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SPARC Handbook
About the installation
Choosing the media
Configuring the network
Preparing the disks
The stage file
Installing base system
Configuring the kernel
Configuring the system
Installing tools
Configuring the bootloader
Working with Gentoo
Portage introduction
USE flags
Portage features
Initscript system
Environment variables
Working with Portage
Files and directories
Mixing software branches
Additional tools
Custom package repository
Advanced features
OpenRC network configuration
Getting started
Advanced configuration
Modular networking
Adding functionality
Dynamic management

Optional: Installing firmware and/or microcode


Linux Firmware

Before getting to configuring kernel sections, it is beneficial to be aware that some hardware devices require additional, sometimes non-FOSS compliant, firmware to be installed on the system before they will operate correctly. This is often the case for wireless network interfaces commonly found in both desktop and laptop computers. Modern video chips from vendors like AMD, Nvidia, and Intel, often also require external firmware files to be fully functional. Most firmware for modern hardware devices can be found within the sys-kernel/linux-firmware package.

It is recommended to have the sys-kernel/linux-firmware package installed before the initial system reboot in order to have the firmware available in the event that it is necessary:

root #emerge --ask sys-kernel/linux-firmware
Installing certain firmware packages often requires accepting the associated firmware licenses. If necessary, visit the license handling section of the Handbook for help on accepting licenses.

It is important to note that kernel symbols that are built as modules (M) will load their associated firmware files from the filesystem when they are loaded by the kernel. It is not necessary to include the device's firmware files into the kernel's binary image for symbols loaded as modules.


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 miscellaneous bugs in CPU hardware.

Microcode updates for AMD CPUs are distributed within the aforementioned sys-kernel/linux-firmware package. Microcode for Intel CPUs can be found within 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. For the purposes of the installation, three approaches to kernel management will be presented, however at any point post-installation a new approach can be employed.

Ranked from least involved to most involved:

Full automation approach: Distribution kernels
A Distribution Kernel is used to configure, automatically build, and install the Linux kernel, its associated modules, and (optionally, but enabled by default) an initramfs file. Future kernel updates are fully automated since they are handled through the package manager, just like any other system package. It is possible provide a custom kernel configuration file if customization is necessary. This is the least involved process and is perfect for new Gentoo users due to it working out-of-the-box and offering minimal involvement from the system administrator.
Full manual approach
New kernel sources are installed via the system package manager. The kernel is manually configured, built, and installed using the eselect kernel and a slew of make commands. Future kernel updates repeat the manual process of configuring, building, and installing the kernel files. This is the most involved process, but offers maximum control over the kernel update process.
Hybrid approach: Genkernel
New kernel sources are installed via the system package manager. System administrators may use Gentoo's genkernel tool to configure, build, and install the Linux kernel, its associated modules, and (optionally, but not enabled by default) an initramfs file. It is possible provide a custom kernel configuration file if customization is necessary. Future kernel configuration, compilation, and installation require the system administrator's involvement in the form of running eselect kernel, genkernel, and potentially other commands for each update.

The core around which all distributions are built is the Linux kernel. It is the layer between the user's programs and the system hardware. Although the handbook provides its users several possible kernel sources, a more comprehensive listing with more detailed descriptions is available at the Kernel overview page.

Kernel installation tasks such as, copying the kernel image to /boot or the EFI System Partition, generating an initramfs and/or Unified Kernel Image, updating bootloader configuration, can be automated with installkernel. Users may wish to configure and install sys-kernel/installkernel before proceeding. See the Kernel installation section below for more more information.

Installing the kernel sources

This section is only relevant when using the following genkernel (hybrid) or manual kernel management approach.

The use of sys-kernel/installkernel is not strictly required, but highly recommended. When this package is installed, the kernel installation process will be delegated to installkernel. This allows for installing several different kernel versions side-by-side as well as managing and automating several tasks relating to kernel installation described later in the handbook. Install it now with:

root #emerge --ask sys-kernel/installkernel

When installing and compiling the kernel for sparc-based systems, Gentoo recommends the sys-kernel/gentoo-sources package.

Choose an appropriate kernel source and install it using emerge:

root #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 the symlink USE flag 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:

root #eselect kernel list
Available kernel symlink targets:
  [1]   linux-6.6.21-gentoo

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

root #eselect kernel set 1
root #ls -l /usr/src/linux
lrwxrwxrwx    1 root   root    12 Oct 13 11:04 /usr/src/linux -> linux-6.6.21-gentoo

Alternative: Manual configuration


In case it was missed, this section requires the kernel sources to be installed. Be sure to obtain the relevant kernel sources, then return here for the rest of section.

Manually configuring a kernel is commonly seen as one of the most difficult procedures a system administrator has to perform. Nothing is less true - after configuring a few 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:

root #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.

root #cd /usr/src/linux
root #make menuconfig

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.

Enabling 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:

KERNEL Enabling Gentoo-specific options
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 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.

Enabling support for typical system components

Make sure that every driver that is vital to the booting of the system (such as SATA controllers, NVMe block device support, filesystem support, etc.) is compiled in the kernel and not as a module, otherwise the system may 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):

KERNEL Enabling devtmpfs support (CONFIG_DEVTMPFS)
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 device support
    <*> SCSI disk support
Device Drivers --->
  <*> Serial ATA and Parallel ATA drivers (libata)  --->
    [*] ATA ACPI Support
    [*] SATA Port Multiplier support
    <*> AHCI SATA support (ahci)
    [*] ATA BMDMA support
    [*] ATA SFF support (for legacy IDE and PATA)
    <*> Intel ESB, ICH, PIIX3, PIIX4 PATA/SATA support (ata_piix)

Verify basic NVMe support has been enabled:

KERNEL Enable basic NVMe support for Linux 4.4.x (CONFIG_BLK_DEV_NVME)
Device Drivers  --->
  <*> NVM Express block device
KERNEL Enable basic NVMe support for Linux 5.x.x (CONFIG_DEVTMPFS)
Device Drivers --->
  NVME Support --->
    <*> NVM Express block device

It does not hurt to enable the following additional NVMe support:

[*] NVMe multipath support
[*] NVMe hardware monitoring
<M> NVM Express over Fabrics FC host driver
<M> NVM Express over Fabrics TCP host driver
<M> NVMe Target support
  [*]   NVMe Target Passthrough support
  <M>   NVMe loopback device support
  <M>   NVMe over Fabrics FC target driver
  < >     NVMe over Fabrics FC Transport Loopback Test driver (NEW)
  <M>   NVMe over Fabrics TCP target 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:

File systems --->
  <*> Second extended fs support
  <*> The Extended 3 (ext3) filesystem
  <*> The Extended 4 (ext4) filesystem
  <*> Btrfs filesystem support
  <*> XFS 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 over Ethernet
    <*> 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):

KERNEL Activating SMP 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:

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
  <*> Unified support for USB4 and Thunderbolt  --->

Optional: Signed kernel modules

To automatically sign the kernel modules enable CONFIG_MODULE_SIG_ALL:

[*] Enable loadable module support  
  -*-   Module signature verification    
    [*]     Automatically sign all modules    
    Which hash algorithm should modules be signed with? (Sign modules with SHA-512) --->

Optionally change the hash algorithm if desired.

To enforce that all modules are signed with a valid signature, enable CONFIG_MODULE_SIG_FORCE as well:

KERNEL Enforce signed kernel modules CONFIG_MODULE_SIG_FORCE
[*] Enable loadable module support  
  -*-   Module signature verification    
    [*]     Require modules to be validly signed
    [*]     Automatically sign all modules
    Which hash algorithm should modules be signed with? (Sign modules with SHA-512) --->

To use a custom key, specify the location of this key in CONFIG_MODULE_SIG_KEY, if unspecified the kernel build system will generate a key. It is recommended to generate one manually instead. This can be done with:

root #openssl req -new -nodes -utf8 -sha256 -x509 -outform PEM -out kernel_key.pem -keyout kernel_key.pem

OpenSSL will ask some questions about the user generating the key, it is recommended to fill in these questions as detailed as possible.

Store the key in a safe location, at the very least the key should be readable only by the root user. Verify this with:

root #ls -l kernel_key.pem
 -r-------- 1 root root 3164 Jan  4 10:38 kernel_key.pem 

If this outputs anything other then the above, correct the permissions with:

root #chown root:root kernel_key.pem
root #chmod 400 kernel_key.pem
-*- Cryptographic API  ---> 
  Certificates for signature checking  --->  
    (/path/to/kernel_key.pem) File name or PKCS#11 URI of module signing key

To also sign external kernel modules installed by other packages via linux-mod-r1.eclass, enable the modules-sign USE flag globally:

FILE /etc/portage/make.confEnable module signing

# Optionally, when using custom signing keys.
MODULES_SIGN_CERT="/path/to/kernel_key.pem" # Only required if the MODULES_SIGN_KEY does not also contain the certificate
MODULES_SIGN_HASH="sha512" # Defaults to sha512
The MODULES_SIGN_KEY and MODULES_SIGN_CERT may be different files. For this example the pem file generated by OpenSSL includes both the key and the accompanying certificate, and thus both variables are set to the same value.

Optional: Signing the kernel image (Secure Boot)

When signing the kernel image (for use on systems with Secure Boot enabled) it is recommended to set the following kernel config options:

KERNEL Lockdown for secureboot
General setup  --->
  Kexec and crash features  --->   
    [*] Enable kexec system call                                                                                          
    [*] Enable kexec file based system call                                                                               
    [*]   Verify kernel signature during kexec_file_load() syscall                                                        
    [*]     Require a valid signature in kexec_file_load() syscall                                                        
    [*]     Enable ""image"" signature verification support  

[*] Enable loadable module support  
  -*-   Module signature verification    
    [*]     Require modules to be validly signed
    [*]     Automatically sign all modules
    Which hash algorithm should modules be signed with? (Sign modules with SHA-512) --->  

Security options  ---> 
[*] Integrity subsystem   
  [*] Basic module for enforcing kernel lockdown                                                                       
  [*]   Enable lockdown LSM early in init                                                                       
        Kernel default lockdown mode (Integrity)  --->            

[*]   Digital signature verification using multiple keyrings                                                            
  [*]     Enable asymmetric keys support                                                                                     
  -*-       Require all keys on the integrity keyrings be signed                                                              
  [*]       Provide keyring for platform/firmware trusted keys                                                                
  [*]       Provide a keyring to which Machine Owner Keys may be added                                                        
  [ ]         Enforce Machine Keyring CA Restrictions

Where ""image"" is a placeholder for the architecture specific image name. These options, from the top to the bottom: enforces that the kernel image in a kexec call must be signed (kexec allows replacing the kernel in-place), enforces that kernel modules are signed, enables lockdown integrity mode (prevents modifying the kernel at runtime), and enables various keychains.

On arches that do not natively support decompressing the kernel (e.g. arm64 and riscv), the kernel must be built with its own decompressor (zboot):

Device Drivers --->                                                                                                                           
  Firmware Drivers --->                                                                                                                       
    EFI (Extensible Firmware Interface) Support --->                                                                                               
      [*] Enable the generic EFI decompressor

After compilation of the kernel, as explained in the next section, the kernel image must be signed. First install app-crypt/sbsigntools and then sign the kernel image:

root #emerge --ask app-crypt/sbsigntools
root #sbsign /usr/src/linux-x.y.z/path/to/kernel-image --cert /path/to/kernel_key.pem --key /path/to/kernel_key.pem --out /usr/src/linux-x.y.z/path/to/kernel-image
For this example the same key that was generated to sign the modules is used to sign the kernel image. It is also possible to generate and use a second sperate key for signing the kernel image. The same OpenSSL command as in the previous section may be used again.

Then proceed with the installation.

To automatically sign EFI executables installed by other packages, enable the secureboot USE flag globally:

FILE /etc/portage/make.confEnable Secure Boot
USE="modules-sign secureboot"

# Optionally, to use custom signing keys.
MODULES_SIGN_CERT="/path/to/kernel_key.pem" # Only required if the MODULES_SIGN_KEY does not also contain the certificate.
MODULES_SIGN_HASH="sha512" # Defaults to sha512

# Optionally, to boot with secureboot enabled, may be the same or different signing key.
The SECUREBOOT_SIGN_KEY and SECUREBOOT_SIGN_CERT may be different files. For this example the pem file generated by OpenSSL includes both the key and the accompanying certificate, and thus both variables are set to the same value.
When generating an Unified Kernel Image with systemd's ukify the kernel image will be signed automatically before inclusion in the unified kernel image and it is not necessary to sign it manually.

Architecture specific options

Activate the correct bus-support:

Console drivers --->
  Frame-buffer support --->
    [*] SBUS and UPA framebuffers             
      [*] Creator/Creator3D support     (Only for UPA slot adapter used in many Ultras)
    [*] CGsix (GX,TurboGX) support      (Only for SBUS slot adapter used in many SPARCStations)

Of course enable support for the OBP:

KERNEL Activating OBP Support
Misc Linux/SPARC drivers --->
  [*]  /dev/openprom device support

Enable SCSI-specific support:

KERNEL Activating SCSI-specific support
SCSI support --->
  SCSI low-level drivers --->
    <*> Sparc ESP Scsi Driver             (Only for SPARC ESP on-board SCSI adapter)
    <*> PTI Qlogic, ISP Driver            (Only for SBUS SCSI controllers from PTI or QLogic)
    <*> SYM53C8XX Version 2 SCSI support  (Only for Ultra 60 on-board SCSI adapter)

To support the network card, select one of the following:

KERNEL Activating networking support
Network device support --->
  Ethernet (10 or 100Mbit) --->
    <*> Sun LANCE support                   (Only for SPARCStation, older Ultra systems, and as Sbus option)
    <*> Sun Happy Meal 10/100baseT support  (Only for Ultra; also supports "qfe" quad-ethernet on PCI and Sbus)
    <*> DECchip Tulip (dc21x4x) PCI support (For some Netras, like N1)
  Ethernet (1000Mbit) --->
    <*> Broadcom Tigon3 support (Modern Netra, Sun Fire machines)

With a 4-port Ethernet machine (10/100 or 10/100/1000) the port order is different from the one used by Solaris. Use sys-apps/ethtool to check the port link status.

When using a qla2xxx disk controller, install sys-block/qla-fc-firmware and add support for loading external firmware.

KERNEL Enabling external firmware
Device Drivers  --->
   Generic Driver Options  --->
   ()  External firmware blobs to build into the kernel binary
   ()  Firmware blobs root directory

Set "External firmware blobs" to ql2200_fw.bin and "Firmware blobs root directory" to /lib/firmware/.

Compiling and installing

With the kernel configured, it is time to compile and install it. Exit the configuration and start the compilation process:

root #make && make modules_install
It is possible to enable parallel builds using make -j N with N being the 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, check the size of the resulting file:

root #ls -lh arch/sparc/boot/image
-rw-r--r--    1 root     root         2.4M Oct 25 14:38 image

If the (uncompressed) size is bigger than 7.5 MB, reconfigure the kernel until it doesn't exceed these limits. One way of accomplishing this is by having most kernel drivers compiled as modules. Ignoring this can lead to a non-booting kernel.

Also, if the kernel is just a tad too big, try stripping it using the strip command:

root #strip -R .comment -R .note arch/sparc/boot/image

Finally copy the kernel image to /boot/.

root #cp arch/sparc/boot/image /boot/kernel-6.6.21-gentoo

Alternative: Genkernel

In case it was missed, this section requires the kernel sources to be installed. Be sure to obtain the relevant kernel sources, then return here for the rest of section.

Genkernel should only be considered by users that have a required need that only Genkernel can meet, otherwise it is recommended to use the Distribution kernel or manually compile your own as it will make maintaining a Gentoo system a lot more simple. An example of why genkernel is more difficult to manage is the lack of integration with sys-kernel/installkernel. This means a user will not get the same level of automation as provided by the other methods, such as Unified Kernel Images will need to be created manually when using Genkernel.

Genkernel provides a generic kernel configuration file and will compile the kernel and initramfs, then install the resulting binaries to the appropriate locations. This results in minimal and generic hardware support for the system's first boot, and allows for additional update control and customization of the kernel's configuration in the future.

Be informed: while using genkernel to maintain the kernel provides system administrators with more update control over the system's kernel, initramfs, and other options, it will require a time and effort commitment to perform future kernel updates as new sources are released. Those looking for a hands-off approach to kernel maintenance should use a distribution kernel.

For additional clarity, it is a misconception to believe genkernel automatically generates a custom kernel configuration for the hardware on which it is run; it uses a predetermined kernel configuration that supports most generic hardware and automatically handles the make commands necessary to assemble and install the kernel, the associate modules, and the initramfs file.

Binary redistributable software license group

If the linux-firmware package has been previously installed, then skip onward to the to the installation section.

As a prerequisite, due to the firwmare USE flag being enabled by default for the sys-kernel/genkernel package, the package manager will also attempt to pull in the sys-kernel/linux-firmware package. The binary redistributable software licenses are required to be accepted before the linux-firmware will install.

This license group can be accepted system-wide for any package by adding the @BINARY-REDISTRIBUTABLE as an ACCEPT_LICENSE value in the /etc/portage/make.conf file. It can be exclusively accepted for the linux-firmware package by adding a specific inclusion via a /etc/portage/package.license/linux-firmware file.

If necessary, review the methods of accepting software licenses available in the Installing the base system chapter of the handbook, then make some changes for acceptable software licenses.

If in analysis paralysis, the following will do the trick:

root #mkdir /etc/portage/package.license
FILE /etc/portage/package.license/linux-firmwareAccept binary redistributable licenses for the linux-firmware package
sys-kernel/linux-firmware @BINARY-REDISTRIBUTABLE


Explanations and prerequisites aside, install the sys-kernel/genkernel package:

root #emerge --ask sys-kernel/genkernel


Compile the kernel sources by running genkernel all. Be aware though, as genkernel compiles a kernel that supports a wide array of hardware for differing computer architectures, this compilation may take quite a while to finish.

If the root partition/volume uses a filesystem other than ext4 or XFS, it may be necessary to manually configure the kernel using genkernel --menuconfig all to add built-in kernel support for the particular filesystem(s) (i.e. not building the filesystem as a module).
Users of LVM2 should add --lvm as an argument to the genkernel command below.
root #genkernel --mountboot --install all

Once genkernel completes, a kernel and an initial ram filesystem (initramfs) will be generated and installed into the /boot directory. Associated modules will be installed into the /lib/modules directory. The initramfs will be started immediately after loading the kernel to perform hardware auto-detection (just like in the live disk image environments).

root #ls /boot/vmlinu* /boot/initramfs*
root #ls /lib/modules

Kernel installation


Installkernel may be used to automate, the kernel installation, initramfs generation, unified kernel image generation and/or bootloader configuration among other things. sys-kernel/installkernel implements two paths of achieving this: the traditional installkernel originating from Debian and systemd's kernel-install. Which one to choose depends, among other things, on the system's bootloader. By default systemd's kernel-install is used on systemd profiles, while the traditional installkernel is the default for other profiles.

If unsure, follow the 'Traditional layout' subsection below.


When using systemd-boot (formerly gummiboot) as the bootloader, systemd's kernel-install must be used. Therefore ensure the systemd and the systemd-boot USE flags are enabled on sys-kernel/installkernel, and then install the relevant package for systemd-boot.

On OpenRC systems:

FILE /etc/portage/package.use/systemd-boot
sys-apps/systemd-utils boot kernel-install
sys-kernel/installkernel systemd systemd-boot
root #emerge --ask sys-apps/systemd-utils

On systemd systems:

FILE /etc/portage/package.use/systemd
sys-apps/systemd boot
sys-kernel/installkernel systemd-boot
root #emerge --ask sys-apps/systemd


Users of GRUB can use either systemd's kernel-install or the traditional Debian installkernel. The systemd USE flag switches between these implementations. To automatically run grub-mkconfig when installing the kernel, enable the grub USE flag.

FILE /etc/portage/package.use/installkernel
sys-kernel/installkernel grub
root #emerge --ask sys-kernel/installkernel

Traditional layout, other bootloaders (e.g. lilo, etc.)

The traditional /boot layout (for e.g. LILO, etc.) is used by default if the grub, systemd-boot and uki USE flags are not enabled. No further action is required.

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. The default configuration of the Project:Distribution Kernel requires an 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.config to 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.

Installkernel can automatically generate an initramfs when installing the kernel if the dracut USE flag is enabled:

FILE /etc/portage/package.use/installkernel
sys-kernel/installkernel dracut

Alternatively, dracut may be called manually to generate an initramfs. Install sys-kernel/dracut first, then have it generate an initramfs:

root #emerge --ask sys-kernel/dracut
root #dracut --kver=6.6.21-gentoo

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

root #ls /boot/initramfs*

Optional: Building an Unified Kernel Image

An Unified Kernel Image (UKI) combines, among other things, the kernel, the initramfs and the kernel command line into a single executable. Since the kernel command line is embedded into the unified kernel image it should be specified before generating the unified kernel image (see below). Note that any kernel command line arguments supplied by the bootloader or firmware at boot are ignored when booting with secure boot enabled.

An unified kernel image requires a stub loader, currently the only one available is systemd-stub. To enable it:

For systemd systems:

FILE /etc/portage/package.use/systemd
sys-apps/systemd boot

For OpenRC systems:

FILE /etc/portage/package.use/systemd-utils
sys-apps/systemd-utils boot kernel-install

Installkernel can automatically generate an unified kernel image using either dracut or ukify, by enabling the respective flag. The uki USE flag should be enabled as well to install the generated unified kernel image to the $ESP/EFI/Linux directory on the EFI system partition (ESP).

For dracut:

FILE /etc/portage/package.use/installkernel
sys-kernel/installkernel dracut uki
FILE /etc/dracut.conf

For ukify:

FILE /etc/portage/package.use/installkernel
sys-apps/systemd ukify          # For systemd systems
sys-apps/systemd-utils ukify    # For OpenRC systems
sys-kernel/installkernel dracut ukify uki
FILE /etc/kernel/cmdline

Note that while dracut can generate both an initramfs and an unified kernel image, ukify can only generate the latter and therefore the initramfs must be generated separately with dracut.

Generic Unified Kernel Image

The prebuilt sys-kernel/gentoo-kernel-bin can optionally install a prebuilt generic unified kernel image containing a generic initramfs that is able to boot most systemd based systems. It can be installed by enabling the generic-uki USE flag, and configuring installkernel to not generate a custom initramfs or unified kernel image:

FILE /etc/portage/package.use/generic-uki
sys-kernel/gentoo-kernel-bin generic-uki
sys-kernel/installkernel -dracut -ukify uki

Secure Boot

The generic Unified Kernel Image optionally distributed by sys-kernel/gentoo-kernel-bin is already pre-signed. How to sign a locally generated unified kernel image depends on whether dracut or ukify is used. Note that the location of the key and certificate should be the same as the SECUREBOOT_SIGN_KEY and SECUREBOOT_SIGN_CERT as specified in /etc/portage/make.conf.

For dracut:

FILE /etc/dracut.conf

For ukify:

FILE /etc/kernel/uki.conf

Rebuilding external kernel modules

External kernel modules installed by other packages via linux-mod-r1.eclass must be rebuilt for each new kernel version. When the distribution kernels are used this may be automated by enabling the dist-kernel flag globally.

FILE /etc/portage/package.use/module-rebuild
*/* dist-kernel

External kernel modules may also be rebuilt manually with:

root #emerge --ask @module-rebuild

Kernel modules

Listing available kernel 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 modules that will be automatically loaded to be listed. Modules cannot be loaded twice; they are either loaded or unloaded. Sometimes exotic hardware requires help to load their drivers.

The modules that need to be loaded during each boot in can be added to /etc/modules-load.d/*.conf files in the format of one module per line. When extra options are needed for the modules, they should be set in /etc/modprobe.d/*.conf files instead.

To view all modules available for a specific kernel version, issue the following find command. Do not forget to substitute "<kernel version>" with the appropriate version of the kernel to search:

root #find /lib/modules/<kernel version>/ -type f -iname '*.o' -or -iname '*.ko' | less

Force loading particular kernel modules

To force load the kernel to 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 within it.

root #mkdir -p /etc/modules-load.d
root #nano -w /etc/modules-load.d/network.conf

Note that the module's .ko file suffix is insignificant to the loading mechanism and left out of the configuration file:

FILE /etc/modules-load.d/network.confForce loading 3c59x module

Continue the installation with Configuring the system.