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+title = "Gentoo kernel upgrade"
+summary = "Little howto on updating the kernel in a Gentoo system"
++++
+
+## Preliminary steps
+
+Fetch the sources of the kernel that you want to install.
+
+```sh {class="cmd-root"}
+emerge -uDUa --with-bdeps=y =sys-kernel/gentoo-sources-X.Y.Z
+```
+You may also need to update @world set before updating the kernel.
+
+```sh {class="cmd-root"}
+emerge -uDUa --with-bdeps=y @world
+```
+
+Using eselect, select the kernel version that you want to upgrade to.
+
+```sh {class="cmd-root"}
+eselect kernel list
+eselect kernel set X
+```
+
+Go to the directory containing kernel sources.
+
+```sh {class="cmd-root"}
+cd /usr/src/linux
+```
+
+## Configuration
+
+Start by copying your `.config` from the old kernel sources.
+
+```sh {class="cmd-root"}
+cp /usr/src/linux-A.B.C-gentoo/.config /usr/src/linux
+```
+
+Then, you will have to update the configuration file. The oldconfig command will interactively prompt you to insert values for the new options.
+
+```sh {class="cmd-root"}
+make oldconfig
+```
+
+Alternatively, the olddefconfig command will automatically insert default values for the new options.
+
+```sh {class="cmd-root"}
+make olddefconfig
+```
+
+To easily change more configuration options you can use the menuconfig command, which will start an interactive ncurses interface.
+
+```sh {class="cmd-root"}
+make menuconfig
+```
+
+{{< note Warning >}}
+Manually editing the `.config` file is highly discouraged.
+{{< /note >}}
+
+## Building
+
+Once you finished setting your configuration, you can build your kernel with make.
+You can speed up the build process by using multiple make jobs (determine the number with nproc).
+
+```sh {class="cmd-root"}
+make -jN
+```
+
+## Intel microcode addendum
+
+Firstly, install the microcode firmware and the tool to manage it.
+
+```sh {class="cmd-root"}
+emerge sys-firmware/intel-microcode
+```
+
+Use the newly installed `iucode_tool` to get the processor signature and search a microcode bundle with a matching signature.
+
+```sh {class="cmd-root"}
+iucode_tool -S
+iucode_tool -S -l /lib/firmware/intel-ucode/*
+```
+
+Enable in the kernel configuration the microcode loading features. Then, add in the `Firmware loading facility` the microcode
+bundle found before as a named firmware blob.
+
+After building the kernel, you can easily verify after the boot that the microcode is loaded with dmesg.
+
+```sh {class="cmd-root"}
+dmesg | grep microcode
+```
+
+## Installation
+
+First of all, mount the boot partition, if it is not already mounted.
+
+```sh {class="cmd-root"}
+mount /dev/XXX /boot
+```
+
+Then, you can install the new kernel and modules.
+
+```sh {class="cmd-root"}
+make install
+make modules_install
+```
+
+Update your bootloader configuration. If you are using GRUB, you can use grub-mkconfig.
+
+```sh {class="cmd-root"}
+grub-mkconfig -o /boot/grub/grub.cfg
+```
+
+Some kernel modules may be installed by packages and live outside of the kernel source tree (e.g. Nvidia drivers), and need to be rebuilt.
+
+```sh {class="cmd-root"}
+emerge @module-rebuild
+```
+
+Additionally, updating the firmware may be required by some drivers.
+
+```sh {class="cmd-root"}
+emerge sys-kernel/linux-firmware
+```
+
+## References
+- https://wiki.gentoo.org/wiki/Kernel
+- https://wiki.gentoo.org/wiki/Kernel/Upgrade
+- https://wiki.gentoo.org/wiki/Kernel/Removal
+- https://wiki.gentoo.org/wiki/Intel_microcode