Custom Kernel

This material is work-in-progress ... Do not follow instructions here until this notice is removed. (Last edited by Orson Teodoro on 16 Mar 2018.)

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You want to build a custom kernel to enable experimental hardware or features or outdated hardware, to reduce bloat further, to tune the kernel to the hardware.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

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.

Working with aports

There are several ways to maintain a kernel. The first option is to create a new kernel package. The other option is to just use the existing vanilla kernel just tweaking the configure-vanilla.ARCH 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
3.7.0	3.7-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 3.7 do:

 git checkout -b 3.7-stable origin/3.7-stable

If you are on Edge do:

 git checkout master

Option B: Vanilla with native settings and minimal edits

Most users will want to use this option.

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

For Alpine Edge:

 git checkout -b my-custom-kernel

For Alpine 3.7:

 git checkout -b my-custom-kernel origin/3.7-stable

Doing it this way, you do less work in maintaining. All you need to do is keep master or 3.7-stable in sync[1][2] 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-vanilla folder where you should see a APKBUILD and some config- files. When you are done with your edits either by editing directly the APKBUILD and copyting the config-vanilla.ARCH as .config in the linux-4.15 folder. You will then move the .config back overriding the config-vanilla.ARCH generated by make menuconfig (discussed below in the Configuring kernel section). After generating your config, you need to abuild checksum. Then, do git add APKBUILD config-vanilla.ARCH where ARCH is whatever architecture (x86, x86_64, ...) you use. Then, you need to do git commit APKBUILD config-NAME.ARCH -m "Enabled these options ...." for your custom _flavor and the ARCHitecture of your system. 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

Attempt to build the kernel first. To do that, you do abuild -rK to install most of the dependencies. 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. There should be two sets one for -vanilla 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 config-NAME.ARCH in the srcdir.

The alternative is to use the kernel configuration menu in the build-NAME folder, but before yo 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 config-NAME.ARCH file. When you are done updating the config-NAME.ARCH, you need to do 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)	100 Hz	No Forced Preemption (Server)	64
ppc64le	Server processors	POWER8 (-mcpu=power8), AltiVec (-Wa,-maltivec to assembler or -maltivec -mabi=altivec), VSX	100 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	250 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)	300 HZ	Voluntary Kernel Preemption (Desktop)	32
x86	586/K5/5x86/6x86/6x86MX	(-mtune=generic ; SIMD assembly modules enabled based on simple compile test and/or presence of CPU flag)	300 HZ	Voluntary Kernel Preemption (Desktop)	32
armhf	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	100 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		300 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

If you want to reboot the kernel fast avoiding the POST test, you need sudo 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 you do hibernation and when it restores back, it should lock down the computer and ask for prompt. 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 is swapped out in plaintext. To increase the security either disable swap (Alpine default) or use an encrypted swap. The swap file/partition is typically used as a dump of the hibernated image.

Building

You should then do an abuild -r to attempt to build it.

Installing

To install it you do a sudo apk add linux-NAME where NAME is your custom kernel release name or vanilla if you used option B.

Bootloader

You need to configure your bootloader to use the kernel. Add a new entry but do not replace the old kernel. The old kernel is your way back if the kernel config was a bad one. The naming scheme should be similar but with the tag. You should make sure that the _flavor name is attached for new kernel packages and do not override the existing vanilla kernel and the existing vanilla initramfs.

For Grub, in your /boot/grub/grub.cfg if mounted should contain the new entry something like

 menuentry 'Alpine Linux (ck1)' {
         set root=(hd0,7)
         linux /vmlinuz-ck1 root=/dev/sda17 rw modules=sd-mod,usb-storage,ext4
         initrd /initramfs-ck1
 }

In the above entry hd0,7 (in one based indexing) is associated with the boot partition /dev/sda7. /dev/sda17 should point to your userland partition containing your Alpine system files (/usr/bin, /bin, ...).

If you haven't yet installed Grub, to install the bootloader with grub, you do something like grub-install --force /dev/sda to install it at your MBR or grub-install --force /dev/sda7 where /dev/sda7 is your bootable drive found with fdisk -l depending on how you set up grub.

Testing

To test, first you should make a bootable Alpine USB image. Then, when you have your rescue USB done, you sudo reboot the computer.

To test it, you basically do trial and error. Sometimes your config is missing something if you want to have a bare minimum setting.