Aunque se instala para una CPU de 32 bits, casi todas las placas base de x86 (desde alrededor de 2006-2007 hasta el presente) que se produjeron con soporte para UEFI tienen 64-bit Firmware UEFI. Algunos usuarios pueden notar "64" en el nombre de los ajustes de configuración y archivos en las siguientes secciones a continuación. Esto es "esperado" en casi todos los casos.
Hay algunas excepciones muy pequeñas a esta regla de firmware UEFI de 64 bits, a saber, algunas de los primeros Apple Mac y algunas tabletas Dell con tecnología Intel Atom  eran compatibles con el firmware UEFI de 32 bits. La gran mayoría de lectores nunca encontrarán firmware UEFI de 32 bits en la naturaleza. Por esta razón, el firmware UEFI de 32 bits no se trata en el manual x86.
Selecting a boot loader
With the Linux kernel configured, system tools installed and configuration files edited, it is time to install the last important piece of a Linux installation: the boot loader.
The boot loader is responsible for firing up the Linux kernel upon boot - without it, the system would not know how to proceed when the power button has been pressed.
In this section of the Handbook a delineation has been made between emerging the boot loader's package and installing a boot loader to a system disk. Here the term emerge will be used to ask Portage to make the software package available to the system. The term install will signify the boot loader copying files or physically modifying appropriate sections of the system's disk drive in order to render the boot loader activated and ready to operate on the next power cycle.
By default, the majority of Gentoo systems now rely upon GRUB (found in the sys-boot/grub package), which is the direct successor to GRUB Legacy. With no additional configuration, GRUB gladly supports older BIOS ("pc") systems. With a small amount of configuration, necessary before build time, GRUB can support more than a half a dozen additional platforms. For more information, consult the Prerequisites section of the GRUB article.
When using an older BIOS system supporting only MBR partition tables, no additional configuration is needed in order to emerge GRUB:
emerge --ask --verbose sys-boot/grub
A note for UEFI users: running the above command will output the enabled GRUB_PLATFORMS values before emerging. When using UEFI capable systems, users will need to ensure
GRUB_PLATFORMS="efi-64" is enabled (as it is the case by default). If that is not the case for the setup,
GRUB_PLATFORMS="efi-64" will need to be added to the /etc/portage/make.conf file before emerging GRUB so that the package will be built with EFI functionality:
echo 'GRUB_PLATFORMS="efi-64"' >> /etc/portage/make.conf
emerge --ask sys-boot/grub
If GRUB was somehow emerged without enabling
GRUB_PLATFORMS="efi-64", the line (as shown above) can be added to make.conf and then dependencies for the world package set can be re-calculated by passing the
--update --newuse options to emerge:
emerge --ask --update --newuse --verbose sys-boot/grub
The GRUB software has now been merged onto the system, but it has not yet been installed as a secondary bootloader.
Next, install the necessary GRUB files to the /boot/grub/ directory via the grub-install command. Presuming the first disk (the one where the system boots from) is /dev/sda, one of the following commands will do:
DOS/Legacy BIOS systems
For DOS/Legacy BIOS systems:
Make sure the EFI system partition has been mounted before running grub-install. It is possible for grub-install to install the GRUB EFI file (grubx64.efi) into the wrong directory without providing any indication the wrong directory was used.
For EFI systems:
grub-install --target=x86_64-efi --efi-directory=
Installing for x86_64-efi platform. Installation finished. No error reported.
When debugging GRUB, there are a couple of quick fixes that may result in a bootable installation without having to reboot to a new live image environment.
In the event that "EFI variables are not supported on this system" is displayed somewhere in the output, it is likely the live image was not booted in EFI mode and is presently in Legacy BIOS boot mode. The solution is to try the removable GRUB step mentioned below. This will overwrite the executable EFI file located at /EFI/BOOT/BOOTX64.EFI. Upon rebooting in EFI mode, the motherboard firmware may execute this default boot entry and execute GRUB.
If grub-install returns an error that says "Could not prepare Boot variable: Read-only file system", and the live environment was correctly booted in EFI mode, then it should be possible to remount the efivars special mount as read-write and then re-run the aforementioned grub-install command:
mount -o remount,rw,nosuid,nodev,noexec --types efivarfs efivarfs /sys/firmware/efi/efivars
This is caused by certain non-official Gentoo environments not mounting the special EFI filesystem by default. If the previous command does not run, then reboot using an official Gentoo live image environment in EFI mode.
Some motherboard manufacturers with poor UEFI implementations seem to only support the /EFI/BOOT directory location for the .EFI file in the EFI System Partition (ESP). The GRUB installer can create the .EFI file in this location automatically by appending the
--removable option to the install command. Ensure the ESP has been mounted before running the following command; presuming it is mounted at (as defined earlier), run:
grub-install --target=x86_64-efi --efi-directory= --removable
This creates the 'default' directory defined by the UEFI specification, and then creates a file with the default name: bootx64.efi.
Next, generate the GRUB configuration based on the user configuration specified in the /etc/default/grub file and /etc/grub.d scripts. In most cases, no configuration is needed by users as GRUB will automatically detect which kernel to boot (the highest one available in /boot/) and what the root file system is. It is also possible to append kernel parameters in /etc/default/grub using the GRUB_CMDLINE_LINUX variable.
To generate the final GRUB configuration, run the grub-mkconfig command:
grub-mkconfig -o /boot/grub/grub.cfg
Generating grub.cfg ... Found linux image: /boot/vmlinuz-5.15.52-gentoo Found initrd image: /boot/initramfs-genkernel-x86-5.15.52-gentoo done
The output of the command must mention that at least one Linux image is found, as those are needed to boot the system. If an initramfs is used or genkernel was used to build the kernel, the correct initrd image should be detected as well. If this is not the case, go to /boot/ and check the contents using the ls command. If the files are indeed missing, go back to the kernel configuration and installation instructions.
The os-prober utility can be used in conjunction with GRUB to detect other operating systems from attached drives. Windows 7, 8.1, 10, and other distributions of Linux are detectable. Those desiring dual boot systems should emerge the sys-boot/os-prober package then re-run the grub-mkconfig command (as seen above). If detection problems are encountered be sure to read the GRUB article in its entirety before asking the Gentoo community for support.
Alternative 1: LILO
LILO, the LInuxLOader, is the tried and true workhorse of Linux boot loaders. However, it lacks features when compared to GRUB. LILO is still used because, on some systems, GRUB does not work and LILO does. Of course, it is also used because some people know LILO and want to stick with it. Either way, Gentoo supports both bootloaders.
Installing LILO is a breeze; just use emerge.
emerge --ask sys-boot/lilo
To configure LILO, first create /etc/lilo.conf:
nano -w /etc/lilo.conf
In the configuration file, sections are used to refer to the bootable kernel. Make sure that the kernel files (with kernel version) and initramfs files are known, as they need to be referred to in this configuration file.
If the root filesystem is JFS, add an
append="ro"line after each boot item since JFS needs to replay its log before it allows read-write mounting.
boot=/dev/sda # Install LILO in the MBR prompt # Give the user the chance to select another section timeout=50 # Wait 5 (five) seconds before booting the default section default=gentoo # When the timeout has passed, boot the "gentoo" section compact # This drastically reduces load time and keeps the map file smaller; may fail on some systems image=/boot/vmlinuz-5.15.52-gentoo label=gentoo # Name we give to this section read-only # Start with a read-only root. Do not alter! root=/dev/sda4 # Location of the root filesystem image=/boot/vmlinuz-5.15.52-gentoo label=gentoo.rescue # Name we give to this section read-only # Start with a read-only root. Do not alter! root=/dev/sda4 # Location of the root filesystem append="init=/bin/bb" # Launch the Gentoo static rescue shell # The next two lines are for dual booting with a Windows system. # In this example, Windows is hosted on /dev/sda6. other=/dev/sda6 label=windows
If a different partitioning scheme and/or kernel image is used, adjust accordingly.
If an initramfs is necessary, then change the configuration by referring to this initramfs file and telling the initramfs where the root device is located:
image=/boot/vmlinuz-5.15.52-gentoo label=gentoo read-only append="root=/dev/sda4" initrd=/boot/initramfs-genkernel-x86-5.15.52-gentoo
If additional options need to be passed to the kernel, use an
append statement. For instance, to add the
video statement to enable framebuffer:
image=/boot/vmlinuz-5.15.52-gentoo label=gentoo read-only root=/dev/sda4 append="video=uvesafb:mtrr,ywrap,1024x768-32@85"
Users that used genkernel should know that their kernels use the same boot options as is used for the installation CD. For instance, if SCSI device support needs to be enabled, add
doscsi as kernel option.
Now save the file and exit.
To finish up, run the /sbin/lilo executable so LILO can apply the /etc/lilo.conf settings to the system (i.e. install itself on the disk). Keep in mind that /sbin/lilo must be executed each time a new kernel is installed or a change has been made to the lilo.conf file in order for the system to boot if the filename of the kernel has changed.
Alternative 2: efibootmgr
Computer systems with UEFI-based firmware technically do not need secondary bootloaders (e.g. GRUB) in order to boot kernels. Secondary bootloaders exist to extend the functionality of UEFI firmware during the boot process. Using GRUB (see the prior section) is typically easier and more robust because it offers a more flexible approach for quickly modifying kernel parameters at boot time.
System administrators who desire to take a minimalist, although more rigid, approach to booting the system can avoid secondary bootloaders and boot the Linux kernel as an EFI stub.
The sys-boot/efibootmgr application is a tool to used interact with UEFI firmware - the system's primary bootloader. Normally this looks like adding or removing boot entries to the firmware's list of bootable entries. It can also update firmware settings so that the Linux kernels that were previously added as bootable entries can be executed with additional options. These interactions are performed through special data structures called EFI variables (hence the need for kernel support of EFI vars).
Ensure the EFI stub kernel article has been reviewed before continuing. The kernel must have specific options enabled to be directly bootable by the UEFI firmware. It may be necessary to recompile the kernel in order to build-in this support.
It is also a good idea to take a look at the efibootmgr article for additional information.
To reiterate, efibootmgr is not a requirement to boot an UEFI system; it is merely necessary to add an entry for an EFI-stub kernel into the UEFI firmware. When built appropriately with EFI stub support, the Linux kernel itself can be booted directly. Additional kernel command-line options can be built-in to the Linux kernel (there is a kernel configuration option called CONFIG_CMDLINE. Similarly, support for initramfs can be 'built-in' to the kernel as well. These decisions must be made prior to kernel compilation, resulting in a more static boot configuration.
Install the efibootmgr software:
emerge --ask sys-boot/efibootmgr
Create the location, and copy the kernel into this location, calling it bootx64.efi:
cp /boot/vmlinuz-* /bootx64.efi
The use of a backslash (\) as directory path separator is mandatory when using UEFI definitions.
Create boot entry called "gentoo" for the freshly compiled EFI stub kernel within the UEFI firmware:
efibootmgr --create --disk /dev/sda --part 2 --label "gentoo" --loader "\bootx64.efi"
If an initial RAM file system (initramfs) is used, add the proper boot option to it:
efibootmgr -c -d /dev/sda -p 2 -L "gentoo" -l "\bootx64.efi" initrd='\initramfs-genkernel-x86-5.15.52-gentoo'
With these changes done, when the system reboots, a boot entry called "gentoo" will be available.
Alternative 3: Syslinux
Syslinux is yet another bootloader alternative for the x86 architecture. It supports MBR and, as of version 6.00, it supports EFI boot. PXE (network) boot and lesser-known options are also supported. Although Syslinux is a popular bootloader for many it is unsupported by the Handbook. Readers can find information on emerging and then installing this bootloader in the Syslinux article.