Revision as of 02:46, 24 May 2011

Genkernel is a tool which help you to build a Linux kernel and deploy it along with a ramdisk image which contains all of the necessary modules to make the kernel being able to mount the different partitions of your harddrives in the VFS at the system startup.

Concepts

TBC

Business case #1: Genkernel applied to the Debian sources

A good candidate to quickly start with Genkernel is the Debian kernel (sys-kernel/debian-sources) mainly because:

1. Configuration files are available for the various platforms supported by the Debian project
2. The configuration makes an intensive use of putting features in modules

First step: emerging the required packages

The first step is to emerge:

1. The Debian sources
2. Genkernel itself

This is achieved with:

# emerge sys-kernel/debian-sources sys-kernel/genkernel

Once the Debian kernel sources are deployed, you should find a directory named linux-debian-version (e.g. linux-debian-2.6.32.30) under /usr/src. Update your the linux symlink to point on this directory:

# cd /usr/src
# rm linux
# ln -s linux-debian-2.6.32.30 linux

Alternatively, emerge the debian-sources with USE="symlink"

Second step: Grabbing a configuration file

If is now time to download the kernel configuration file. For this tutorial we will use a configuration file for AMD64 (several others architectures like MIPS or SPARC64 are available) as published by the Debian project, if you plan to use the Debian kernel you are advised to review it adjust it to your needs although it is tailored for a vast range of machines.

# cd linux
# wget http://merkel.debian.org/~jurij/2.6.32-30/amd64/config-2.6.32-5-amd64.gz
# gunzip config-2.6.32-5-amd64.gz

Alternatively, you can use Funtoo's config-extract tool to generate an official Debian kernel configuration. Use it as follows:

# cd linux
# ./config-extract amd64
Wrote amd64_none_amd64 kernel configuration to /usr/src/linux-debian-2.6.32.30/.config.

config-extract also allows you to extract special Debian featuresets, such as settings for Xen and OpenVZ kernels:

# ./config-extract amd64 openvz
Wrote amd64_openvz_amd64 kernel configuration to /usr/src/linux-debian-2.6.32.30/.config.

To see a full list of available Debian kernel configurations, type ./config-extract --list from within the root kernel source tree.

After using config-extract, run make oldconfig and accept all default options by hitting Enter at all prompts.

Third step: Building and installing the kernel

This is simply achieved by:

# genkernel --kernel-config=config-2.6.32-5-amd64 all

• --kernel-config: use the given name located in the kernel source tree (/usr/src/linux by default is taken unless overridden by --kerndir)
• all: rebuild the kernel image and the initramfs ramdisk image (aside of kernel modules, the ramdisk image contains tools such as BusyBox and some generic startup scripts, depending on options you use on the command line several additional tools like lvm or raid volume management can be incorporated as well).

If you use Genkernel to rebuild a Linux kernel on SPARC64, remember to either:

• Set sparc64-unknown-linux-gnu- in General setup --> Cross-compiler tool prefix
• Put --kernel-cross-compile=sparc64-unknown-linux-gnu- on the Genkernel command line

One the kernel has been compiled and the ram disk has been generated, the kernel image plus its companion files (initramfs image and System.map) are placed in the /boot directory, You can use your favourite tool to update your bootloader configuration files.

Business case #2: Recompiling the Gentoo kernel sources from a SystemRescue CD chroot

A second case that many Funtoo users will face to is to compile their own kernel when installing a brand new Funtoo instance from a stage 3 archive (the most common scenario is to boot the machine with SystemRescue CD).

First step emerging the required packages

The first step is to emerge:

1. The Gentoo kernel sources
2. Genkernel itself

This is achieved with:

# emerge sys-kernel/gentoo-sources sys-kernel/genkernel

Once the Gentoo kernel sources are deployed, you should find a directory named linux-version-gentoo (e.g. linux-2.6.39-gentoo) under /usr/src. Update the linux symlink to point on this directory:

# cd /usr/src
# rm linux
# ln -s linux-2.6.39-gentoo linux

Second step: Grabbing and tweaking a configuration file

How to start your kernel configuration? Simply by using the same configuration template the running System Rescue CD kernel had been built with! Before chrooting in your Funtoo instance, you did something like:

# mount -o bind /proc /mnt/gentoo/proc

Or:

# mount -t proc none /mnt/gentoo/proc

In your chroot environment (or from a System Rescue CD virtual terminal) if you look what /proc contains you will notice a file named config.gz:

# ls /proc
...
dr-xr-xr-x  7 root       root                     0 May 23 03:13 952
dr-xr-xr-x  7 root       root                     0 May 23 03:13 953
dr-xr-xr-x  7 root       root                     0 May 23 18:42 9834
...
-r--r--r--  1 root       root                 16024 May 23 22:27 config.gz
-r--r--r--  1 root       root                     0 May 23 22:27 consoles
-r--r--r--  1 root       root                     0 May 23 22:27 cpuinfo
...

config.gz holds the running kernel (System Rescue CD) configuration, just copy the gunziped content in the Gentoo sources directory:

# cd /usr/src/linux
# zcat /proc/config.gz > .config

Next, run make oldconfig to set all newly added options:

# make oldconfig

Next, tweak the kernel configuration in the way you prefer (manually edition of the .config file, make nconfig, make menuconfig....) if you wish. You are not ready yet! A final step is required: you must either set CONFIG_INITRAMFS_SOURCE to a black value (CONFIG_INITRAMFS_SOURCE="") either delete the statement in the .config file. Forgotting to do that will make Genkernel abort the compilation process with a message like:

/usr/src/linux-2.6.39-gentoo/scripts/gen_initramfs_list.sh: Cannot open '/var/tmp/genkernel/initramfs-2.6.32.14-std155-i386.cpio.gz'
make[1]: *** [usr/initramfs_data.cpio.lzma] Error 1

Third step: Building and installing the kernel

This is simply achieved by:

# genkernel --no-mrproper all

The same remarks written in the third paragraph of the first use case are still valid here.