o a configuration that can be used with the new kernel. This article shows how to make a new kernel from new kernel sources with converting the configuration file of the old kernel.
Make a backup of the current kernel configuration
It is wise to make a backup of the kernel configuration so that the previous configurations are not lost. After all, many users devote considerable time to figure out the best configuration for the system, and losing that information is definitely not wanted.
It is easy to make a backup of the current kernel configuration:
cp .config ~/kernel-config-`uname -r`
Provided that the symlink to the kernel sources has been set correctly, this copies the configuration of the currently used kernel to the home directory of root, renaming the configuration to kernel-config- followed by the version of the current running Linux kernel.
The symlink /usr/src/linux should always point to the directory that holds the sources of the kernel which currently runs. This can be done in one of three ways:
- Installing the kernel sources with
- Setting the link with eselect
- Manually updating the symbolic link
This will make the /usr/src/linux point to the newly installed kernel sources.
If necessary, it can still be modified later with one of the other two methods.
To set the symlink with eselect:
eselect kernel list
Available kernel symlink targets:  linux-3.14.14-gentoo *  linux-3.16.3-gentoo
This outputs the available kernel sources. The asterisk indicates the chosen sources.
To change the kernel sources, e.g. to the second entry, do:
eselect kernel set 2
To set the symbolic link manually:
ln -sf /usr/src/linux-3.16.3 /usr/src/linux
ls -l /usr/src/linux
lrwxrwxrwx 1 root root 19 Oct 4 10:21 /usr/src/linux -> linux-3.16.3-gentoo
Copy previous kernel configuration
The configuration of the old kernel needs to be copied to the new one. It can be found in several places:
- In the procfs filesystem, if the kernel option Enable access to .config through /proc/config.gz was activated in the present kernel:
zcat /proc/config.gz > /usr/src/linux/.config
- In the /boot directory, if the configuration was installed installed there:
cp /boot/config-3.14.14-gentoo /usr/src/linux/.config
- In the kernel directory of the currently-running kernel:
cp /usr/src/linux-3.14.14-gentoo/.config /usr/src/linux/
Configure the new kernel
To use the configuration of the old kernel with the new kernel, it needs to be converted. The conversion can be done by running either make oldconfig or make silentoldconfig.
make oldconfig gives many choices, make silentoldconfig does not. Use either, not both.
The following configuration is like the text based configuration with make config. For new configuration options, it gives a choice. For example:
The string (NEW) at the end of the line marks this option as new. Left to the string in square brackets are the possible answers: Yes, no, module or ? to show the help. The recommend answer is capitalized (here Y). The help explains the option or driver.
Unfortunately make oldconfig doesn't show - next to the help - a lot more information for each option, like the context, so that it is sometimes difficult to give the right answer. In this case the best way to go is to remember the option name and revise it afterwards through one of the graphical kernel configuration tools.
If interactivity is not wanted (no questions should be asked), then use make silentoldconfig:
When external kernel modules are installed (like nvidia or zfs), it may be necessary to run make modules_prepare as described below before bulding the kernel. Some modules cannot be installed or prepared before the kernel has been built.
Do not forget to reconfigure the bootloader to account for the new kernel filenames, and rebuild the initramfs if one is used as well.
For this step, follow the steps in the manual configuration article.
Reinstall external kernel modules
Any external kernel modules, such as binary kernel modules, need to be rebuilt for each new kernel. If the kernel hasn't been built yet, it has to first be prepared for the bu