Revision as of 08:19, 29 October 2013

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Unified Extensible Firmware Interface (or UEFI for short) is a new type of firmware that was initially designed by Intel (known as EFI then) mainly for its Itanium based systems. It introduces new ways of booting an OS that is distinct from the commonly used "MBR boot code" method followed for BIOS systems. It started as Intel's EFI in versions 1.x and then a group of companies called the UEFI Forum took over its development from which it was called Unified EFI starting with version 2.0. As of 24 July 2013, UEFI Specification 2.4 (released July 11, 2013) is the most recent version.

BIOS

A BIOS or Basic Input-Output System is the very first program (firmware) that is executed once the system is switched on. In most cases it is stored in a flash memory in the motherboard itself and independent of the system storage. BIOS launches the first 440 bytes (Master Boot Record) of the first disk in the BIOS disk order. Since very little can be achieved by a program that fits into the 440-byte boot code area, usually a common boot loader like GRUB or Syslinux or LILO would be loaded by the BIOS, and it would load an operating system by either chain-loading or directly loading the kernel. See Arch Boot Process for detail.

UEFI

UEFI has support for reading both the partition table as well as understanding filesystems. Hence it is not limited by 440 byte code limitation (MBR boot code) as in BIOS systems. It does not use the MBR boot code at all.

The commonly used UEFI firmwares support both MBR and GPT partition table. EFI in Apple-Intel Macs are known to also support Apple Partition Map besides MBR and GPT. Most UEFI firmwares have support for accessing FAT12 (floppy disks), FAT16 and FAT32 filesystems in HDDs and ISO9660 (and UDF) in CD/DVDs. EFI in Apple-Intel Macs can access HFS/HFS+ filesystems also apart from the mentioned ones.

UEFI does not launch any boot code in the MBR whether it exists or not. Instead it uses a special partition in the partition table called EFI SYSTEM PARTITION in which files required to be launched by the firmware are stored. Each vendor can store its files under <EFI SYSTEM PARTITION>/EFI/<VENDOR NAME>/ folder and can use the firmware or its shell (UEFI shell) to launch the boot program. An EFI System Partition is usually formatted as FAT32 (mostly) or FAT16.

Under UEFI, every program whether it is an OS loader or a utility (e.g. a memory testing app or recovery tool), should be a UEFI Application corresponding to the EFI firmware bitness/architecture. The vast majority of UEFI firmwares, including recent Apple Macs, use x86_64 EFI firmware. The only known devices that use IA32 (32-bit) EFI are older (pre 2008) Apple Macs, some recent Intel Cloverfield ultrabooks and some older Intel Server boards are known to operate on Intel EFI 1.10 firmware.

An x86_64 EFI firmware does not include support for launching 32-bit EFI apps (unlike x86_64 Linux and Windows versions which include such support). Therefore the UEFI application must be compiled for that specific firmware processor bitness/architecture.

Boot Process under UEFI

1. System switched on - Power On Self Test, or POST process.
2. UEFI firmware is loaded. Firmware initializes the hardware required for booting.
3. Firmware then reads its Boot Manager data to determine which UEFI application to be launched and from where (i.e. from which disk and partition).
4. Firmware then launches the UEFI application as defined in the boot entry in the firmware's boot manager.
5. The launched UEFI application may launch another application (in case of UEFI Shell or a boot manager like rEFInd) or the kernel and initramfs (in case of a boot loader like GRUB) depending on how the UEFI application was configured.

Multibooting in UEFI

Since each OS or vendor can maintain its own files within the EFI System Partition without affecting the other, multi-booting using UEFI is just a matter of launching a different UEFI application corresponding to the particular OS's bootloader. This removes the need for relying on chainloading mechanisms of one boot loader to load another to switch OSes.

Booting Microsoft Windows

64-bit Windows Vista (SP1+), Windows 7 and Windows 8 versions support booting using x86_64 EFI firmware. Windows forces type of partitioning depending on the firmware used, i.e. if Windows is booted in UEFI mode, it can be installed only to a GPT disk. If the Windows is booted in Legacy BIOS mode, it can be installed only to a MBR disk. This is a limitation enforced by Windows installer. Thus Windows supports either UEFI-GPT boot or BIOS-MBR boot only, not UEFI-MBR or BIOS-GPT boot.

This limitation is not enforced by Linux kernel itself, but rather depends on how the bootloader is configured. However this Windows limitation should be considered if the user wishes to boot Windows and Linux from the same disk, since setting up the bootloader itself depends on the firmware type and disk partitioning used. In case of Windows and Linux dual boot in the same disk, it is advisable to follow the method used by Windows, either go for UEFI-GPT boot or BIOS-MBR boot only, not the other two cases.

32-bit Windows versions only support BIOS-MBR booting. So, in case of Linux and 32-bit Windows booting from the same disk, the disk can use only MBR. See http://support.microsoft.com/kb/2581408 for more info.

Detecting UEFI Firmware bitness

Non Macs

Check whether the dir /sys/firmware/efi exists, if it exists it means the kernel has booted in EFI mode. In that case the UEFI bitness is same as kernel bitness. (ie. i686 or x86_64)

Apple Macs

Pre-2008 Macs mostly have i386-efi firmware while >=2008 Macs have mostly x86_64-efi. All Macs capable of running Mac OS X Snow Leopard 64-bit Kernel have x86_64 EFI 1.x firmware.

To find out the arch of the efi firmware in a Mac, type the following into the Mac OS X terminal:

ioreg -l -p IODeviceTree | grep firmware-abi

If the command returns EFI32 then it is IA32 (32-bit) EFI firmware. If it returns EFI64 then it is x86_64 EFI firmware. Most of the Macs do not have UEFI 2.x firmware as Apple's EFI implementation is not fully compliant with UEFI 2.x Specification.

Linux Kernel Config options for UEFI

The required Linux Kernel configuration options for UEFI systems are :

CONFIG_RELOCATABLE=y
CONFIG_EFI=y
CONFIG_EFI_STUB=y
CONFIG_FB_EFI=y
CONFIG_FRAMEBUFFER_CONSOLE=y

UEFI Runtime Variables Support (efivarfs filesystem - /sys/firmware/efi/efivars). This option is important as this is required to manipulate UEFI Runtime Variables using tools like /usr/bin/efibootmgr. The below config option has been added in kernel 3.10 and above.

CONFIG_EFIVAR_FS=y

UEFI Runtime Variables Support (old efivars sysfs interface - /sys/firmware/efi/vars). This option should be disabled.

CONFIG_EFI_VARS=n

GUID Partition Table GPT config option - mandatory for UEFI support

CONFIG_EFI_PARTITION=y

Retrieved from https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/plain/Documentation/x86/x86_64/uefi.txt .

UEFI Variables

UEFI defines variables through which an operating system can interact with the firmware. UEFI Boot Variables are used by the boot-loader and used by the OS only for early system start-up. UEFI Runtime Variables allow an OS to manage certain settings of the firmware like the UEFI Boot Manager or managing the keys for UEFI Secure Boot Protocol etc. You can get the list using

$ efivar -l

UEFI Variables Support in Linux Kernel

Linux kernel exposes EFI variables data to userspace via 2 interfaces:

• OLD sysfs-efivars interface - populated by efivars kernel module at /sys/firmware/efi/vars - 1024 byte maximum per-variable data size limitation, no UEFI Secure Boot variables support (due to the size limitation) and not recommended by kernel upstream anymore. Still supported by kernel upstream but completely disabled in Arch's official kernels.

• NEW efivarfs (EFI VARiable FileSystem) interface - mounted using efivarfs kernel module at /sys/firmware/efi/efivars - replacement for the OLD sysfs-efivars interface, has no maximum per-variable size limitation, supports UEFI Secure Boot variables and recommended by kernel upstream. Introduced in kernel 3.8 and NEW efivarfs module split from OLD efivars kernel module in kernel 3.10 .

Inconsistency between efivarfs and sysfs-efivars

Enabling both OLD sysfs-efivars and NEW efivarfs can cause data inconsistency issues (see See https://lkml.org/lkml/2013/4/16/473 for more info). Due to this OLD sysfs-efivars is completely disabled in Arch's official kernels (since core/linux-3.11 and core/linux-lts-3.10) and only NEW efivarfs is enabled/supported going forward. All the UEFI Variables related tools and utilities in official repositories support efivarfs as of 01 October 2013.

Requirements for UEFI Variables support to work properly

1. EFI Runtime Services support should be present in the kernel (CONFIG_EFI=y)
2. Kernel processor bitness/arch and EFI processor bitness/arch should match
3. Kernel should be booted in EFI mode (via EFISTUB or any EFI boot loader, not via BIOS/CSM or Apple's "bootcamp" which is also BIOS/CSM)
4. EFI Runtime Services in the kernel SHOULD NOT be disabled via kernel cmdline, i.e. noefi kernel parameter SHOULD NOT be used
5. efivarfs filesystem should be mounted at /sys/firmware/efi/efivars, otherwise follow #Mount_efivarfs section below.
6. efivar should list (option -l) the EFI Variables without any error. For sample output see #Sample_List_of_UEFI_Variables.

If EFI Variables support does not work even after the above conditions are satisfied, try the below workarounds:

1. If any userspace tool is unable to modify efi variables data, check for existence of /sys/firmware/efi/efivars/dump-* files. If they exist, delete them, reboot and retry again.
2. If the above step does not fix the issue, try booting with efi_no_storage_paranoia kernel parameter to disable kernel efi variable storage space check that may prevent writing/modification of efi variables.

Mount efivarfs

If efivarfs is not automatically mounted at /sys/firmware/efi/efivars by systemd during boot, then you need to manually mount it to expose UEFI Variable support to the userspace tools like efibootmgr etc.:

# mount -t efivarfs efivarfs /sys/firmware/efi/efivars

It is also a good idea to auto-mount efivarfs during boot via /etc/fstab as follows:

/etc/fstab

efivarfs    /sys/firmware/efi/efivars    efivarfs    defaults    0    0

Userspace Tools

There are few tools that can access/modify the UEFI variables, namely

1. efivar - Library and Tool to manipulate UEFI Variables (used by vathpela's efibootmgr) - https://github.com/vathpela/efivar - efivar or efivar-git^AUR
2. efibootmgr - Tool to manipulate UEFI Firmware Boot Manager Settings. Upstream (http://linux.dell.com/git/efibootmgr.git) efibootmgr code does not support efivarfs. A fork of efibootmgr by Fedora's Peter Jones (vathpela) supports both efivarfs and sysfs-efivars. It is currently used in official core/efibootmgr pkg and AUR pkg efibootmgr-pjones-git^AUR - https://github.com/vathpela/efibootmgr/tree/libefivars
3. uefivars - Dumps list of EFI variables with some additional PCI related info (uses efibootmgr code internally) - https://github.com/fpmurphy/Various/tree/master/uefivars-2.0 supports only efivarfs and https://github.com/fpmurphy/Various/tree/master/uefivars-1.0 supports only sysfs-efivars . AUR package uefivars-git^AUR
4. efitools - Tools to Create and Setup own UEFI Secure Boot Certificates, Keys and Signed Binaries (requires efivarfs) - efitools-git^AUR
5. Ubuntu's Firmware Test Suite - https://wiki.ubuntu.com/FirmwareTestSuite/ - fwts^AUR (along with fwts-efi-runtime-dkms^AUR) or fwts-git^AUR

efibootmgr

Assuming the boot-loader file to be launched is /boot/efi/EFI/refind/refind_x64.efi, /boot/efi/EFI/refind/refind_x64.efi can be split up as /boot/efi and /EFI/refind/refind_x64.efi, wherein /boot/efi is the mountpoint of the EFI System Partition, which is assumed to be /dev/sdXY (here X and Y are just placeholders for the actual values - eg:- in /dev/sda1 , X==a Y==1).

To determine the actual device path for the EFI System Partition (assuming mountpoint /boot/efi for example) (should be in the form /dev/sdXY), try :

# findmnt /boot/efi
TARGET SOURCE  FSTYPE OPTIONS
/boot/efi  /dev/sdXY  vfat         rw,flush,tz=UTC

Verify that uefi variables support in kernel is working properly by running:

# efivar -l

If efivar lists the uefi variables without any error, then you can proceed. If not, check whether all the conditions in #Requirements_for_UEFI_Variables_support_to_work_properly are met.

Then create the boot entry using efibootmgr as follows:

# efibootmgr -c -d /dev/sdX -p Y -l /EFI/refind/refind_x64.efi -L "rEFInd"

In the above command /boot/efi/EFI/refind/refind_x64.efi translates to /boot/efi and /EFI/refind/refind_x64.efi which in turn translate to drive /dev/sdX -> partition Y -> file /EFI/refind/refind_x64.efi.

The 'label' is the name of the menu entry shown in the UEFI boot menu. This name is user's choice and does not affect the booting of the system. More info can be obtained from efibootmgr GIT README .

FAT32 filesystem is case-insensitive since it does not use UTF-8 encoding by default. In that case the firmware uses capital 'EFI' instead of small 'efi', therefore using \EFI\refind\refindx64.efi or \efi\refind\refind_x64.efi does not matter (this will change if the filesystem encoding is UTF-8).

EFI System Partition

The EFI System Partition (also called ESP or EFISYS) is a FAT32 formatted physical partition (in the main partition table of the disk, not LVM or software raid etc.) from where the UEFI firmware launches the UEFI bootloader and application. It is a OS independent partition that acts as the storage place for the EFI bootloaders and applications which the firmware launches them. It is mandatory for UEFI boot. It should be marked as EF00 or ef00 type code in gdisk, or boot flag in case of GNU Parted (only for GPT disk). It is recommended to keep ESP size at 512 MiB although smaller/larger sizes are fine (smaller sizes provided it is higher than the minimum FAT32 FS partition size limit (as mandated by FAT32 specification from Microsoft). For more info visit link.

GPT partitioned disks

• Create a partition with partition type ef00 or EF00 using gdisk (from gptfdisk pkg). Then format that partition as FAT32 using mkfs.vfat -F32 /dev/<THAT_PARTITION>

(or)

• Create a FAT32 partition and in GNU Parted set/activate the boot flag (not legacy_boot flag) on that partition

MBR partitioned disks

Create a partition with partition type 0xEF using fdisk (from util-linux pkg). Then format that partition as FAT32 using mkfs.vfat -F32 /dev/<THAT_PARTITION>

UEFI Shell

The UEFI Shell is a shell/terminal for the firmware which allows launching uefi applications which include uefi bootloaders. Apart from that, the shell can also be used to obtain various other information about the system or the firmware like memory map (memmap), modifyiang boot manager variables (bcfg), running partitioning programs (diskpart), loading uefi drivers, editing text files (edit), hexedit etc.

Obtaining UEFI Shell

You can download a BSD licensed UEFI Shell from Intel's Tianocore UDK/EDK2 Sourceforge.net project.

• AUR uefi-shell-svn^AUR pkg (recommended) - provides x86_64 Shell in x86_64 system and IA32 Shell in i686 system - compiled directly from latest Tianocore EDK2 SVN source
• Precompiled x86_64 UEFI Shell v2 binary (may not be up-to-date)
• Precompiled x86_64 UEFI Shell v1 binary (not updated anymore upstream)
• Precompiled IA32 UEFI Shell v2 binary (may not be up-to-date)
• Precompiled IA32 UEFI Shell v1 binary (not updated anymore upstream)

Shell v2 works best in UEFI 2.3+ systems and is recommended over Shell v1 in those systems. Shell v1 should work in all UEFI systems irrespective of the spec. version the firmware follows. More info at ShellPkg and this mail

Launching UEFI Shell

Few Asus and other AMI Aptio x86_64 UEFI firmware based motherboards (from Sandy Bridge onwards) provide an option called "Launch EFI Shell from filesystem device" . For those motherboards, download the x86_64 UEFI Shell and copy it to your EFI System Partition as <EFI_SYSTEM_PARTITION>/shellx64.efi (mostly /boot/efi/shellx64.efi) .

Systems with Phoenix SecureCore Tiano UEFI firmware are known to have embedded UEFI Shell which can be launched using either F6, F11 or F12 key.

Important UEFI Shell Commands

UEFI Shell commands usually support -b option which makes output pause after each page. map lists recognized filesystems (fs0, ...) and data storage devices (blk0, ...). Run help -b to list available commands.

More info at http://software.intel.com/en-us/articles/efi-shells-and-scripting/

bcfg

BCFG command is used to modify the UEFI NVRAM entries, which allow the user to change the boot entries or driver options. This command is described in detail in page 83 (Section 5.3) of "UEFI Shell Specification 2.0" PDF document.

To dump a list of current boot entries:

Shell> bcfg boot dump -v

To add a boot menu entry for rEFInd (for example) as 4th (numbering starts from zero) option in the boot menu:

Shell> bcfg boot add 3 fs0:\EFI\refind\refind_x64.efi "rEFInd"

where fs0: is the mapping corresponding to the EFI System Partition and fs0:\EFI\refind\refind_x64.efi is the file to be launched.

To remove the 4th boot option:

Shell> bcfg boot rm 3

To move the boot option #3 to #0 (i.e. 1st or the default entry in the UEFI Boot menu):

Shell> bcfg boot mv 3 0

For bcfg help text:

Shell> help bcfg -v -b

or:

Shell> bcfg -? -v -b

edit

EDIT command provides a basic text editor with an interface similar to nano text editor, but slightly less functional. It handles UTF-8 encoding and takes care or LF vs CRLF line endings.

To edit, for example rEFInd's refind.conf in the EFI System Partition (fs0: in the firmware)

Shell> fs0:
FS0:\> cd \EFI\arch\refind
FS0:\EFI\arch\refind\> edit refind.conf

Type Ctrl-E for help.

UEFI Linux Hardware Compatibility

See HCL/Firmwares/UEFI for the main article.

UEFI Bootable Media

Create UEFI bootable USB from ISO

In Linux

• First create either a MBR or GPT (recommended) partition table and at least one partition in the USB (so it is fine to use an already partitioned USB).
  Note: Using a GPT partition table is recommended as some firmwares don't support booting from MBR devices in full UEFI mode (e.g. Gigabyte).

• Mount the ISO image from the Arch Linux download page.

# mkdir -p /mnt/{usb,iso}
# mount -o loop archlinux-2013.10.01-dual.iso /mnt/iso

• Then create a FAT32 filesystem in the partition on the USB (unmount before if necessary) with LABEL as used in the Archiso configuration. Obtain the label from /mnt/iso/loader/entries/archiso-x86_64.conf; this is used by the archiso hook in initramfs to identify the udev path to the installation media. mkfs.vfat is part of package dosfstools.
  Note: The filesystem should be either FAT32 (recommended), FAT16, or FAT12.

# awk 'BEGIN {FS="="} /archisolabel/ {print $3}' /mnt/iso/loader/entries/archiso-x86_64.conf | xargs mkfs.vfat -F32 /dev/sdXY -n

• Mount the newly created FAT32 USB partition, and copy the contents of the installation media to the USB media.

# mount /dev/sdXY /mnt/usb
# cp -a /mnt/iso/* /mnt/usb
# sync
# umount /mnt/{usb,iso}

In Windows

• Format the USB drive as FAT32.
  Note: The filesystem should be either FAT32 (recommended), FAT16, or FAT12.

• Extracted the ISO (similar to extracting ZIP archive) to the USB drive using 7-Zip.

• Change the Volume Label of the USB drive to match the LABEL mentioned in archisolabel= part in <USB>\loader\entries\archiso-x86_64.conf .
  Note: The above step is required for Official ISO (archiso) but not required for Archboot.

Remove UEFI boot support from ISO

Most of the 32-bit EFI Macs and some 64-bit EFI Macs refuse to boot from a UEFI(X64)+BIOS bootable CD/DVD. If one wishes to proceed with the installation using optical media, it might be necessary to remove UEFI support first.

Mount the official installation media and obtain the archisolabel as shown in the previous section.

Rebuild the ISO using xorriso from libisoburn:

$ xorriso -as mkisofs -iso-level 3 \
    -full-iso9660-filenames\
    -volid "ARCH_201212" \
    -appid "Arch Linux CD" \
    -publisher "Arch Linux <https://www.archlinux.org>" \
    -preparer "prepared like a BAWSE" \
    -eltorito-boot isolinux/isolinux.bin \
    -eltorito-catalog isolinux/boot.cat \
    -no-emul-boot -boot-load-size 4 -boot-info-table \
    -isohybrid-mbr "/mnt/iso/isolinux/isohdpfx.bin" \
    -output "~/archiso.iso" "/mnt/iso/"

Burn ~/archiso.iso to optical media and proceed with installation normally.

Testing UEFI in systems without native support

OVMF for Virtual Machines

OVMF [1] is a tianocore project to enable UEFI support for Virtual Machines. OVMF contains a sample UEFI firmware for QEMU.

You can build OVMF (with Secure Boot support) from AUR ovmf-svn^AUR and run it as follows:

qemu-system-x86_64 -enable-kvm -net none -m 1024 -bios /usr/share/ovmf/x86_64/bios.bin 

DUET for BIOS only systems

DUET is a tianocore project that enables chainloading a full UEFI environment from a BIOS system, in a way similar to BIOS OS booting. This method is being discussed extensively in http://www.insanelymac.com/forum/topic/186440-linux-and-windows-uefi-boot-using-tianocore-duet-firmware/. Pre-build DUET images can be downloaded from one of the repos at https://gitorious.org/tianocore_uefi_duet_builds. Specific instructions for setting up DUET is available at https://gitorious.org/tianocore_uefi_duet_builds/tianocore_uefi_duet_installer/blobs/raw/master/Migle_BootDuet_INSTALL.txt.

You can also try http://sourceforge.net/projects/cloverefiboot/ which provides modified DUET images that may contain some system specific fixes and is more frequently updated compared to the gitorious repos.

Troubleshooting

Windows 7 won't boot in UEFI Mode

If you have installed Windows to a different harddisk with GPT partitioning and still have a MBR partitioned harddisk in your computer, then it is possible that the UEFI BIOS is starting it's CSM support (for booting MBR partitions) and therefor Windows won't boot. To solve this merge your MBR harddisk to GPT partitioning or disable the SATA port where the MBR harddisk is plugged in or unplug the SATA connector from this harddisk.

Mainboards with this kind of problem:

Gigabyte Z77X-UD3H rev. 1.1 (UEFI BIOS version F19e)

- UEFI BIOS option for booting UEFI Only doesn't pretend the UEFI BIOS from starting CSM

See also

• Wikipedia's page on UEFI
• Wikipedia's page on EFI SYSTEM Partition
• Linux Kernel x86_64 UEFI Documentation
• UEFI Forum - contains the official UEFI Specifications - GUID Partition Table is part of UEFI Specification
• Intel's Tianocore Project for Open-Source UEFI firmware which includes DuetPkg for direct BIOS based booting and OvmfPkg used in QEMU and Oracle VirtualBox
• Intel UEFI Community Resource Center
• Intel's page on EFI
• FGA: The EFI boot process
• Microsoft's Windows and GPT FAQ - Contains info on Windows UEFI booting also
• Convert Windows Vista SP1+ or 7 x86_64 boot from BIOS-MBR mode to UEFI-GPT mode without Reinstall
• Create a Linux BIOS+UEFI and Windows x64 BIOS+UEFI bootable USB drive
• Rod Smith - A BIOS to UEFI Transformation
• UEFI Boot problems on some newer machines (LKML)
• EFI Shells and Scripting - Intel Documentation
• UEFI Shell - Intel Documentation
• UEFI Shell - bcfg command info
• UEFI Shell v2 binary with bcfg modified to work with UEFI pre-2.3 firmware - from Clover efiboot
• LPC 2012 Plumbing UEFI into Linux
• LPC 2012 UEFI Tutorial : part 1
• LPC 2012 UEFI Tutorial : part 2