Configuring the Linux kernel
Optional: Installing firmware and/or microcode
Before getting to configuring kernel sections, it is beneficial to be aware that some devices require additional firmware to be installed on the system before they will operate correctly. This is often the case for network interfaces, especially wireless network interfaces commonly used in both desktop and laptop computers. Modern video chips from vendors like AMD, Nvidia, and Intel, often need external firmware files to be fully functional. Most firmware for modern hardware is available in the sys-kernel/linux-firmware package. On systems using graphics cards from these vendors, it is wise to emerge this firmware package in order to have it available before configuring and compiling the kernel.
emerge --ask sys-kernel/linux-firmware
In addition to discrete graphics hardware and network interfaces, CPUs also can require firmware updates. Typically this kind of firmware is referred to as microcode. Newer revisions of microcode are sometimes necessary to patch instability, security concerns, or other bugs in CPU hardware.
Microcode updates for AMD CPUs are typically distributed with the aforementioned linux-firmware package. Microcode for Intel CPUs can be found in the sys-firmware/intel-microcode package, which will need to be installed separately. See the Microcode article for more information on how to apply microcode updates.
Kernel configuration and compilation
Now it is time to configure and compile the kernel sources. There are three approaches for this:
- The kernel is manually configured and built.
- A tool called genkernel is used to automatically build and install the Linux kernel.
- A Distribution Kernel is used to automatically build and install the Linux kernel like any other package.
Manual configuration is explained as the default choice since it is the best way to optimize an environment.
The core around which all distributions are built is the Linux kernel. It is the layer between the user programs and the system hardware. Gentoo provides its users several possible kernel sources. A full listing with description is available at the Kernel overview page.
Installing the sources
When manually installing and compiling the kernel for amd64-based systems, Gentoo recommends the sys-kernel/gentoo-sources package.
Choose an appropriate kernel source and install it using emerge:
emerge --ask sys-kernel/gentoo-sources
This will install the Linux kernel sources in /usr/src/ using the specific kernel version in the path. It will not create a symbolic link by itself without
USE=symlink being enabled on the chosen kernel sources package.
It is conventional for a /usr/src/linux symlink to be maintained, such that it refers to whichever sources correspond with the currently running kernel. However, this symbolic link will not be created by default. An easy way to create the symbolic link is to utilize eselect's kernel module.
For further information regarding the purpose of the symlink, and how to manage it, please refer to Kernel/Upgrade.
First, list all installed kernels:
eselect kernel list
Available kernel symlink targets:  linux-5.15.52-gentoo
In order to create a symbolic link called linux, use:
eselect kernel set 1
ls -l /usr/src/linux
lrwxrwxrwx 1 root root 12 Oct 13 11:04 /usr/src/linux -> linux-5.15.52-gentoo
Manually configuring a kernel is often seen as the most difficult procedure a Linux user ever has to perform. Nothing is less true - after configuring a couple of kernels no one remembers that it was difficult!
However, one thing is true: it is vital to know the system when a kernel is configured manually. Most information can be gathered by emerging sys-apps/pciutils which contains the lspci command:
emerge --ask sys-apps/pciutils
Inside the chroot, it is safe to ignore any pcilib warnings (like pcilib: cannot open /sys/bus/pci/devices) that lspci might throw out.
Another source of system information is to run lsmod to see what kernel modules the installation CD uses as it might provide a nice hint on what to enable.
Now go to the kernel source directory and execute make menuconfig. This will fire up menu-driven configuration screen.
The Linux kernel configuration has many, many sections. Let's first list some options that must be activated (otherwise Gentoo will not function, or not function properly without additional tweaks). We also have a Gentoo kernel configuration guide on the Gentoo wiki that might help out further.
Activating required options
When using sys-kernel/gentoo-sources, it is strongly recommend the Gentoo-specific configuration options be enabled. These ensure that a minimum of kernel features required for proper functioning is available:
Gentoo Linux ---> Generic Driver Options ---> [*] Gentoo Linux support [*] Linux dynamic and persistent device naming (userspace devfs) support [*] Select options required by Portage features Support for init systems, system and service managers ---> [*] OpenRC, runit and other script based systems and managers [*] systemd
When using sys-kernel/vanilla-sources, the additional selections for init systems will be unavailable. Enabling support is possible, but goes beyond the scope of the handbook.
Make sure that every driver that is vital to the booting of the system (such as SCSI controller, etc.) is compiled in the kernel and not as a module, otherwise the system will not be able to boot completely.
Next select the exact processor type. It is also recommended to enable MCE features (if available) so that users are able to be notified of any hardware problems. On some architectures (such as x86_64), these errors are not printed to dmesg, but to /dev/mcelog. This requires the app-admin/mcelog package.
Also select Maintain a devtmpfs file system to mount at /dev so that critical device files are already available early in the boot process (CONFIG_DEVTMPFS and CONFIG_DEVTMPFS_MOUNT):
Device Drivers ---> Generic Driver Options ---> [*] Maintain a devtmpfs filesystem to mount at /dev [*] Automount devtmpfs at /dev, after the kernel mounted the rootfs
Verify SCSI disk support has been activated (CONFIG_BLK_DEV_SD):
Device Drivers ---> SCSI device support ---> <*> SCSI disk support
Now go to File Systems and select support for the filesystems that will be used by the system. Do not compile the file system that is used for the root filesystem as module, otherwise the system may not be able to mount the partition. Also select Virtual memory and /proc file system. Select one or more of the following options as needed by the system (CONFIG_EXT2_FS, CONFIG_EXT3_FS, CONFIG_EXT4_FS, CONFIG_MSDOS_FS, CONFIG_VFAT_FS, CONFIG_PROC_FS, and CONFIG_TMPFS):
File systems ---> <*> Second extended fs support <*> The Extended 3 (ext3) filesystem <*> The Extended 4 (ext4) filesystem <*> Reiserfs support <*> JFS filesystem support <*> XFS filesystem support <*> Btrfs filesystem support DOS/FAT/NT Filesystems ---> <*> MSDOS fs support <*> VFAT (Windows-95) fs support Pseudo Filesystems ---> [*] /proc file system support [*] Tmpfs virtual memory file system support (former shm fs)
If PPPoE is used to connect to the Internet, or a dial-up modem, then enable the following options (CONFIG_PPP, CONFIG_PPP_ASYNC, and CONFIG_PPP_SYNC_TTY):
Device Drivers ---> Network device support ---> <*> PPP (point-to-point protocol) support <*> PPP support for async serial ports <*> PPP support for sync tty ports
The two compression options won't harm but are not definitely needed, neither does the PPP over Ethernet option, that might only be used by ppp when configured to do kernel mode PPPoE.
Don't forget to include support in the kernel for the network (Ethernet or wireless) cards.
Most systems also have multiple cores at their disposal, so it is important to activate Symmetric multi-processing support (CONFIG_SMP):
Processor type and features ---> [*] Symmetric multi-processing support
In multi-core systems, each core counts as one processor.
If USB input devices (like keyboard or mouse) or other USB devices will be used, do not forget to enable those as well (CONFIG_HID_GENERIC and CONFIG_USB_HID, CONFIG_USB_SUPPORT, CONFIG_USB_XHCI_HCD, CONFIG_USB_EHCI_HCD, CONFIG_USB_OHCI_HCD):
Device Drivers ---> HID support ---> -*- HID bus support <*> Generic HID driver [*] Battery level reporting for HID devices USB HID support ---> <*> USB HID transport layer [*] USB support ---> <*> xHCI HCD (USB 3.0) support <*> EHCI HCD (USB 2.0) support <*> OHCI HCD (USB 1.1) support
Architecture specific kernel configuration
Make sure to select IA32 Emulation if 32-bit programs should be supported (CONFIG_IA32_EMULATION). Gentoo installs a multilib system (mixed 32-bit/64-bit computing) by default, so unless a no-multilib profile is used, this option is required.
Processor type and features ---> [ ] Machine Check / overheating reporting [ ] Intel MCE Features [ ] AMD MCE Features Processor family (AMD-Opteron/Athlon64) ---> ( ) Opteron/Athlon64/Hammer/K8 ( ) Intel P4 / older Netburst based Xeon ( ) Core 2/newer Xeon ( ) Intel Atom ( ) Generic-x86-64 Binary Emulations ---> [*] IA32 Emulation
Enable GPT partition label support if that was used previously when partitioning the disk (CONFIG_PARTITION_ADVANCED and CONFIG_EFI_PARTITION):
-*- Enable the block layer ---> Partition Types ---> [*] Advanced partition selection [*] EFI GUID Partition support
Enable EFI stub support, EFI variables and EFI Framebuffer in the Linux kernel if UEFI is used to boot the system (CONFIG_EFI, CONFIG_EFI_STUB, CONFIG_EFI_MIXED, CONFIG_EFI_VARS, and CONFIG_EFI_FB):
Processor type and features ---> [*] EFI runtime service support [*] EFI stub support [*] EFI mixed-mode support Device Drivers Firmware Drivers ---> EFI (Extensible Firmware Interface) Support ---> <*> EFI Variable Support via sysfs Graphics support ---> Frame buffer Devices ---> <*> Support for frame buffer devices ---> [*] EFI-based Framebuffer Support
Compiling and installing
With the configuration now done, it is time to compile and install the kernel. Exit the configuration and start the compilation process:
make && make modules_install
It is possible to enable parallel builds using make -jX with
Xbeing an integer number of parallel tasks that the build process is allowed to launch. This is similar to the instructions about /etc/portage/make.conf earlier, with the MAKEOPTS variable.
When the kernel has finished compiling, copy the kernel image to /boot/. This is handled by the make install command:
This will copy the kernel image into /boot/ together with the System.map file and the kernel configuration file.
Optional: Building an initramfs
In certain cases it is necessary to build an initramfs - an initial ram-based file system. The most common reason is when important file system locations (like /usr/ or /var/) are on separate partitions. With an initramfs, these partitions can be mounted using the tools available inside the initramfs.
Without an initramfs, there is a risk that the system will not boot properly as the tools that are responsible for mounting the file systems require information that resides on unmounted file systems. An initramfs will pull in the necessary files into an archive which is used right after the kernel boots, but before the control is handed over to the init tool. Scripts on the initramfs will then make sure that the partitions are properly mounted before the system continues booting.
If using genkernel, it should be used for both building the kernel and the initramfs. When using genkernel only for generating an initramfs, it is crucial to pass
--kernel-config=/path/to/kernel.configto genkernel or the generated initramfs may not work with a manually built kernel. Note that manually built kernels go beyond the scope of support for the handbook. See the kernel configuration article for more information.
To install an initramfs, install sys-kernel/dracut first, then have it generate an initramfs:
emerge --ask sys-kernel/dracut
The initramfs will be stored in /boot/. The resulting file can be found by simply listing the files starting with initramfs:
Now continue with Kernel modules.
Alternative: Using genkernel
If a manual configuration looks too daunting, then consider using genkernel. It will configure and build the kernel automatically.
genkernel works by configuring a kernel nearly identically to the way the installation CD kernel is configured. This means that when genkernel is used to build the kernel, the system will generally detect hardware it was built for support with at boot-time, just like the installation CD does. genkernel may be a useful solution for those users who may not be comfortable compiling their own kernels. Note that genkernel does not automatically generate a custom kernel configuration the hardware on which it is being run.
Now, let's see how to use genkernel. First, emerge the sys-kernel/genkernel ebuild:
emerge --ask sys-kernel/genkernel
Next, edit the /etc/fstab file so that the line containing /boot/ as second field has the first field pointing to the right device. If the partitioning example from the handbook is followed, then this device is most likely /dev/sda1 with the ext4 file system. This would make the entry in the file look like so:
nano -w /etc/fstab
/dev/sda1 /boot ext4 defaults 0 2
Further in the Gentoo installation, /etc/fstab will be configured again. The /boot setting is needed right now as the genkernel application reads in this configuration.
Now, compile the kernel sources by running genkernel all. Be aware though, as genkernel compiles a kernel that supports almost all hardware, this compilation will take quite a while to finish!
If the root partition/volume doesn't use ext4 as filesystem it might be necessary to manually configure the kernel using genkernel --menuconfig all and add support for this particular filesystem in the kernel (i.e. not as a module). Users of LVM2 will probably want to add
--lvmas an argument as well.
Once genkernel completes, a kernel, full set of modules and initial ram disk (initramfs) will be created. We will use the kernel and initrd when configuring a boot loader later in this document. Write down the names of the kernel and initrd as this information is used when the boot loader configuration file is edited. The initrd will be started immediately after booting to perform hardware autodetection (just like on the installation CD) before the "real" system starts up.
ls /boot/vmlinu* /boot/initramfs*
Alternative: Using distribution kernels
Distribution Kernels are ebuilds that cover the complete process of unpacking, configuring, compiling, and installing the kernel. The primary advantage of this method is that the kernels are upgraded to new versions as part of @world upgrade without a need for manual action. Distribution kernels default to a configuration supporting the majority of hardware but they can be customized via /etc/portage/savedconfig.
There are other methods available to customize the kernel config such as config snippets.
Installing correct installkernel
Before using the distribution kernels, please verify that the correct installkernel package for the system is installed. When using systemd-boot (formerly gummiboot), install:
emerge --ask sys-kernel/installkernel-systemd-boot
When using a traditional /boot layout (e.g. GRUB, LILO, etc.), the gentoo variant should be installed by default. If in doubt:
emerge --ask sys-kernel/installkernel-gentoo
Installing a distribution kernel
To build a kernel with Gentoo patches from source, type:
emerge --ask sys-kernel/gentoo-kernel
System administrators who want to avoid compiling the kernel sources locally can instead use precompiled kernel images:
emerge --ask sys-kernel/gentoo-kernel-bin
Upgrading and cleaning up
Once the kernel is installed, the package manager will automatically upgrade it to newer versions. The previous versions will be kept until the package manager is requested to clean up stale packages. Please remember to periodically run:
to save space. Alternatively, to specifically clean up old kernel versions:
emerge --prune sys-kernel/gentoo-kernel sys-kernel/gentoo-kernel-bin
Distribution kernels are now capable of rebuilding kernel modules installed by other packages.
USE=dist-kernel which controls a subslot dependency on virtual/dist-kernel.
Manually rebuilding the initramfs
If required, manually trigger such rebuilds by, after a kernel upgrade, executing:
emerge --ask @module-rebuild
If any of these modules (e.g. ZFS) are needed at early boot, rebuild the initramfs afterward:
emerge --config sys-kernel/gentoo-kernel
emerge --config sys-kernel/gentoo-kernel-bin
Configuring the modules
Hardware modules are optional to be listed manually. udev will normally load all hardware modules that are detected to be connected in most cases. However, it is not harmful for automatically detected modules to be listed. Sometimes exotic hardware requires help to load their drivers.
List the modules that need to be loaded automatically in /etc/modules-load.d/*.conf files one module per line. Extra options for the modules, if necessary, should be set in /etc/modprobe.d/*.conf files.
To view all available modules, run the following find command. Don't forget to substitute "<kernel version>" with the version of the kernel just compiled:
find /lib/modules/<kernel version>/ -type f -iname '*.o' -or -iname '*.ko' | less
For instance, to automatically load the 3c59x.ko module (which is the driver for a specific 3Com network card family), edit the /etc/modules-load.d/network.conf file and enter the module name in it. The actual file name is insignificant to the loader.
mkdir -p /etc/modules-load.d
nano -w /etc/modules-load.d/network.conf
Continue the installation with Configuring the system.