nixos: nixos/doc/manual/configuration/linux-kernel.xml to CommonMark

nixos/doc/manual/configuration/linux-kernel.chapter.md

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+# Linux Kernel {#sec-kernel-config}
+
+You can override the Linux kernel and associated packages using the
+option `boot.kernelPackages`. For instance, this selects the Linux 3.10
+kernel:
+
+```nix
+boot.kernelPackages = pkgs.linuxPackages_3_10;
+```
+
+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.
+
+The default Linux kernel configuration should be fine for most users.
+You can see the configuration of your current kernel with the following
+command:
+
+```ShellSession
+zcat /proc/config.gz
+```
+
+If you want to change the kernel configuration, you can use the
+`packageOverrides` feature (see [](#sec-customising-packages)). For
+instance, to enable support for the kernel debugger KGDB:
+
+```nix
+nixpkgs.config.packageOverrides = pkgs:
+  { linux_3_4 = pkgs.linux_3_4.override {
+      extraConfig =
+        ''
+          KGDB y
+        '';
+    };
+  };
+```
+
+`extraConfig` takes a list of Linux kernel configuration options, one
+per line. The name of the option should not include the prefix
+`CONFIG_`. The option value is typically `y`, `n` or `m` (to build
+something as a kernel module).
+
+Kernel modules for hardware devices are generally loaded automatically
+by `udev`. You can force a module to be loaded via
+[`boot.kernelModules`](options.html#opt-boot.kernelModules), e.g.
+
+```nix
+boot.kernelModules = [ "fuse" "kvm-intel" "coretemp" ];
+```
+
+If the module is required early during the boot (e.g. to mount the root
+file system), you can use [`boot.initrd.kernelModules`](options.html#opt-boot.initrd.kernelModules):
+
+```nix
+boot.initrd.kernelModules = [ "cifs" ];
+```
+
+This causes the specified modules and their dependencies to be added to
+the initial ramdisk.
+
+Kernel runtime parameters can be set through
+[`boot.kernel.sysctl`](options.html#opt-boot.kernel.sysctl), e.g.
+
+```nix
+boot.kernel.sysctl."net.ipv4.tcp_keepalive_time" = 120;
+```
+
+sets the kernel's TCP keepalive time to 120 seconds. To see the
+available parameters, run `sysctl -a`.
+
+## Customize your kernel {#sec-linux-config-customizing}
+
+The first step before compiling the kernel is to generate an appropriate
+`.config` configuration. Either you pass your own config via the
+`configfile` setting of `linuxManualConfig`:
+
+```nix
+custom-kernel = super.linuxManualConfig {
+  inherit (super) stdenv hostPlatform;
+  inherit (linux_4_9) src;
+  version = "${linux_4_9.version}-custom";
+
+  configfile = /home/me/my_kernel_config;
+  allowImportFromDerivation = true;
+};
+```
+
+You can edit the config with this snippet (by default `make
+   menuconfig` won\'t work out of the box on nixos):
+
+```ShellSession
+nix-shell -E 'with import <nixpkgs> {}; kernelToOverride.overrideAttrs (o: {nativeBuildInputs=o.nativeBuildInputs ++ [ pkg-config ncurses ];})'
+```
+
+or you can let nixpkgs generate the configuration. Nixpkgs generates it
+via answering the interactive kernel utility `make config`. The answers
+depend on parameters passed to
+`pkgs/os-specific/linux/kernel/generic.nix` (which you can influence by
+overriding `extraConfig, autoModules,
+   modDirVersion, preferBuiltin, extraConfig`).
+
+```nix
+mptcp93.override ({
+  name="mptcp-local";
+
+  ignoreConfigErrors = true;
+  autoModules = false;
+  kernelPreferBuiltin = true;
+
+  enableParallelBuilding = true;
+
+  extraConfig = ''
+    DEBUG_KERNEL y
+    FRAME_POINTER y
+    KGDB y
+    KGDB_SERIAL_CONSOLE y
+    DEBUG_INFO y
+  '';
+});
+```
+
+## Developing kernel modules {#sec-linux-config-developing-modules}
+
+When developing kernel modules it\'s often convenient to run
+edit-compile-run loop as quickly as possible. See below snippet as an
+example of developing `mellanox` drivers.
+
+```ShellSession
+$ 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
+```