Configuring the Linux kernel
Installing the sources
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.
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/ in which a symbolic link called linux will be pointing to the installed kernel source:
ls -l /usr/src/linux
lrwxrwxrwx 1 root root 12 Oct 13 11:04 /usr/src/linux -> linux-4.9.16-gentoo
Now it is time to configure and compile the kernel sources. There are two approaches for this:
- The kernel is manually configured and built.
- A tool called genkernel is used to automatically build and install the Linux kernel.
We explain the manual configuration as the default choice here as it is the best way to optimize an environment.
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 even 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
If you are using sys-kernel/gentoo-sources, we strongly recommend you enable the Gentoo-specific configuration options. 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
Naturally your choice in the last two lines depends on your choice of init system (OpenRC vs. Systemd).
If you are using sys-kernel/vanilla-sources, you will have to find the required options on your own.
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 you use. Don't compile the file system that is used for the root filesystem as module, otherwise the Gentoo system will 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 Executable file formats / 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 and EFI variables in the Linux kernel if UEFI is used to boot the system (CONFIG_EFI, CONFIG_EFI_STUB, CONFIG_EFI_MIXED, and CONFIG_EFI_VARS):
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
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 huge risk that the system will not boot up properly as the tools that are responsible for mounting the file systems need information that resides on those 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.
It is recommended to use genkernel for both, building kernel and initramfs. If you decide to use genkernel only for generating initramfs it is crucial to pass
--kernel-config=/path/to/kernel.configto genkernel or generated initramfs may not work with your manually built kernel.
To install an initramfs, install sys-kernel/genkernel first, then have it generate an initramfs:
emerge --ask sys-kernel/genkernel
genkernel --install --kernel-config=/path/to/used/kernel.config initramfs
In order to enable specific support in the initramfs, such as LVM or RAID, add in the appropriate options to genkernel. See genkernel --help for more information. In the next example support is enabled for LVM and software RAID (mdadm):
genkernel --lvm --mdadm --install --kernel-config=/path/to/used/kernel.config initramfs
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 using genkernel is recommended. 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 all hardware at boot-time, just like the installation CD does. Because genkernel doesn't require any manual kernel configuration, it is an ideal solution for those users who may not be comfortable compiling their own kernels.
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 ext2 file system. This would make the entry in the file look like so:
nano -w /etc/fstab
/dev/sda1 /boot ext2 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 ext2 or ext3 or 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.
Optional: Installing firmware
Some drivers require additional firmware to be installed on the system before they work. This is often the case for network interfaces, especially wireless network interfaces. Also, modern video chips, from vendors like AMD, NVidia, and Intel when using open source drivers, often need external firmware files. Most of the firmware is packaged in sys-kernel/linux-firmware:
emerge --ask sys-kernel/linux-firmware