Startup Configuration

This documents the boot process in Gentoo Linux.

UEFI

• Computers that implement UEFI don't really need GRUB, as UEFI implements a boot manager. You still need a bootloader after the OS has been chosen, but this can be done directly by the kernel image if you compile it with the EFI_STUB option. You probably also need to setup the Built-in kernel command line (at the bottom of Processor type and features).
• So on a computer implementing UEFI, the best choice is to not install GRUB at all. Generally the BIOS will offer an interface to add / modify / delete UEFI entries. However, you may need to pass some kernel parameters on the command line (such as root=/dev/sdaX). You may build those directly on the kernel.
• Be careful when entering the new EFI entry; it should use the syntax fsX:\pathfilename.efi where X is the UEFI mapping (drive number). So it can be fs1:\ or fs0:\ for instance. WARNING: On ASUS models it seems you have to omit the fsX: part altogether. Just enter the path to the EFI file.
• In case of problems, running the EFI Shell can be helpful. It should be copied to the root of the ESP partition and renamed to shellx64.efi (at least for ASUS models).
• You need the first partition to be a FAT32 partition in order for UEFI to work. I also think the partition table should be GPT (not sure).

efibootmgr

• If you cannot directly create the entry via the BIOS, you can use efibootmgr. Be careful that in this case the computer must be booted in UEFI mode, not in BIOS legacy mode.

GRUB

GRUB 2

• You just have to run grub2-mkconfig -o /boot/grub/grub.cfg, which should generate the proper configuration file. Be sure to first emerge os-prober if you have a Windows boot partition, as it will autodetect it.

• Then run grub2-install /dev/sda to install Grub 2 on the MBR. For UEFI computers things are of course different but usually you then do not need GRUB at all.

• If you have a RAID array, just running grub2-install /dev/sda should be enough. However, you need to emerge mdadm or grub2-install will report an error and exit.

• If you have a setup where the boot partition is distinct from the root partition, double check if there is a symlink (boot that points to .) in the boot partition. Most of the time in grub.cfg the path to the kernel is given as /boot/kernel-xxx and this will fail without the symlink.

Checking GRUB

• Checking GRUB can only be done visually or through a (v)KVM. Note that GRUB does not write to the filesystem when ran, only to the boot sectors of the hard drive. If GRUB boots on a vKVM, it means it is OK, even if the hardware is emulated inside the vKVM.

• Note that repartitioning a disk does not necessarily wipe out the bootloader (GRUB) which is installed in the Master Boot Record (MBR). To completely erase a previously installed bootloader:

dd if="/dev/zero" of="${HARD_DISK_DEVICE}" count="1" bs="2048"

Note that the previous command will destroy the partition table, so will destroy all data on the hard drive!

Obtaining a log of the boot messages

Kernel log

• It is available at /var/log/dmesg. It will give you a log of the kernel messages.

• You can still get some information by editing /etc/conf.d/rc, changing RC_BOOTLOG="yes" and emerging the package showconsole. The log file will be written to /var/log/boot.msg. But it will miss some parts of the boot process - that's normal.

With OpenRC

• In /etc/rc.conf:
  • Enable rc_logger="YES", logs will go to /var/log/rc.log.
  • Enable rc_interactive="YES" and press "I" as soon as you can if you have errors at boot.
  • Increasing rc_start_wait can also help with debugging.

Log System

You need to emerge a log system (and activate it, you need to add the init script to a runlevel) such as syslog-ng. It can also give you some information.

udev activating services

• If udev is activating services you don't want (and possibly also loading kernel modules), try to change RC_COLDPLUG value in /etc/conf.d/rc (and possibly RC_HOTPLUG too).

Mounting filesystems

• Filesystems listed in /etc/fstab are either mounted by the localmount script, or if they are considered network filesystems, by the netmount script. The list of filesystems considered network are in the variable NET_FS_LIST, normally defined in /sbin/functions.sh. You can override it in /etc/conf.d/localmount and /etc/conf.d/netmount.

Required /dev files

• With udev most of the /dev directory can be empty, however you still need those three nodes or Gentoo won't boot:

mknod --mode=600 console c 5 1 
mknod --mode=666 null c 1 3 
mknod --mode=666 zero c 1 5