What is linux kernel configuration

Kernel/Traditional compilation

This article is an introduction to building custom kernels from kernel.org sources. This method of compiling kernels is the traditional method common to all distributions. It can be, depending on your background, more complicated than using the Kernels/Arch Build System. Consider the Arch Build System tools are developed and maintained to make repeatable compilation tasks efficient and safe.

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Preparation

It is not necessary (or recommended) to use the root account or root privileges (i.e. via Sudo) for kernel preparation.

Install the core packages

Install the base-devel package group, which contains necessary packages such as make and gcc . It is also recommended to install the following packages, as listed in the default Arch kernel PKGBUILD: xmlto , kmod , inetutils , bc , libelf , git , cpio , perl , tar , xz .

Create a kernel compilation directory

It is recommended to create a separate build directory for your kernel(s). In this example, the directory kernelbuild will be created in the home directory:

Download the kernel source

Download the kernel source from https://www.kernel.org. This should be the tarball ( tar.xz ) file for your chosen kernel.

It can be downloaded by simply right-clicking the tar.xz link in your browser and selecting Save Link As. , or any other number of ways via alternative graphical or command-line tools that utilise HTTP, TFTP, Rsync, or Git.

In the following command-line example, wget has been installed and is used inside the

/kernelbuild directory to obtain kernel 4.8.6:

You should also verify the correctness of the download before trusting it. First grab the signature, then use that to grab the fingerprint of the signing key, then use the fingerprint to obtain the actual signing key:

Note the signature was generated for the tar archive (i.e. extension .tar ), not the compressed .tar.xz file that you have downloaded. You need to decompress the latter without untarring it. Verify that you have xz installed, then you can proceed like so:

Do not proceed if this does not result in output that includes the string «Good signature».

If wget was not used inside the build directory, it will be necessary to move the tarball into it, e.g.

Unpack the kernel source

Within the build directory, unpack the kernel tarball:

To finalise the preparation, ensure that the kernel tree is absolutely clean; do not rely on the source tree being clean after unpacking. To do so, first change into the new kernel source directory created, and then run the make mrproper command:

Kernel configuration

This is the most crucial step in customizing the default kernel to reflect your computer’s precise specifications. Kernel configuration is set in its .config file, which includes the use of Kernel modules. By setting the options in .config properly, your kernel and computer will function most efficiently.

You can do a mixture of two things:

• Use the default Arch settings from an official kernel (recommended)
• Manually configure the kernel options (optional, advanced and not recommended)

Default Arch configuration

This method will create a .config file for the custom kernel using the default Arch kernel settings. If a stock Arch kernel is running, you can use the following command inside the custom kernel source directory:

Otherwise, the default configuration can be found online in the official Arch Linux kernel package.

Advanced configuration

There are several tools available to fine-tune the kernel configuration, which provide an alternative to otherwise spending hours manually configuring each and every one of the options available during compilation.

Those tools are:

• make menuconfig : Command-line ncurses interface superseded by nconfig
• make nconfig : Newer ncurses interface for the command-line
• make xconfig : User-friendly graphical interface that requires packagekit-qt5 to be installed as a dependency. This is the recommended method — especially for less experienced users — as it is easier to navigate, and information about each option is also displayed.
• make gconfig : Graphical configuration similar to xconfig but using gtk. This requires gtk2 , glib2 and libgladeAUR .

The chosen method should be run inside the kernel source directory, and all will either create a new .config file, or overwrite an existing one where present. All optional configurations will be automatically enabled, although any newer configuration options (i.e. with an older kernel .config ) may not be automatically selected.

Once the changes have been made save the .config file. It is a good idea to make a backup copy outside the source directory. You may need to do this multiple times before you get all the options right.

If unsure, only change a few options between compilations. If you cannot boot your newly built kernel, see the list of necessary config items here.

Running lspci -k # from liveCD lists names of kernel modules in use. Most importantly, you must maintain cgroups support. This is necessary for systemd. For more detailed information, see Gentoo:Kernel/Gentoo Kernel Configuration Guide and Gentoo:Intel#Kernel or Gentoo:Ryzen#Kernel for Intel or AMD Ryzen processors.

Compilation

Compilation time will vary from as little as fifteen minutes to over an hour, depending on your kernel configuration and processor capability. Once the .config file has been set for the custom kernel, within the source directory run the following command to compile:

Installation

Install the modules

Once the kernel has been compiled, the modules for it must follow. First build the modules:

Then install the modules. As root or with root privileges, run the following command to do so:

This will copy the compiled modules into /lib/modules/ — . For example, for kernel version 4.8 installed above, they would be copied to /lib/modules/4.8.6-ARCH . This keeps the modules for individual kernels used separated.

Copy the kernel to /boot directory

The kernel compilation process will generate a compressed bzImage (big zImage) of that kernel, which must be copied to the /boot directory and renamed in the process. Provided the name is prefixed with vmlinuz- , you may name the kernel as you wish. In the examples below, the installed and compiled 4.8 kernel has been copied over and renamed to vmlinuz-linux48 :

Make initial RAM disk

If you do not know what making an initial RAM disk is, see Initramfs on Wikipedia and mkinitcpio.

Automated preset method

An existing mkinitcpio preset can be copied and modified so that the custom kernel initramfs images can be generated in the same way as for an official kernel. This is useful where intending to recompile the kernel (e.g. where updated). In the example below, the preset file for the stock Arch kernel will be copied and modified for kernel 4.8, installed above.

First, copy the existing preset file, renaming it to match the name of the custom kernel specified as a suffix to /boot/vmlinuz- when copying the bzImage (in this case, linux48 ):

Second, edit the file and amend for the custom kernel. Note (again) that the ALL_kver= parameter also matches the name of the custom kernel specified when copying the bzImage :

Finally, generate the initramfs images for the custom kernel in the same way as for an official kernel:

Manual method

Rather than use a preset file, mkinitcpio can also be used to generate an initramfs file manually. The syntax of the command is:

• -k ( —kernel ): Specifies the modules to use when generating the initramfs image. The name will be the same as the name of the custom kernel source directory (and the modules directory for it, located in /usr/lib/modules/ ).
• -g ( —generate ): Specifies the name of the initramfs file to generate in the /boot directory. Again, using the naming convention mentioned above is recommended.

For example, the command for the 4.8 custom kernel installed above would be:

Copy System.map

The System.map file is not required for booting Linux. It is a type of «phone directory» list of functions in a particular build of a kernel. The System.map contains a list of kernel symbols (i.e function names, variable names etc) and their corresponding addresses. This «symbol-name to address mapping» is used by:

• Some processes like klogd, ksymoops, etc.
• By OOPS handler when information has to be dumped to the screen during a kernel crash (i.e info like in which function it has crashed).

If your /boot is on a filesystem which supports symlinks (i.e., not FAT32), copy System.map to /boot , appending your kernel’s name to the destination file. Then create a symlink from /boot/System.map to point to /boot/System.map- :

After completing all steps above, you should have the following 3 files and 1 soft symlink in your /boot directory along with any other previously existing files:

• Kernel: vmlinuz-
• Initramfs: Initramfs- .img
• System Map: System.map-
• System Map kernel symlink

Bootloader configuration

Add an entry for your new kernel in your bootloader’s configuration file. See Arch boot process#Feature comparison for possible boot loaders, their wiki articles and other information.

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Configuring the Linux kernel

Contents

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 :

This will install the Linux kernel sources in /usr/src/ using the specific kernel version in the path. It won’t 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:

In order to create a symbolic link called linux , use:

Now it is time to configure and compile the kernel sources. There are three approaches for this:

1. The kernel is manually configured and built.
2. A tool called genkernel is used to automatically build and install the Linux kernel.
3. A Distribution Kernel is used to automatically build and install the Linux kernel like any other package.

We explain the manual configuration as the default choice here as it is the best way to optimize an environment.

Kernel configuration and compilation

Manual configuration

Introduction

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:

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:

Naturally the choice in the last two lines depends on the selected init system (OpenRC vs. systemd). It does not hurt to have support for both init systems enabled.

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 ):

Verify SCSI disk support has been activated ( CONFIG_BLK_DEV_SD ):

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 ):

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 ):

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 ):

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 ):

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.

Enable GPT partition label support if that was used previously when partitioning the disk ( CONFIG_PARTITION_ADVANCED and CONFIG_EFI_PARTITION ):

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 ):

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:

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.

To install an initramfs, install sys-kernel/genkernel first, then have it generate an 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 ):

The initramfs will be stored in /boot/ . The resulting file can be found by simply listing the files starting with initramfs:

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 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. 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:

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:

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!

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.

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:

When using a traditional /boot layout (e.g. GRUB, LILO, etc.), the gentoo variant should be installed by default. If in doubt:

Installing a distribution kernel

To build a kernel with Gentoo patches from source, type:

System administrators who want to avoid compiling the kernel sources locally can instead use precompiled kernel images:

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:

Post-install/upgrade tasks

Distribution kernels are now capable of rebuilding kernel modules installed by other packages. linux-mod.eclass provides USE=dist-kernel which controls a subslot dependency on virtual/dist-kernel.

Enabling this on packages like sys-fs/zfs and sys-fs/zfs-kmod allows them to automatically be rebuilt against the new kernel and re-generate the initramfs if applicable accordingly!

Manually rebuilding the initramfs

If required, manually trigger such rebuilds by, after a kernel upgrade, executing:

If any of these modules (e.g. ZFS) are needed at early boot, rebuild the initramfs afterward:

Kernel modules

Configuring the modules

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 » » with the version of the kernel just compiled:

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.

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:

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