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This article describes the steps to upgrade to a new kernel.

Installing and using a new kernel

A kernel upgrade may be a good idea when new kernel sources are installed. New kernel sources are sometimes installed while updating the system by running the following command:

root #emerge --ask --update --deep --with-bdeps=y --newuse @world

Of course, they can be installed directly using the next command (replace gentoo-sources with hardened-sources when using a hardened profile):

root #emerge --ask --update --deep --with-bdeps=y --newuse sys-kernel/gentoo-sources

Installing new kernel sources doesn't provide the user with a new kernel. It is necessary to make and install a new kernel from the new sources and then reboot the system to actually run the new kernel.

Making a new kernel from the new sources is basically the same process as making a kernel when installing the system. The difference is that one can use the configuration of the old kernel to create a configuration for the new kernel. Using the old configuration saves the user from going through all the kernel options (like make menuconfig) again.

The configuration of the kernel is saved in a file named .config in the directory that holds the kernel sources. A new kernel may have options or features the old kernel does not have, or it might not have a feature or option anymore which the old kernel still has. The kernel configuration specifies whether the features and options of a kernel are to be enabled or not, perhaps built into the kernel, or perhaps built as modules which can be loaded into the running kernel on demand. Hence the configuration file of the new kernel may have new entries the configuration file of the old kernel doesn't have, and it might not have entries anymore which are present in the configuration file of the old kernel.

To deal with such changes of the configuration file, the configuration file of the old kernel needs to be converted to 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:

root #cd /usr/src/linux
root #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.

Set symlink to new kernel sources

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:

  1. Installing the kernel sources with USE="symlink"
  2. Setting the link with eselect
  3. Manually updating the symbolic link

Installing the kernel sources with the symlink USE flag

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.

Setting the link with eselect

To set the symlink with eselect:

root #eselect kernel list
Available kernel symlink targets:
 [1] linux-3.14.14-gentoo *
 [2] 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:

root #eselect kernel set 2

Manually updating the symbolic link

To set the symbolic link manually:

root #ln -sf /usr/src/linux-3.16.3 /usr/src/linux
root #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:
root #zcat /proc/config.gz > /usr/src/linux/.config
  • From the old kernel. This will only work when the old kernel was compiled with CONFIG_IKCONFIG:
root #/usr/src/linux/scripts/extract-ikconfig /path/to/old/kernel >/usr/src/linux/.config
  • In the /boot directory, if the configuration was installed there:
root #cp /boot/config-3.14.14-gentoo /usr/src/linux/.config
  • In the kernel directory of the currently-running kernel:
root #cp /usr/src/linux-3.14.14-gentoo/.config /usr/src/linux/
  • In the /etc/kernels/ directory, if SAVE_CONFIG="yes" is set in /etc/genkernel.conf and genkernel was previously used:
root #cp /etc/kernels/kernel-config-x86_64-3.14.14-gentoo /usr/src/linux/.config

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 silentoldconfig or make olddefconfig. Use either, not both.

make silentoldconfig

The following configuration is like the text based configuration with make config. For new configuration options, the user is asked for a decision. For example:

root #cd /usr/src/linux
root #make silentoldconfig
Anticipatory I/O scheduler (IOSCHED_AS) [Y/n/m/?] (NEW)

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 (i.e. default) answer is capitalized (here Y). The help explains the option or driver.

Unfortunately make silentoldconfig doesn't show a lot more information for each option, such as the context, so 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.

make olddefconfig

If all new configuration options should be set to their recommended (i.e. default) values use make olddefconfig:

root #cd /usr/src/linux
root #make olddefconfig

make help

Use make help to see other conversion methods available:

root #make help


When external kernel modules are installed (like nvidia or zfs), it may be necessary to run make modules_prepare as described below before building 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.

Automated build and installation

It is possible to automatically build and install the newly emerged kernel using portage hooks. While other approaches are also possible, the following is based on genkernel, gentoo-sources and requires the following prerequisites:

  1. genkernel all is able to build and install the kernel to which the /usr/src/linux symlink points into $BOOTDIR and your bootloader.
  2. the symlink use flag is set for the kernel ebuild

If those are fulfilled, simply install a post_pkg_postinst portage hook as shown below.

FILE /etc/portage/env/sys-kernel/gentoo-sourcesAutomated kernel build and installation portage hook
post_pkg_postinst() {
	CURRENT_KV=$(uname -r)

	# This assumes that the kernel ebuild was emerged with the
	# 'symlink' use flag set, so that /usr/src/linux points to the
	# newly emerged kernel.
	cp "${EROOT}/usr/src/linux-${CURRENT_KV}/.config" "${EROOT}/usr/src/linux"

	pushd .
	cd "${EROOT}/usr/src/linux"
	make olddefconfig

	genkernel all

Reinstalling external kernel modules

Any external kernel modules, such as binary kernel modules, need to be rebuilt for each new kernel. If the kernel has not been built yet, it has to first be prepared for the building of the external kernel modules:

root #make modules_prepare

Packages containing kernel modules can be rebuilt using the @module-rebuild set:

root #emerge --ask @module-rebuild

Solving build problems

When experiencing build problems while rebuilding the current kernel, it might help to sanitize the kernel sources. Make sure to backup the .config file first, as the operation will remove it. Make sure not to use a .bak or ~ suffix as backup as make distclean will clean those up as well.

root #cp .config /usr/src/kernel_config_bk
root #make distclean
root #mv /usr/src/kernel_config_bk .config

Removing old kernels

See the kernel removal article.

External resources