Linux Kernel Boot Process

• BIOS loads Linux Kernel to memory and split to 2 parts:
  • Real-mode kernel: Below 640K of memory
  • Proctected-mode kernel: Start from 1MB of memory

Real-mode kernel:

• http://lxr.linux.no/#linux+v2.6.25.6/arch/x86/boot/header.S#L110

  • Real-mode kernel will be start by BIOS at _start:
  • It will jump directly to start_of_setup:

• http://lxr.linux.no/#linux+v2.6.25.6/arch/x86/boot/header.S#L229

  • _start jumps to start_of_setup:
  • Reset disk controller
  • Setup stack
  • Zero bss (https://en.wikipedia.org/wiki/.bss)
  • Call to a C main()

• http://lxr.linux.no/#linux+v2.6.25.6/arch/x86/boot/main.c#L122

  • start_of_setup: calls main()
  • Prepare/copy boot parameters
  • Initilize memory heap
  • Detect memory layout
  • Query MCA
  • Set video mode
  • Invoke method go_to_protected_mode()

• http://lxr.linux.no/#linux+v2.6.25.6/arch/x86/boot/pm.c#L153

  • main() calls go_to_protected_mode()
  • The method purpose is used to prepare steps before switch to protected mode
  • Switch hook
  • Move compressed kernel to correct place
  • Resejt co-processors
  • Setup IDT https://en.wikipedia.org/wiki/Interrupt_descriptor_table
  • Setup GDT https://en.wikipedia.org/wiki/Global_Descriptor_Table
  • Jump to protected_mode_jump:

Protected-mode:

• http://lxr.linux.no/#linux+v2.6.25.6/arch/x86/boot/pmjump.S#L31

  • go_to_protected_mode() calls protected_mode_jump:
  • Prepare/copy boot parameters
  • Enable protected mode
  • Disable paging https://en.wikipedia.org/wiki/Memory_paging
  • Jump to startup_32:

• http://lxr.linux.no/#linux+v2.6.25.6/arch/x86/boot/compressed/head_32.S#L35

  • protected_mode_jump: jumps to startup_32:
  • Doing some stuffs
  • Call to decompress_kernel()
  • Clears the bss segment
  • Setup final GDT https://en.wikipedia.org/wiki/Global_Descriptor_Table
  • Enables paging https://en.wikipedia.org/wiki/Memory_paging
  • Initializes a stack
  • Setup final IDT https://en.wikipedia.org/wiki/Interrupt_descriptor_table
  • Call to start_kernel()

• http://lxr.linux.no/#linux+v2.6.25.6/arch/x86/boot/compressed/misc.c#L368

  • startup_32: calls to decompress_kernel()
  • Uncompressed kernel will replace at place of existed compressed kernel. Start at 1MB

• http://lxr.linux.no/#linux+v2.6.25.6/init/main.c#L507

  • startup_32: calls to start_kernel()
  • Set PID = 0
  • Initialize stuffs, such as: scheduler, memory zones, time keeping, cpu, cgroup
  • Call to rest_init()

• http://lxr.linux.no/#linux+v2.6.25.6/init/main.c#L432

  • start_kernel() calls to rest_init()
  • Create another thread to call kernel_init()
  • Create scheduling
  • Sleep by call cpu_idle()

• http://lxr.linux.no/linux+v2.6.25.6/init/main.c#L808

  • start_kernel() create thread and call to kernel_init()
  • Set PID = 1
  • Init the remaining CPUs
  • Init work queue
  • Init user mode
  • Init driver
  • Call to init_post()

• http://lxr.linux.no/#linux+v2.6.25.6/init/main.c#L769

  • kernel_init() calls to init_post()
  • Execute /sbin/init, /etc/init, /bin/init, and /bin/sh in order