EFI System Partition

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The EFI system partition (ESP) is a FAT formatted partition containing the primary EFI boot loader(s) for installed operating systems.

Kernel

Advanced partition selection (CONFIG_PARTITION_ADVANCED) and EFI GUID Partition support (CONFIG_EFI_PARTITION) must be enabled:

KERNEL Enable support for GPT
-*- Enable the block layer --->
   Partition Types --->
      [*] Advanced partition selection
      [*] EFI GUID Partition support

ISO8859-1 codepage must be enabled too, in order to mount the FAT EFI partition:

KERNEL Enable ISO8859-1 codepage and support for VFAT
-*- File Systems --->
   DOS/FAT/EXFAT/NT Filesystems  --->
      <*> VFAT (Windows-95) fs support
      (437) Default codepage for FAT
      (iso8859-1) Default iocharset for FAT
   Native Language support --->
      [*] NLS ISO 8859-1  (Latin 1; Western European Languages)

Characteristics

For creation instructions see Handbook.

parted (sys-block/parted) will show it with the boot, esp flags:

root #parted /dev/sda print
Model: ATA SAMSUNG SSD SM84 (scsi)
Disk /dev/sda: 256GB
Sector size (logical/physical): 512B/512B
Partition Table: gpt
Disk Flags: 
 
Number  Start   End     Size    File system  Name                          Flags
 1      1049kB  99.6MB  98.6MB  fat32        EFI System Partition          boot, esp

gdisk (sys-apps/gptfdisk) will show it with partition code EF00:

root #gdisk -l /dev/sda
GPT fdisk (gdisk) version 1.0.1
 
Partition table scan:
  MBR: protective
  BSD: not present
  APM: not present
  GPT: present
 
Found valid GPT with protective MBR; using GPT.
Disk /dev/sda: 500118192 sectors, 238.5 GiB
Logical sector size: 512 bytes
Disk identifier (GUID): 1B59C2C8-8795-4625-9718-4D636B005AC1
Partition table holds up to 128 entries
First usable sector is 34, last usable sector is 500118158
Partitions will be aligned on 2048-sector boundaries
Total free space is 2669 sectors (1.3 MiB)
 
Number  Start (sector)    End (sector)  Size       Code  Name
   1            2048          194559   94.0 MiB    EF00  EFI System Partition

Its filesystem can be created using the mkfs.fat command:

root #mkfs.fat -F 32 /dev/sda1

Size considerations

Gentoo Handbook recommends to allocating 1 GiB for ESP, which is more than enough for any bootloader payloads like EFI stub kernels or Windows.

Optional: Mount point

An entry in /etc/fstab might be useful for manually mounting the ESP but is not needed for booting.

Optional: autofs

Mounting the ESP to /boot/efi/, as was traditionally done, is not recommended. A nested setup complicates implementation of best-practice autofs-style mounts, as establishing the inner autofs will trigger the outer one. Mounting these partitions via autofs (and by extension keeping them unmounted whenever possible) is recommended due to the data integrity and security characteristics of VFAT file systems being effectively nonexistent.

Where bootloader support is available use /boot for the XBOOTLDR partition and /efi for the ESP. If it is not possible to do so, a monolithic ESP should be mounted at /boot; autofs-style mounts should still be used.

Note
systemd, when partitions are configured according to the Discoverable Partitions Specification, can automatically mount the ESP used for the current boot to /boot or /efi unless a different partition is mounted there [possibly via /etc/fstab] or the mount point directory is not empty. If both ESP and XBOOTLDR exist, the /efi mount point is used.

For systemd (if not using the GPT auto generator):

FILE /etc/fstabConfiguring the ESP mountpoint for systemd
UUID=56FE-81E0        /efi       vfat    defaults,noatime,uid=0,gid=0,umask=0077,x-systemd.automount,x-systemd.idle-timeout=600 0 2

For OpenRC, use AutoFS to mount on-demand:

root #emerge --ask net-fs/autofs

Setup a Direct AutoFS Mount for the ESP.

FILE /etc/autofs/auto.masterConfiguring autofs to read the 'boot' file
/- /etc/autofs/auto.boot --timeout=600,sync,nodev

Tell AutoFS to watch for access to the paths /boot and /efi and mount them with options from device.

FILE /etc/autofs/auto.bootConfiguring the ESP mountpoint
/boot    -fstype=vfat,uid=0,gid=0,umask=0077     UUID=AB12-CD34
/efi     -fstype=vfat,uid=0,gid=0,umask=0077     UUID=EF00-000A

AutoFS needs to be running to watch mountpoints:

root #rc-update add autofs default

To use the automounter before rebooting, start it manually:

root #/etc/init.d/autofs start

There is no need to add these partitions to /etc/fstab.

Standard layout

There is a standard layout for the ESP. Vendors and distributions are supposed to put their stuff into vendor specific directories.

user $tree -L 3 /efi
 /efi
 └── EFI
     ├── BOOT
     │   └── BOOTX64.EFI
     ├── Gentoo
     │   ├── grubx64.efi
     │   ├── initramfs-x.y.z.img
     │   ├── kernel-x.y.z.efi
     │   ├── mmx64.efi
     │   └── shimx64.efi
     ├── Linux
     │   └── gentoo-x.y.z.efi
     ├── Microsoft
     │   ├── Boot
     │   └── Recovery
     ├── refind
     │   └── refind_x64.efi
     └── systemd
         └── systemd-bootx64.efi

Here the Microsoft subtree - and also the Boot subtree[1] - was created by an earlier installation of Windows 10. The Boot subtree is the fallback directory. If UEFI can't find any vendor specific directories it will boot from here. In a multiboot environment with properly set up vendor specific subtrees the Boot subtree can be deleted.

UEFI boot items

Computers with UEFI provide a boot menu for bootloaders on the ESP. This boot menu is a function of the firmware and is not shown by default. Moreover there is no standard of how to get to the boot menu, but the most common way is to hold (or continuously press) a key on UEFI firmware initialization (also POST, power-on self test). Most UEFI firmware vendors will show the boot menu when one of the function keys on the connected keyboard is pressed. The boot menu may also be available from within the firmware setup ("BIOS setup"), which is also accessible when pressing a predefined key. Most commonly Esc, Del, F1, F2, F10, F11 and F12, and on tablets also the Volume Up and Volume Down keys are used to either enter the firmware setup or display the boot menu.[2] Please consult your system or mainboard manual for the exact keys and further details.

An installation tool will not only manage the bootloader and other required files on the ESP, it will also manage the addition of an "EFI boot entry". EFI boot entries, stored in NVRAM, are like the registration of the bootloader to the firmware. EFI will normally only list bootloaders with a boot entry in the boot menu, therefore it is not sufficient to only copy a bootloader or EFI executable to the ESP. Linux bootloaders will normally automatically add an EFI boot entry on installation of an (U)EFI bootloader, like e.g. GRUB with grub-install.

Alternatively, a configuration tool for creating, sorting and deleting boot items is often included on UEFI compatible operating systems. The contents of the ESP is visible to these tools and creating a boot item is like choosing the medium from a given selection, then surfing through the ESP and selecting the item, e.g bzImage-4.9.76-r1-gentoo.efi. On Linux, efibootmgr can be used for managing the UEFI boot items.

root #efibootmgr
BootCurrent: 0001
Timeout: 0 seconds
BootOrder: 0001,0000,2001,2002,2003
Boot0000* Windows Boot Manager  HD(1,GPT,6d98f360-cb3e-4727-8fed-5ce0c040365d,0x800,0x1400000)/\EFI\Microsoft\Boot\bootmgfw.efiRC
Boot0001* Linux Boot Manager    HD(1,GPT,6d98f360-cb3e-4727-8fed-5ce0c040365d,0x800,0x1400000)/\EFI\systemd\systemd-bootx64.efi
Boot2001* EFI USB Device        RC
Boot2002* EFI DVD/CDROM RC
Boot2003* EFI Network   RC

In case a bootloader is deleted from the ESP, UEFI will normally delete the EFI boot entry as well on startup, and the bootloader will no longer be available from the boot menu, even when the file is afterwards restored, i.e. copied to the same path on the ESP.

One single exception, where no EFI boot entry is required, is the removable media boot path. On internal (fixed) media, such as the internal HDD or SDD, it is also used as the fallback boot path on most UEFI systems.

Removable media

EFI bootloaders on removable media are not configured as boot entries, so tools like efibootmgr are not required. Instead the computer firmware identifies removable boot options by looking for a predefined file name unique to the system architecture in use, in a predefined path.[3]

Most (U)EFI implementations support the use of the removable media boot path as a fallback on internal drives.[4] Even though this is against specification, most systems use it in case the configured EFI boot entries are not working (i.a. a fallback), e.g. in case the boot entries are defunct (like missing files), or the EFI variables (accessible on Linux using efivarfs) are lost, e.g. failure of the persistent store (like when the NVRAM is reset, cleared or becomes corrupted, which may occur when the "CMOS battery" fails). With a buggy (U)EFI it may even be the only usable bootloader on that system. Only one such fallback bootloader is possible on a specific system (i.e. architecture), due to the standardized boot path and file names.

Architecture Abbreviation File name PE executable machine type
x86 32-bit IA-32 \efi\boot\bootia32.efi 0x14c
x86-64 (amd64) x64 \efi\boot\bootx64.efi 0x8664
Itanium IA-64 \efi\boot\bootia64.efi 0x200
ARM 32-bit AArch32 \efi\boot\bootarm.efi 0x01c2
ARM 64-bit AArch64 \efi\boot\bootaa64.efi 0xAA64
RISC-V 32-bit \efi\boot\bootriscv32.efi 0x5032
RISC-V 64-bit \efi\boot\bootriscv64.efi 0x5064
RISC-V 128-bit \efi\boot\bootriscv128.efi 0x5128
Loongson 32-bit LoongArch32 \efi\boot\bootloongarch32.efi 0x6232
Loongson 64-bit LoongArch64 \efi\boot\bootloongarch64.efi 0x6264
Note
To boot from removable media, the firmware has to look for compatible bootloaders on supported devices, which can be configured in the firmware setup. Like most firmwares, (U)EFI provides a hotkey to show a boot selection (formally known as "BIOS Boot Specification" or BBS), otherwise (U)EFI will use the configured default boot entry. However, a buggy (U)EFI may default to the removable media boot path even on internal drives.

To use the removable media boot path it is sufficient to copy the EFI bootloader to the required path and file name. Should the firmware make use of this fallback, this may also remove the ability to select between configured boot entries, meaning that boot options (as listed with efibootmgr) through (U)EFI will not work. Even though every EFI bootloader can be used as fallback, it may be advisable to use a boot manager that itself has the ability to select between boot options, such as GRUB or rEFInd. From the above example for x64 (amd64):

root #cp /efi/EFI/Gentoo/grubx64.efi /efi/EFI/boot/bootx64.efi
Note
The FAT file system of the EFI System Partition (ESP) is not case-sensitive, but case-preserving (VFAT). When VFAT has been mounted with strict case checking, check=s, the path /efi/EFI/boot/bootx64.efi from the above example may not work. See the case sensitivity section in the FAT article for further details. Running tree -L 3 /efi (when the ESP is mounted under /efi) will show the names in use on the individual system, to which the above command has to be changed accordingly.
Important
The fallback bootloader is not automatically updated! Every time e.g. GRUB is re-installed (like after a version update, which may contain security fixes), it has to be copied to the fallback boot path again, overwriting (updating) the previous version!

The boot manager included in systemd, systemd-boot (formally Gummiboot), will automatically install to the removable media boot path. When sys-apps/systemd with the boot USE flag is updated, it is necessary to run bootctl again in order to update both bootloader files.

root #bootctl update

See also

References

  1. ↑ https://wiki.debian.org/UEFI#Force_grub-efi_installation_to_the_removable_media_path
  2. ↑ https://docs.microsoft.com/en-us/windows-hardware/manufacture/desktop/boot-to-uefi-mode-or-legacy-bios-mode
  3. ↑ UEFI 2.10 Specification, 3.5.1.1 Removable Media Boot Behavior
  4. ↑ Debian Wiki – UEFI: Force grub-efi installation to the removable media path
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