ "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
<option>boot.kernel.sysctl</option>, e.g.
<programlisting>
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>
  <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><![CDATA[
$ 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
]]></screen>

</section>

</chapter>
