Kconfig start configuration

If we just cloned the kernel source tree and we want to start tweaking the kernel for our system… which Kconfig configuration we should choose?

1. Default Configuration

By default Kconfig will look for kernel .config configuration files in your system:

scripts/kconfig/Makefile:

export KCONFIG_DEFCONFIG_LIST :=
ifndef cross_compiling
kernel-release := $(shell uname -r)
KCONFIG_DEFCONFIG_LIST += \
	/lib/modules/$(kernel-release)/.config \
	/etc/kernel-config \
	/boot/config-$(kernel-release)
endif
KCONFIG_DEFCONFIG_LIST += arch/$(SRCARCH)/configs/$(KBUILD_DEFCONFIG)

/lib/modules/$(uname -r)/.config
/etc/kernel-config
/boot/config-$(uname -r)

And if not found, it will load the default configuration for your architecture, found in

arch/$(SRCARCH)/configs/$(KBUILD_DEFCONFIG):

[xubuntu@vm ../x86/configs/]$ pwd
/home/xubuntu/linux-6.1.14/arch/x86/configs
[xubuntu@vm ../x86/configs/]$ ls
i386_defconfig  tiny.config  x86_64_defconfig  xen.config

Default, xen, tiny, etc.

For example, tiny (literally only this):

CONFIG_NOHIGHMEM=y
# CONFIG_HIGHMEM4G is not set
# CONFIG_HIGHMEM64G is not set
CONFIG_UNWINDER_GUESS=y
# CONFIG_UNWINDER_FRAME_POINTER is not set

While x86-64 is 279 lines long:

CONFIG_WERROR=y
CONFIG_SYSVIPC=y
CONFIG_POSIX_MQUEUE=y
CONFIG_AUDIT=y
CONFIG_NO_HZ=y
CONFIG_HIGH_RES_TIMERS=y
[..]
CONFIG_SCHEDSTATS=y
CONFIG_BLK_DEV_IO_TRACE=y
CONFIG_PROVIDE_OHCI1394_DMA_INIT=y
CONFIG_EARLY_PRINTK_DBGP=y
CONFIG_DEBUG_BOOT_PARAMS=y

2. localmodconfig approach

Another configuration approach is to get a list of current loaded kernel modules via lsmod and then passing them to kconfig so it configures the base kernel plus the functionality provided by these modules (disabling not loaded modules):

[xubuntu@vm ~/linux-6.1.14/]$ lsmod > lsmod
[xubuntu@vm ~/linux-6.1.14/]$ make LSMOD=lsmod localmodconfig
[xubuntu@vm ~/linux-6.1.14/]$ make LSMOD=lsmod localmodconfig
using config: '/proc/config.gz'
glue_helper config not found!!
module vboxsf did not have configs CONFIG_VBOXSF_FS
.config:5312:warning: symbol value '1' invalid for KASAN_STACK

Look how it’s understanding the module vboxsf for example.

Another thing to mention is that configurations that are believed to be new from the previous configuration are still asked:

Compile also drivers which will not load (COMPILE_TEST) [N/y/?] n
Compile the kernel with warnings as errors (WERROR) [N/y/?] (NEW)
Local version - append to kernel release (LOCALVERSION) [-llkd01] -llkd01

See the NEW mark appearing.