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# Linux kernel {#sec-linux-kernel}
The Nix expressions to build the Linux kernel are in [`pkgs/os-specific/linux/kernel`](https://github.com/NixOS/nixpkgs/blob/master/pkgs/os-specific/linux/kernel).
The function that builds the kernel has an argument `kernelPatches` which should be a list of `{name, patch, extraConfig}` attribute sets, where `name` is the name of the patch (which is included in the kernel’s `meta.description` attribute), `patch` is the patch itself (possibly compressed), and `extraConfig` (optional) is a string specifying extra options to be concatenated to the kernel configuration file (`.config`).
The kernel derivation exports an attribute `features` specifying whether optional functionality is or isn’t enabled. This is used in NixOS to implement kernel-specific behaviour. For instance, if the kernel has the `iwlwifi` feature (i.e. has built-in support for Intel wireless chipsets), then NixOS doesn’t have to build the external `iwlwifi` package:
```nix
modulesTree = [kernel]
++ pkgs.lib.optional (!kernel.features ? iwlwifi) kernelPackages.iwlwifi
++ ...;
```
How to add a new (major) version of the Linux kernel to Nixpkgs:
1. Copy the old Nix expression (e.g. `linux-2.6.21.nix`) to the new one (e.g. `linux-2.6.22.nix`) and update it.
2. Add the new kernel to `all-packages.nix` (e.g., create an attribute `kernel_2_6_22`).
3. Now we’re going to update the kernel configuration. First unpack the kernel. Then for each supported platform (`i686`, `x86_64`, `uml`) do the following:
1. Make an copy from the old config (e.g. `config-2.6.21-i686-smp`) to the new one (e.g. `config-2.6.22-i686-smp`).
2. Copy the config file for this platform (e.g. `config-2.6.22-i686-smp`) to `.config` in the kernel source tree.
3. Run `make oldconfig ARCH={i386,x86_64,um}` and answer all questions. (For the uml configuration, also add `SHELL=bash`.) Make sure to keep the configuration consistent between platforms (i.e. don’t enable some feature on `i686` and disable it on `x86_64`).
4. If needed you can also run `make menuconfig`:
```ShellSession
$ nix-env -i ncurses
$ export NIX_CFLAGS_LINK=-lncurses
$ make menuconfig ARCH=arch
```
5. Copy `.config` over the new config file (e.g. `config-2.6.22-i686-smp`).
4. Test building the kernel: `nix-build -A kernel_2_6_22`. If it compiles, ship it! For extra credit, try booting NixOS with it.
5. It may be that the new kernel requires updating the external kernel modules and kernel-dependent packages listed in the `linuxPackagesFor` function in `all-packages.nix` (such as the NVIDIA drivers, AUFS, etc.). If the updated packages aren’t backwards compatible with older kernels, you may need to keep the older versions around.
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