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<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>
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 = [ "fuse" "kvm-intel" "coretemp" ];
</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 = [ "cifs" ];
</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."net.ipv4.tcp_keepalive_time" = 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 = "${base_kernel.version}-custom";
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 <nixpkgs> {}; 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="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
'';
});
</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 '<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
</programlisting>
</section>
</chapter>
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