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Compiling and installing the Gentoo Linux kernel on emerge without genkernel (part 2)


The first install of a Gentoo kernel needs to be somewhat manual if you want to optimize the kernel for the (virtual) system it boots on.

In part 1 I laid out how to improve the subsequent emerges of sys-kernel/gentoo-sources with a small drop in script to build the kernel as part of the ebuild.

Since end of last year Gentoo also supports a less manual way of emerging a kernel:

The following kernel blends are available:

  • sys-kernel/gentoo-kernel (the Gentoo kernel you can configure and compile locally - typically this is what you want if you run Gentoo)
  • sys-kernel/gentoo-kernel-bin (a pre-compiled Gentoo kernel similar to what genkernel would get you)
  • sys-kernel/vanilla-kernel (the upstream Linux kernel, again configurable and locally compiled)

So a quick walk-through for the gentoo-kernel variant:

1. Set up the correct package USE flags

We do not want an initrd and we want our own config to be re-used so:

echo "sys-kernel/gentoo-kernel -initramfs savedconfig" >> /etc/portage/package.use/gentoo-kernel

2. Preseed the saved config

The current kernel config needs to be saved as the initial savedconfig so it is found and applied for our emerge below:

mkdir -p /etc/portage/savedconfig/sys-kernel
cp -n "/usr/src/linux-$(uname -r)/.config" /etc/portage/savedconfig/sys-kernel/gentoo-kernel

3. Emerge the new kernel

emerge sys-kernel/gentoo-kernel

4. Update grub and reboot

Unfortunately this ebuild does not update grub, so we have to run grub-mkconfig manually. This can again be automated via a post_pkg_postinst() script. See the step 7 below.

But for now, let's do it manually:

grub-mkconfig -o /boot/grub/grub.cfg
# All fine? Time to reboot the machine:

5. (Optional) Prepare for the next kernel build

Run etc-update and merge the new kernel config entries into your savedconfig.

Screenshot of etc-update

The kernel should auto-build once new versions become available via portage.

Again the etc-update can be automated if you feel that is sufficiently safe to do in your environment. See step 7 below for details.

6. (Optional) Remove the old kernel sources

If you want to switch from the method based on gentoo-sources to the gentoo-kernel one, you can remove the kernel sources:

emerge -C "=sys-kernel/gentoo-sources-5*"

Be sure to update the /usr/src/linux symlink to the new kernel sources directory from gentoo-kernel, e.g.:

rm /usr/src/linux; ln -s "/usr/src/$(uname -r)" /usr/src/linux

This may be a good time for a bit more house-keeping: Clean up a bit in /usr/src/ to remove old build artefacts, /boot/ to remove old kernels and /lib/modules/ to get rid of old kernel modules.

7. (Optional) Further automate the ebuild

In part 1 we automated the kernel compile, install and a bit more via a helper function for post_pkg_postinst().

We can do the similarly for what is (currently) missing from the gentoo-kernel ebuilds:

Create /etc/portage/env/sys-kernel/gentoo-kernel with the following:

post_pkg_postinst() {
        etc-update --automode -5 /etc/portage/savedconfig/sys-kernel
        grub-mkconfig -o /boot/grub/grub.cfg

The upside of gentoo-kernel over gentoo-sources is that you can put "config override files" in /etc/kernel/config.d/. That way you theoretically profit from config improvements made by the upstream developers. See the Gentoo distribution kernel documentation for a sample snippet. I am fine with savedconfig for now but it is nice that Gentoo provides the flexibility to support both approaches.

Compiling and installing the Gentoo Linux kernel on emerge without genkernel (part 1)


Gentoo emerges of sys-kernel/gentoo-sources will nicely install the current kernel into /usr/src/linux-* but it will not compile them.

The Gentoo wiki kernel documentation has a script snippet to automate the kernel build with genkernel.

I do not like to use genkernel as it brings in lots of firmware files to build initrds that are not needed on virtual hardware. It also makes building the kernel slower.

So, the plain approach:

Make emerge sys-kernel/gentoo-sources symlink the latest kernel to /usr/src/linux so we can find it easily:

echo "sys-kernel/gentoo-sources symlink" >> /etc/portage/package.use/gentoo-sources

Create /etc/portage/env/sys-kernel/gentoo-sources with the following:

post_pkg_postinst() {
        CURRENT_KV=$(uname -r)
        unset ARCH
        if [[ -f "${EROOT:-/}usr/src/linux-${CURRENT_KV}/.config" ]] ; then
                cp -n "${EROOT:-/}usr/src/linux-${CURRENT_KV}/.config" "${EROOT:-/}usr/src/linux/.config"
                cd "${EROOT:-/}usr/src/linux/" && \
                make olddefconfig && \
                make -j5 && make modules_install && make install && \
                grub-mkconfig -o /boot/grub/grub.cfg

This will compile the next kernel on the basis of the config of the currently running kernel, install the modules and the kernel bzImage and update grub so it knows about the new kernel for the next reboot.

If you forget to unset ARCH the Linux build system will complain like:

Makefile:583: arch/amd64/Makefile: No such file or directory
make: *** No rule to make target 'arch/amd64/Makefile'.  Stop.

You can test the new magic by re-emerging the latest kernel, e.g. currently emerge =sys-kernel/gentoo-sources-5.4.80-r1: