Процесс загрузки EFI

Процесс загрузки при использовании EFI отличается от процесса загрузки компьютера с BIOS

В случае использования EFI происходит следующее:

1. Запускается загрузчик EFI. Его задача — выбрать устройство для дальнейшей загрузки, аналогично выбору пользователем устройства при использовании BIOS .
2. После выбора устройства, загрузчик находит на диске системный раздел (EFI System partition) и загружает с него файл, отвечающий за дальнейшую загрузку операционной системы. Данный файл всегда расположен по одному адресу. Для amd64 это /efi/boot/bootx64.efi.
   ESR определяется загрузчиком по определенному GUID и отформатирована в системе FAT.
3. После передачи управления загрузчику ОС из системного раздела ESR, дальнейший процесс продолжается как обычно и зависит от операционной системы.

Описание от Redhat

Платформы с UEFI осуществляют чтение таблицы разделов и подключают раздел ESP (EFI System Partition) — раздел VFAT с уникальным идентификатором GUID (Globally Unique Identifier). ESP содержит приложения EFI — загрузчики и служебные программы. В Red Hat Enterprise Linux 6 это раздел /boot/efi/ , а программы EFI расположены в /boot/efi/EFI/redhat/ .

/boot/efi/EFI/redhat/ содержит grub.efi — версию загрузчика, скомпилированную специально для EFI. В самом простом случае менеджер загрузки EFI выбирает этот файл в качестве загрузчика и загружает его в память.

Если раздел ESP содержит другие программы EFI, менеджер загрузки предложит выбрать программу для выполнения.

Как только GRUB определит операционную систему или ядро для загрузки, они будут загружены в память и им будет передано управление.

Так как производители оборудования обычно добавляют собственные каталоги в раздел ESP, необходимо создать условия для цепной загрузки. Менеджер загрузки EFI может запустить загрузчики любых операционных систем, если они расположены в ESP.

Booting an OS using UEFI

UEFI firmware does not support booting through the above mentioned method which is the only way supported by BIOS . UEFI has support for reading both the partition table as well as understanding filesystems.

The commonly used UEFI firmwares support both MBR and GPT partition table . EFI in Apple-Intel Macs are known to support Apple Partition Map also apart from MBR and GPT . Most of the UEFI firmwares have support for accessing FAT12 (floppy disks) , FAT16 and FAT32 filesystems in HDD 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/ / 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 .

Under UEFI, every program whether they are OS loaders or some utilities (like memory testing apps) or recovery tools outside the OS, should be a UEFI Application corresponding to the EFI firmware architecture . Most of the UEFI firmware in the market, including recent Apple Macs use x86_64 EFI firmware . Only some older macs use i386 EFI firmware while no non-Apple UEFI system is known to use i386 EFI firmware .

A x86_64 EFI firmware does not include support for launching 32-bit EFI apps unlike the 64-bit Linux and Windows which include such support . Therefore the bootloader must be compiled for that architecture correctly .

Multibooting on 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 bootloader to load another to switch OSes .

Linux Windows x86_64 UEFI-GPT Multiboot

Windows Vista (SP1+) and 7 pr 8 x86_64 versions support booting natively using UEFI firmware . But for this they need GPT partitioning of the disk used for UEFI booting .Windows x86_64 versions support either UEFI-GPT booting or BIOS-MBR booting .Windows 32-bit versions support only BIOS-MBR booting . Follow the instructions provided in the forum link given in the references sections as to how to do this . Seehttp://support.microsoft.com/default.aspx ?scid=kb;EN-US;2581408 for more info .

This limitation does not exist in Linux Kernel but rather depends on the bootloader used .For the sake of Windows UEFI booting, the Linux bootloader used should also be installed in UEFI-GPT mode if booting from the same disk .

Boot Process under UEFI

1. System switched on — Power On Self Test, or POST process .
2. UEFI firmware is loaded .
3. Firmware reads its Boot Manager to determine which UEFI application to be launched and from where (ie . from which disk and partition) .
4. Firmware launches the UEFI application from the FAT32 formatted UEFISYS partition as defined in the boot entry in the firmware’s boot manager .
5. 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 bootloader like GRUB) depending on how the UEFI application was configured .

Detecting UEFI Firmware Arch

If you have a non-mac UEFI system, then you have a x86_64 (aka 64-bit) UEFI 2 .x firmware .

Some of the known x86_64 UEFI 2.x firmwares are Phoenix SecureCore Tiano, AMI Aptio, Insyde H2O .

Some of the known systems using these firmwares are Asus EZ Mode BIOS (in Sandy Bridge P67 and H67 motherboards), MSI ClickBIOS, HP EliteBooks, Sony Vaio Z series, many Intel Server and Desktop motherboards

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, boot into Mac OS X and type the following command

If the command returns EFI32 then it is i386 EFI 1 .x firmware. If it returns EFI64 then it is x86_64 EFI 1 .x firmware. Macs do not have UEFI 2 .x firmware as Apple’s EFI implementation is not fully compliant with UEFI Specification .

UEFI Support in Linux Kernel

Linux Kernel config options for UEFI

The required Linux Kernel configuration options for UEFI systems are :

UEFI Runtime Variables/Services Support — ‘efivars’ kernel module . This option is important as this is required to manipulate UEFI Runtime Variables using tools likeefibootmgr .

GUID Partition Table GPT config option — mandatory for UEFI support

UEFI Variables Support

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 .

Access to UEFI Runtime services is provided by «efivars» kernel module which is enabled through the CONFIG_EFI_VAR=m kernel config option . This module once loaded exposes the variables under the directory /sys/firmware/efi/vars . One way to check whether the system has booted in UEFI boot mode is to load the «efivars» kernel module and check for the existence of /sys/firmware/efi/vars directory with contents similar to :

The UEFI Runtime Variables will not be exposed to the OS if you have used «noefi» kernel parameter in the boot-loader menu . This parameter instructs the kernel to completely ignore UEFI Runtime Services .

Userspace Tools

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

1. efibootmgr — Used to create/modify boot entries in the UEFI Boot Manager —efibootmgr or efibootmgr-git
2. uefivars — simply dumps the variables — uefivars-git — uses efibootmgr library
3. Ubuntu’s Firmware Test Suite — fwts — fwts-git — uefidump command — fwts uefidump

Non-Mac UEFI systems

efibootmgr

Initially the user may be required to manually launch the boot-loader from the firmware itself (using maybe the UEFI Shell) if the UEFI boot-loader was installed when the system is booted in BIOS mode . Then efibootmgr should be run to make the UEFI boot-loader entry as the default entry in the UEFI Boot Manager .

To use efibootmgr, first load the ‘efivars’ kernel module:

If you get no such device found error for this command, that means you have not booted in UEFI mode or due to some reason the kernel is unable to access UEFI Runtime Variables (noefi ?) .

Verify whether there are files in /sys/firmware/efi/vars/ directory . This directory and its contents are created by «efivars» kernel module and it will exist only if you have booted in UEFI mode, without the «noefi» kernel parameter .

If /sys/firmware/efi/vars/ directory is empty or does not exist, then efibootmgr command will not work . If you are unable to make the ISO/CD/DVD/USB boot in UEFI mode try#Create_UEFI_bootable_USB_from_ISO .

Assume the boot-loader file to be launched is /boot/efi/EFI/gummiboot/gummibootx64.efi . /boot/efi/EFI/gummiboot/gummibootx64.efi can be split up as /boot/efi and /EFI/gummiboot/gummibootx64.efi , wherein /boot/efi is the mountpoint of the UEFI 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 UEFI System Partition (should be in the form /dev/sdXY ), try :

Then create the boot entry using efibootmgr as follows :

In the above command /boot/efi/EFI/gummiboot/gummibootx64.efi translates to /boot/efi and /EFI/gummiboot/gummibootx64.efi which in turn translate to drive /dev/sdX -> partition Y -> file /EFI/gummiboot/gummibootx64.efi .

UEFI uses backward slash as path separator (similar to Windows paths) .

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\gummiboot\gummibootx64.efi or \efi\gummiboot\gummibootx64.efi does not matter (this will change if the filesystem encoding is UTF-8) .

Linux Bootloaders for UEFI

Create an UEFI System Partition in Linux

For GPT partitioned disks

• Using GNU Parted/GParted: Create a FAT32 partition . Set «boot» flag on for that partition .
• Using GPT fdisk (aka gdisk): Create a partition with gdisk type code «EF00» . Then format that partition as FAT32 using mkfs.vfat -F32 /dev/

For MBR partitioned disks

• Using GNU Parted/GParted: Create FAT32 partition . Change the type code of that partition to 0xEF using fdisk, cfdisk or sfdisk .
• Using fdisk: Create a partition with partition type 0xEF and format it as FAT32 using mkfs.vfat -F32 /dev/

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), modifying boot manager variables (bcfg), running partitioning programs (diskpart), loading uefi drivers, editing text files (edit), hexedit etc .

UEFI Shell download links

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

Shell 2.0 works only in UEFI 2.3+ systems and is recommended over Shell 1.0 in those systems. Shell 1 .0 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 UEFI SYSTEM PARTITION as /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 .

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 —

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

where fs0: is the mapping corresponding to the UEFI System Partition and \EFI\arch\refind\refindx64.efi is the file to be launched .

To remove the 4th boot option

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

For bcfg help text

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 UEFI System Partition (fs0: in the firmware)

Type Ctrl-E for help .

Hardware Compatibility

Create UEFI bootable USB from ISO

First create a MBR partition table in the USB using fdisk . Mount the USB partition and create a FAT32 filesystem 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 .

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

If you find the error: «No loader found . Configuration files in /loader/entries/*.conf are needed .« A possible fix is to use a different uefi bootloader to the included one, gummiboot .

Download refind-efi pkg and extract the file /usr/lib/refind/refind_x64.efi from within the package to (USB)/EFI/boot/bootx64.efi (overwrite or rename any existing (USB)/EFI/boot/bootx64.efi file) .

Then copy this text to EFI/boot/refind.conf . Take care that the label in the Arch menu section ( ARCH_201302 here) matches that of your usb’s .

You should now be able to successfully boot, and you can choose which EFI you’d like to load .

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 :