Custom Kernel

This material is work-in-progress ... Do not follow instructions here until this notice is removed. (Last edited by Prabuanand on 31 Dec 2025.)

This process of building a custom configured kernel assumes you are running on Alpine Linux utilizing abuild & aports.

But why?

You want to build a custom kernel to enable experimental hardware or features or outdated hardware, to reduce bloat further, to tune the kernel to the hardware.

The lts kernel for most Alpine ARCHs uses defaults to balance throughput at the expense of some responsiveness, and support for many devices. You can tweak the kernel for desktop use and low latency and responsiveness.

You should disable modules to increase security. By default, Alpine will install modules but not disable most of them. Disabling modules will reduce an DMA attack but not eliminate it completely. If you have a newer processor with VT-d, you can mitigate as long as you:

Leave CONFIG_INTEL_IOMMU_DEFAULT_ON=y or pass intel_iommu=on as a kernel parameter and disable kernel logging so the attacker doesn't gain DMAR address information through dmesg.[1] Also remove references to the kernel version to calculate the IOMMU addresses.[2]

You may also want to harden your kernel by adding at least some of the config changes recommended by kernel-hardening-checker [3]

To increase the security of the boot process, if you have a TPM, you could set CONFIG_INTEL_TXT=y (Enable Intel(R) Trusted Execution Technology (Intel(R) TXT)) (which is not enabled in the hardened kernel by default), then you would need the SINIT module (provided only by Intel)[4], a possibly compiled TrustedGrub2[5], trousers[6], tboot[7]. These packages are not in aports and it is unknown if these tools work on musl. It's not recommended for Edge. Also, there would be trigger packages to generate hashes for the kernel and the mkinitfs updates.

Setting up the Alpine Build System

First, you need to follow the steps in Setup your system and account for building packages. You also need to configure your /etc/apk/repositories so that they search locally for your apks. See Testing the package locally for details.

After setting up accounts and repos, change your shell's current working directory to aports that you just cloned.

$ cd aports

Working with aports

We will try using an existing lts kernel just tweaking the lts.ARCH.config file.

Switching to the proper release version

You need to switch to the proper branch that matches the release so that the kernel compiles against the dependencies properly.

Alpine version	Remote branch
Edge	master
3.23	3.23-stable

The following is required to get access to the APKBUILD released for that version of Alpine and which you will create a commit for.

If you are on 3.23 do:

$ git checkout -b 3.23-stable origin/3.23-stable

If you are on Edge do:

$ git checkout master

Creating your config

You can use linux-lts but what you should do is create a local branch by doing:

For linux-lts kernel on Alpine 3.23:

$ git checkout -b my-custom-kernel origin/3.23-stable

For Alpine Edge:

$ git checkout -b my-custom-kernel

Doing it this way, you do less work in maintaining. All you need to do is keep master or 3.23-stable in sync[8][9] and merge any conflicts.

First switch to the branch by doing:

$ git checkout my-custom-kernel

Then, you need to navigate to the main/linux-lts folder where you should see a APKBUILD and some config- files.

It is a good idea to use custom `FLAVOR` in order to change the name of your kernel and package (custom flavor or revision) so you newly build kernel will not override the existing linux-lts and it's modules. Flavor build will only use your config and omit files that are not for your cpu architectures or virt* configs.(Virt configs provide optimized kernel configurations for running Alpine as a guest in virtual machines.) This will also speed up the process of applying kernel patches by applying it for your architecture only. (Obviously ARCH in the following example is whatever architecture x86, x86_64...you use.)

First make a copy of your ARCH config file:

cp lts.x86_64.config lts-my_custom.x86_64.config

Edit your custom config file and add your changes. When you are done with your edits you need to run checksum to update APKBUILD:

FLAVOR=lts-my_custom abuild checksum

Then commit your changes:

git commit -a -m "Enabled these options ...."

You do this so that git can keep your code separate from Alpine's and so your changes float forward between kernel updates.

Adding custom patches

Custom patches should be added to sources=.

After you added the URL, you need to produce a checksum by doing abuild checksum.

The custom patches may not be autopatched, due to being distributed as an archive or different patch level, so you need to define what to do with it in the prepare().

Configuring kernel

There are multiple ways to prepare and change kernel config. The basic one is to just edit your custom config file. You can also attempt to create a config tailored for your currently running system and modules.

To create a kernel config based on your currently running PC, use:

make localmodconfig

This command reads your current kernel's configuration from /proc/config.gz (if available) or /boot/config-* and creates a .config file that includes only the modules currently loaded on your system. This results in a minimal configuration tailored to your hardware. Alternatively, you can use:

make localyesconfig

This is similar to localmodconfig, but instead of building features as modules (m), it builds them directly into the kernel (y), which can result in a larger kernel image but eliminates the need for separate module files. Note: Before running either command, make sure you have all the hardware you want to support actively in use (USB devices plugged in, network cards active, etc.) so their modules are loaded and included in the configuration.

The alternative is to use the kernel configuration menu in the build-NAME folder, but before you do that you need to sudo apk add ncurses-dev

After you are done using the menu in the build-NAME folder by doing:

make menuconfig

You want to remove ncurses-dev. When you are done, it will be stored in .config which you need to again override the lts.ARCH.config file.

When you are done updating the config-NAME.ARCH, you need to update sums:

abuild checksum

The options in the kernel config are typically defaults. If your device is old, it may be set to n by default.

Vanilla targets and tuning

ARCH	Processor Type / CPU Selection / System Type	Code Generation / Instruction Extensions	Timer Frequency	Preemption Model	Bitness
s390x	IBM zEnterprise 114 and 196	IBM zBC12 and zEC12 (-march=zEC12 -mtune=zEC12)	1000 Hz	No Forced Preemption (Server)	64
ppc64le	Server processors	POWER8 (-mcpu=power8), AltiVec (-Wa,-maltivec to assembler or -maltivec -mabi=altivec), VSX	1000 HZ	No Forced Preemption (Server)	64
ppc	512x/52xx/6xx/7xx/74xx/82xx/83xx/86xx Apple PowerMac based machines	AltiVec (-Wa,-maltivec to assembler or -maltivec -mabi=altivec) on >=74xx	1000 HZ	No Forced Preemption (Server)	32
x86_64	Generic-x86-64	(-mtune=generic ; SIMD assembly modules enabled based on simple compile test and/or presence of CPU flag)	1000 HZ	Voluntary Kernel Preemption (Desktop)	64
x86	586/K5/5x86/6x86/6x86MX	(-mtune=generic ; SIMD assembly modules enabled based on simple compile test and/or presence of CPU flag)	1000 HZ	Voluntary Kernel Preemption (Desktop)	32
armv7	ARMv7 based platforms (Cortex-A, PJ4, Scorpion, Krait) Freescale i.MX family -- Cortex A (i.MX51, i.MX53, i.MX6 Quad/DualLite, i.MX6 SoloLite, i.MX6 SoloX, i.MX6 UltraLite, i.MX7 Dual) Qualcomm -- (MSM8X60, MSM8960, MSM8974) Allwinner SoCs -- (A10 (sun4i), A10s / A13 (sun5i), A31 (sun6i), A20 (sun7i), sun8i Family, (sun9i)) ARM Ldt Versatile Express family --	Either -march=armv7-a or -march=armv5t -Wa,-march=armv7-a based on a compile test. -mfpu=vfp	1000 Hz	Voluntary Kernel Preemption (Desktop)	32
aarch64	Allwinner sunxi 64-bit SoC Family Broadcom BCM2835 family Marvell Berlin SoC Family ARMv8 based Samsung Exynos SoC family ARMv8 based Freescale Layerscape SoC family Hisilicon SoC Family Mediatek MT65xx & MT81xx ARMv8 SoC Marvell EBU SoC Family Qualcomm Platforms Rockchip Platforms AMD Seattle SoC Family Altera's Stratix 10 SoCFPGA Family NVIDIA Tegra SoC Family Spreadtrum SoC platform Cavium Inc. Thunder SoC Family ARMv8 software model (Versatile Express) AppliedMicro X-Gene SOC Family Xilinx ZynqMP Family		1000 HZ	Voluntary Kernel Preemption (Desktop)	64

If you do desktop multitasking, you may want to switch to Voluntary Kernel Preemption (Desktop) or Preemptible Kernel (Low-Latency Desktop) and up the Timer Frequency. If you run a dedicated render farm node or a dedicated bitcoin miner use No Forced Preemption (Server) and decrease the Timer Frequency.

Optimized modules (most are already compiled as modules):

• raid6 -- altivec, avx512, ssse3, avx2, mmx, sse, sse2, neon
• some operations of raid5 -- mmx (32 bit), sse (64 bit), avx

For Kernel API:

• 32-bit memcpy -- 3dnow
• 32-bit memory page clearing and copying -- sse (Athlon/K7 only), mmx

From x86/crypto, arm/crypto, powerpc/crypto:

• CAMELLIA -- avx2, avx, aes-ni
• CHACHA20 -- avx2, neon
• CAST5 -- avx
• CAST6 -- avx
• TWOFISH -- avx
• SERPENT -- avx2, avx, sse2
• SHA1 -- avx2, ssse3, neon, spe
• SHA2 -- avx2
• SHA256 -- ssse3, neon, spe
• SHA512 -- avx2, ssse3, neon
• POLY1305 -- avx2
• GHASH -- pclmulqdq (part of aes-ni), vmx (power8)
• AES -- aes-ni, neon, vmx (power8), spe
• CRC32 -- pclmulqdq, sse, neon, vmx (power8)
• CRCT10DIF -- pclmulqdq, sse, neon, vmx (power8)

Fast reboots with kexec

Main article: kexec

If you want to reboot the kernel fast avoiding the POST test, you need doas apk add kexec-tools and enable kexec in the kernel:

 Processor type and features
   [*] kexec system call

Hibernation to prevent data loss

 Power management and ACPI options
   [*] Hibernation (aka 'suspend to disk')

Hibernation should be used if you have a laptop. You don't want the laptop to suddenly shut off resulting in data loss, you want it to save your work based on a percentage of battery life (this requires special script). When hibernation resumes, should lock and ask for credentials. Depending on your needs, the hibernated image can be encrypted/decrypted which again requires additional customization to scripts.

Hibernation with an unsanitized swap file is generally insecure because data and unlocked memory pages are swapped out in plaintext. To increase the security either disable swap or use an encrypted swap. The swap file/partition is typically used as the hibernation resume image.

Building

Before building, make sure you have ccache installed. This should reduce compile time on multiple builds. You may also want to read up on compile flags - perhaps you want your build to be optimized for speed (or size which is the default). Review /etc/abuild.conf and set:

PACKAGER_PRIVKEY="/home/MY_USER_HERE/.abuild/my-email@mydomain.com-ID.rsa"
USE_CCACHE=1

Please note that kernel build process may not use some of the settings you set in `abuild.conf` so in order to customize compiler or linker and/or it's flags you may need to edit APKBUILD. If you have your config ready, first try building with:

FLAVOR=lts-my_custom abuild -rK 2>&1 | tee build1.log

This will install most of the dependencies and keep buildtime temp dirs and files (srcdir/pkgdir/deps).

If it complains about a dependency like elfutils-dev use -rKd. Then, when it prompts for values for new found config options just hold enter till it starts compiling the kernel. Unless you use FLAVOR or removed or commented out virt.*.config configs in APKBUILD there should be two sets one for -lts and the other for the -virt. Just Ctrl+C out of the compilation process after the second set so you can further customize the config. Then you go into the src/linux-VER and edit the config file. Copy the .config file overriding the lts.ARCH.config in the srcdir.

Installing

If the build was successful the kernel packages are located in ~/packages/main/ARCH You probably already know how to install a package in your Alpine Linux...

Testing

In case your new kernel may be missing a module and can't boot it is generally a good idea to keep the default linux-lts, so make sure you have it installed using the command:

# apk add linux-lts

Normally during the installation using with apk, there are tools that will update initramfs and a boot loader automatically for you. For easier debugging you may want to change boot loader config and remove quiet from linux command line in /etc/default/grub or /etc/update-extlinux.conf. Or perhaps you want to change other kernel options in /etc/mkinitfs/mkinitfs.conf. Please remember to generate initramfs and update your bootloader manually, eg:

# mkinitfs # grub-mkconfig -o /boot/grub/grub.cfg

In case something goes wrong with a boot process it is also a good idea to have a bootable rescue Alpine USB ready.

Once you have the default lts kernel and rescue USB reboot the computer.

If you are curious about correctness testing, some kernel modules or components do preform self tests at the beginning of the boot process. The tools may have test suites that you run with the make command.

See Also

• Kernels
• Archwiki Kernels
• Gentoo Wiki Kernel
• Gentoo Wiki Kernel Configuration
• How to build the Alpine Linux kernel
• Kernel Live Patching (KLP)