Configuration

The kernel is configured by Kconfig. The configuration is materialized into a .config file at the repository root which the build reads. Modules and built-ins, debug options, sanitizers, and architecture features are all controlled by Kconfig.

The configuration cycle

graph LR
    A[Kconfig files in tree] -->|read by| B[scripts/kconfig]
    B -->|writes| C[.config]
    C -->|materialized into| D[include/generated/autoconf.h, Kconfig include files]
    D -->|consumed during build| E[Kbuild]
    E --> F[vmlinux + modules]

Common targets

Target	What it does
make defconfig	Default config for the host arch
make ARCH=arm64 defconfig	Default config for arm64
make <name>_defconfig	Pick a arch/<arch>/configs/<name>_defconfig
make menuconfig	ncurses UI
make nconfig, xconfig, gconfig	Other UIs
make olddefconfig	Accept defaults for newly added options
make oldconfig	Interactive prompt for new options
make savedefconfig	Write a minimal defconfig that reproduces the current .config
make localmodconfig	Trim .config to only what your machine has loaded
make allyesconfig, allnoconfig, allmodconfig	Build everything / nothing / everything as modules
make randconfig	Random config (used by build farms)
make help	List all targets

The Kconfig program lives in scripts/kconfig/.

Kconfig file structure

Each directory has its own Kconfig file, included by parent Kconfigs via source. The hierarchy starts at the top-level Kconfig.

config CONFIG_FOO
    bool "User-friendly name"
    depends on BAR && !BAZ
    default y
    help
      Multi-line help text shown in menuconfig.

Types: bool, tristate (y/m/n), string, int, hex. Tristate options can be built into the kernel (y) or as modules (m).

Example defconfigs

File	Use
arch/x86/configs/x86_64_defconfig	A reasonable x86_64 config
arch/arm64/configs/defconfig	The big arm64 defconfig
arch/riscv/configs/defconfig	RISC-V
kernel/configs/*.config	Fragments to overlay (tiny.config, kvm_guest.config, rt.config, …)

Apply a fragment:

make x86_64_defconfig
./scripts/kconfig/merge_config.sh -m .config kernel/configs/kvm_guest.config
make olddefconfig

Important Kconfig groups (tip-of-the-iceberg)

The full menu is enormous. A few Kconfig "spheres" that come up often:

• Architecture — arch/<arch>/Kconfig and per-platform Kconfigs.
• Processor type and features — CPU family, NUMA, NR_CPUS.
• Memory model — sparsemem vs. flatmem; vmemmap; HIGHMEM (32-bit); page size on archs that allow it.
• Kernel hacking — sanitizers, lockdep, debug objects, KASAN, UBSAN.
• Security options — LSMs (CONFIG_LSM=...), seccomp, lockdown, randstruct.
• Networking — IPv6 hash, BPF, XDP, kTLS, MPTCP, netfilter modules.
• Device drivers — the bulk of any defconfig.
• General setup — preempt model, init system flavor, BPF, kallsyms, IKCONFIG.
• General architecture-dependent options — page size, addressable memory, CMA.
• Cryptographic API — every algorithm.

Build system

Kbuild is the recursive make engine driven by make from the top. See Subsystems → Scripts for the engine itself.

Common Make variables

Variable	Purpose
ARCH	Cross-build target architecture
CROSS_COMPILE	Toolchain prefix (e.g. aarch64-linux-gnu-)
LLVM=1	Use clang/lld toolchain
O=path	Out-of-tree build directory
KBUILD_OUTPUT	Same effect as O= (env var form)
CC=, LD=, AR=, STRIP=, OBJCOPY=	Override individual tools
KAFLAGS=, KCFLAGS=, KCPPFLAGS=	Extra flags
V=1	Verbose build (show every command)
W=1, W=2, W=3	Increasing warning level
C=1, C=2	Run sparse

Out-of-tree build

mkdir /tmp/build
make O=/tmp/build defconfig
make O=/tmp/build -j$(nproc)

This keeps the source tree clean.

Module signing

CONFIG_MODULE_SIG=y requires the kernel to verify modules. The build embeds the public key from CONFIG_SYSTEM_TRUSTED_KEYS= (defaulting to a generated signing_key.pem under the build directory). See Subsystems → Certs.

Compressed kernel image

Each architecture compresses vmlinux with one of gzip, bzip2, lzma, xz, lzo, lz4, zstd (selectable in General setup → Kernel compression mode). The compressed image is then wrapped in arch-specific glue to make bzImage / Image / vmlinuz.

Where to look

• Documentation/kbuild/ — Kconfig and Kbuild reference.
• Documentation/admin-guide/kernel-parameters.txt — boot parameters.
• Documentation/admin-guide/sysctl/ — /proc/sys knobs.

Related pages

• Subsystems → Scripts — Kbuild engine.
• Tooling — checkpatch, sparse.
• Subsystems → Certs — module signing.