Genkernel Quick Start Tutorial

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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.


[edit] Concepts


[edit] 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).

Note: If you want to use the System rescue CD sources provided by Funtoo sys-kernel/sysrescue-std-sources, the philosophy remains exactly the same.

[edit] First step emerging the required packages

The first step is to emerge:

  1. The Gentoo kernel sources: sys-kernel/gentoo-sources
  2. Genkernel itself: sys-kernel/genkernel

This is achieved by running the following:

# 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 to this directory:

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

[edit] 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


# 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 unziped content into 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 blank 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/ Cannot open '/var/tmp/genkernel/initramfs-'
make[1]: *** [usr/initramfs_data.cpio.lzma] Error 1

[edit] 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.