linux 内核启动过程

For x86 64bits architecture platform, linux-2.6.35.4

1. arch/x86/boot/header.S: the first instruction that runs in kernel image in real mode is at label "_start", then call "start_of_setup", then call "main".
2. arch/x86/boot/main.c: "main" function is called by "start_of_setup", after "main" doing lots of operation, at last, call "go_to_protected_mode", 
3. arch/x86/boot/pm.c: "go_to_protected_mode" is defined here, it will call "protected_mode_jump".
4. arch/x86/boot/pmjump.S: "protected_mode_jump" is in this file. After executing this function, the processor is in protected mode.
5. arch/x86/boot/compressed/head_64.S: the entry point is "startup_32", call "decompress_kernel"
6. arch/x86/boot/compressed/misc.c: executing "decompress_kernel".
7. arch/x86/boot/compressed/head_64.S: jump to the decompressed kernel entry point "startup_64"
8. arch/x86/kernel/head_64.S: start to execute at label "startup_64", then call "x86_64_start_kernel".
9. arch/x86/kernel/head64.c: executing "x86_64_start_kernel", it call "x86_64_start_reservations", "x86_64_start_reservations" call "start_kernel".
10. init/main.c: start executing "start_kernel".

In find init-mm.c:
The kernel static define struct mm_struct init_mm, which belongs kernel, kernel accessing physical address:
pgd_t *pgd = init_mm->pgd + pgd_index(addr);
pud_t *pud = pgd + pud_index(addr);
pmd_t *pmd = pud + pmd_index(addr);
pte_t *pte = pmd + pte_index(addr);

// physical_addr the physical address of pfn responding to addr.
long physical_addr = pte_val(*pte) & PAGE_SHIFT;

struct mm_struct init_mm = {
    .mm_rb        = RB_ROOT,
    .pgd        = swapper_pg_dir,
    .mm_users    = ATOMIC_INIT(2),
    .mm_count    = ATOMIC_INIT(1),
    .mmap_sem    = __RWSEM_INITIALIZER(init_mm.mmap_sem),
    .page_table_lock = __SPIN_LOCK_UNLOCKED(init_mm.page_table_lock),
    .mmlist        = LIST_HEAD_INIT(init_mm.mmlist),
    .cpu_vm_mask    = CPU_MASK_ALL,
};

IN pgtable_64.h:

#define swapper_pg_dir init_level4_pgt

init_level4_pgt is a global variable defined in head_64.S, it is in data segmentation of kernel image.

/* Automate the creation of 1 to 1 mapping pmd entries */
#define PMDS(START, PERM, COUNT)            \
    i = 0 ;                        \
    .rept (COUNT) ;                    \
    .quad    (START) + (i << PMD_SHIFT) + (PERM) ;    \
    i = i + 1 ;                    \
    .endr





    .data
    /*
     * This default setting generates an ident mapping at address 0x100000
     * and a mapping for the kernel that precisely maps virtual address
     * 0xffffffff80000000 to physical address 0x000000. (always using
     * 2Mbyte large pages provided by PAE mode)
     */
NEXT_PAGE(init_level4_pgt)
    .quad    level3_ident_pgt - __START_KERNEL_map + _KERNPG_TABLE
    .org    init_level4_pgt + L4_PAGE_OFFSET*8, 0
    .quad    level3_ident_pgt - __START_KERNEL_map + _KERNPG_TABLE
    .org    init_level4_pgt + L4_START_KERNEL*8, 0
    /* (2^48-(2*1024*1024*1024))/(2^39) = 511 */
    .quad    level3_kernel_pgt - __START_KERNEL_map + _PAGE_TABLE

NEXT_PAGE(level3_ident_pgt)
    .quad    level2_ident_pgt - __START_KERNEL_map + _KERNPG_TABLE
    .fill    511,8,0

NEXT_PAGE(level3_kernel_pgt)
    .fill    L3_START_KERNEL,8,0
    /* (2^48-(2*1024*1024*1024)-((2^39)*511))/(2^30) = 510 */
    .quad    level2_kernel_pgt - __START_KERNEL_map + _KERNPG_TABLE
    .quad    level2_fixmap_pgt - __START_KERNEL_map + _PAGE_TABLE

NEXT_PAGE(level2_fixmap_pgt)
    .fill    506,8,0
    .quad    level1_fixmap_pgt - __START_KERNEL_map + _PAGE_TABLE
    /* 8MB reserved for vsyscalls + a 2MB hole = 4 + 1 entries */
    .fill    5,8,0

NEXT_PAGE(level1_fixmap_pgt)
    .fill    512,8,0

NEXT_PAGE(level2_ident_pgt)
    /* Since I easily can, map the first 1G.
     * Don

NEXT_PAGE(level2_kernel_pgt)
    /*
     * 512 MB kernel mapping. We spend a full page on this pagetable
     * anyway.
     *
     * The kernel code+data+bss must not be bigger than that.
     *
     * (NOTE: at +512MB starts the module area, see MODULES_VADDR.
     *  If you want to increase this then increase MODULES_VADDR
     *  too.)
     */
    PMDS(0, __PAGE_KERNEL_LARGE_EXEC,
        KERNEL_IMAGE_SIZE/PMD_SIZE)

NEXT_PAGE(level2_spare_pgt)
    .fill   512, 8, 0


In function start_kernel(), call

mm_init_owner(&init_mm, &init_task);

to assign init_mm to init_task.


In head_64.S:

    /* Setup early boot stage 4 level pagetables. */
    movq    $(init_level4_pgt - __START_KERNEL_map), %rax
    addq    phys_base(%rip), %rax
    movq    %rax, %cr3

The address of kernel page table is loaded to cr3 register.