From UEFI Shell to Linux

- UEFI Linux BootLoader

Zhang RuiSoftware EngineerSep 28th 2011

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BootLoader

Tridition Linux BootLoader

UEFI BootLoader

UEFI Linux BootLoader

Elilo – the standard UEFI Linux BootLoader

Agenda

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• The program invoked by the BIOS to load the image of an operating system kernel into RAM

• Why BootLoader- CPU can only execute program code found in ROM, RAM.- Modern operating systems and application program code and data are

stored on nonvolatile data storage devices, such as hard disk, CD, DVD, flash memory cards, USB, foppy, etc…

BootLoader

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Traditional Linux Bootloader

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• UEFI application– Loaded by Boot Manager/Other UEFI applications.

– Firmware allocates memory and copies the image.

– Unloaded when the application returns from its entry points or when it calls the boot service Exit().

• UEFI driver– Same as UEFI application but

• Optionally unloaded unless EFI_SUCCESS is not returned.

• UEFI OS loader– A special type of UEFI application that normally takes over control of the

system from firmware.

– Never unloaded unless something bad happens

– If success, take control of the system by using ExitBootService().

UEFI BootLoader

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UEFI conceptual overview

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• A UEFI OS loader

• A Linux BootLoader

• No MBR involved

• Single stage

• Follows Linux/X86 Boot Protocol

• Elilo/Grub EFI/efiLinux, etc.

UEFI Linux BootLoader

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• Elilo is the standard Linux boot loader for UEFI-based PC hardware

• Elilo is a UEFI OS loader that can be launched in EFI shell

• Source code : http://elilo.sourceforge.net/cgi-bin/blosxom

• Can be built directly in Linux

Elilo – the uEfi LInux LOader

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• Build a UEFI system partition

• Copy elilo.efi, Linux kernel image, init ramdisk, and elilo configure file to the system partition

• Boot into UEFI shell

• “elilo.efi –C elilo.conf”

How to launch Elilo

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• Get Boot services table and RuntimeServices table.

• Get application options, “-C elilo.conf” <- LoadedImageProtocol

• Parse the options

• Install the FileSystem <- FileSystemProtocol

• Read elilo configuration file, “elilo.conf”.

• Parse config file

• Load kernel image (bzImage)

• Load init ramdisk

• Setup boot parameters for Linux kernel

• Uninstall the FileSystem

• Jump to Linux kernel

How Elilo works

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• bzImage.– setup.bin

• .bstext + .bsdata + .header + .entrytext + …

– .bstext & .bsdata : legacy code for booting from floppy

– .header : setup_header (kernel attributes, used by setup)

– .entrytext: start_of_setup – the real mode kernel entry

• Boot sector : the first 512 bytes of setup.bin

– vmlinux.bin

• Protect mode kernel entry + Compressed Linux kernel

Linux kernel image

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bzImage

Code for floppy

.header(setup_header)

Real mode entry:start_of_setup

…

Protect mode entry:startup_32

Compressed kernel:start_kernel

bzImage

Setup.bin

Vmlinux.bin

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Linux Boot process

start_of_setup: arch/x86/boot/header.S

main: arch/x86/boot/main.c

mount rootfs : /init

startup_32: arch/x86/boot/compress/header_32.S

decompress_kernel: arch/x86/boot/compress/misc.c

start_kernel: init/main.c

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• Every field in it has a story– kernel_src/Documentation/x86/boot.txt

• struct boot_params– APM BIOS information

– EFI information

• System table

• Memory map

– Setup header (offset 0x1F1)

– E820 entries

– …

boot_params structure

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• Read boot sector

• Check if it is bzImage

• If no, fail

• Read setup data

• Get the kernel protect mode entry in setup data

• Allocate memory for kernel image

• Copy kernel image to Memory

Loading Linux kernel image

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• create new boot_params for Linux kernel– Allocate memory for boot_params

– Copy kernel command line

– Get setup header from setup data

– Set loader type

– Set the kernel start address in boot_params

– Set initrd start address in boot_params

– Set cmdline address in boot_params

– Set EFI system table pointer in boot_params

– Set Memory Map <- GetMemoryMap()

Initializing boot_params

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• Now we are ready!

• Kernel image/initrd image/cmdline/boot_params are all in memory now

• Jump to kernel image right now? NO!

• Invoke ExitBootServices

• Save boot_params so that the kernel can get it (esi)

• Initialize GDT

• asm volatile ( “movl %0, %%ecx” : : “m” (kernel_entry) );

• asm volatile ( “jmp *%%ecx” : : );

• startup_32() – the protect mode kernel entry

Yeah!

Jumping to Linux

References

• Unified Extensible Firmware Interface Specification 2.3.1

• Understanding Linux kernel, 3rd Edition

• http://en.wikipedia.org/wiki/Bootloader#Boot_loader

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Q&A