本文以ARM为例，深入探讨Linux内核3.2版本的启动过程。从arch/arm/kernel/head.s开始，讲解了enable_mmu、page tables的创建以及如何进入C环境的详细步骤。
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以arm为例，分析一下kernel的启动过程;
内核版本：linux-3.2.tar.gz
一、arch/arm/kernel/head.s
1
19 .arm
20
21 __HEAD @#define __HEAD.section ".head.text","ax"
22 ENTRY(stext)
23
24 THUMB( adr r9, BSYM(1f) ) @ Kernel is always entered in ARM.
25 THUMB( bx r9 ) @ If this is a Thumb-2 kernel,
26 THUMB( .thumb ) @ switch to Thumb now.
27 THUMB(1: )
28
29 setmode PSR_F_BIT | PSR_I_BIT | SVC_MODE, r9 @ ensure svc mode 关闭普通中断，快速中断，使能svc模式
30 @ and irqs disabled
31 mrc p15, 0, r9, c0, c0 @ get processor id 获得芯片ID
32 bl __lookup_processor_type @ r5=procinfo r9=cpuid 获得处理器型号，r5 == id，#1
33 movs r10, r5 @ invalid processor (r5=0)? 校验正确性，0错误
34 THUMB( it eq ) @ force fixup-able long branch encoding
35 beq __error_p @ yes, error 'p'
36
37 #ifndef CONFIG_XIP_KERNEL
38 adr r3, 2f
39 ldmia r3, {r4, r8}
40 sub r4, r3, r4 @ (PHYS_OFFSET - PAGE_OFFSET)
41 add r8, r8, r4 @ PHYS_OFFSET
42 #else
43 ldr r8, =PHYS_OFFSET @ always constant in this case #define PHYS_OFFSETUL(CONFIG_DRAM_BASE)
44 #endif
#1 :arch/arm/kernel/head-common.h
1
14 __CPUINIT
15 __lookup_processor_type:
16 adr r3, __lookup_processor_type_data @adr 相对偏移读取，读取下面type_data地址
17 ldmia r3, {r4 - r6} @将该地址存放的值 放入r4(.)，r5(begin)，r6(end)
18 sub r3, r3, r4 @ get offset between virt&phys 链接地址-实际地址=偏移量
19 add r5, r5, r3 @ convert virt addresses to 得到begin的虚拟起始地址
20 add r6, r6, r3 @ physical address space 得到end的虚拟起始地址
21 1: ldmia r5, {r3, r4} @ value, mask 得到r5的两个值到r3，r4
22 and r4, r4, r9 @ mask wanted bits 确定id值
23 teq r3, r4
24 beq 2f @相等返回，不等读取下一组
25 add r5, r5, #PROC_INFO_SZ @ sizeof(proc_info_list)
26 cmp r5, r6
27 blo 1b
28 mov r5, #0 @ unknown processor @没有找到返回r5==0
29 2: mov pc, lr
30 ENDPROC(__lookup_processor_type)
31
32
35 .align 2
36 .type __lookup_processor_type_data, %object
37 __lookup_processor_type_data:
38 .long .
39 .long __proc_info_begin
40 .long __proc_info_end
41 .size __lookup_processor_type_data, . - __lookup_processor_type_data
42
43 __error_p:
44 #ifdef CONFIG_DEBUG_LL
45 adr r0, str_p1
46 bl printascii
47 mov r0, r9
48 bl printhex8
49 adr r0, str_p2
50 bl printascii
51 b __error
52 str_p1: .asciz "\nError: unrecognized/unsupported processor variant (0x"
53 str_p2: .asciz ").\n"
54 .align
55 #endif
56 ENDPROC(__error_p)
57
58 __error:
59 #ifdef CONFIG_ARCH_RPC
60
63 mov r0, #0x02000000
64 mov r3, #0x11
65 orr r3, r3, r3, lsl #8
66 orr r3, r3, r3, lsl #16
67 str r3, [r0], #4
68 str r3, [r0], #4
69 str r3, [r0], #4
70 str r3, [r0], #4
71 #endif
继续arch/arm/kernel/head.s
1
5 bl __vet_atags @#1,head-common.s
6 #ifdef CONFIG_SMP_ON_UP
7 bl __fixup_smp @略
8 #endif
9 #ifdef CONFIG_ARM_PATCH_PHYS_VIRT
10 bl __fixup_pv_table @略
11 #endif
12 bl __create_page_tables @#2
#1 :arch/arm/kernel/head-common.h
1
12 __vet_atags:
13 tst r2, #0x3 @ aligned? 不对其就返回
14 bne 1f
15
16 ldr r5, [r2, #0] @读到r5
17 #ifdef CONFIG_OF_FLATTREE
18 ldr r6, =OF_DT_MAGIC @ is it a DTB? 过滤DTB
19 cmp r5, r6
20 beq 2f
21 #endif
22 cmp r5, #ATAG_CORE_SIZE @ is first tag ATAG_CORE? must be first
23 cmpne r5, #ATAG_CORE_SIZE_EMPTY
24 bne 1f
25 ldr r5, [r2, #4]
26 ldr r6, =ATAG_CORE
27 cmp r5, r6
28 bne 1f
29
30 2: mov pc, lr @ atag/dtb pointer is ok
31
32 1: mov r2, #0
33 mov pc, lr
34 ENDPROC(__vet_atags)
@#2：__create_page_tables
1
12 __create_page_tables:
13 pgtbl r4, r8 @ page table address
@#define PG_DIR_SIZE
#define PMD_ORDER
@vmlinux.lds.s,#define LOAD_OFFSET TEXT_OFFSET,0x30008000?
@.macropgtbl, rd, phys
@ add \rd, \phys, #TEXT_OFFSET - PG_DIR_SIZE r4= offset+ 0x30008000-0x4000,page_dir_size = 0x4000 = 16k
@.endm
14
15
18 mov r0, r4 @r0==r4
19 mov r3, #0 @r3==0
20 add r6, r0, #PG_DIR_SIZE @r6==page_dir_end
21 1: str r3, [r0], #4 @clear 0,四个一组
22 str r3, [r0], #4
23 str r3, [r0], #4
24 str r3, [r0], #4
25 teq r0, r6
26 bne 1b @不到结尾返回继续
27
28 ldr r7, [r10, #PROCINFO_MM_MMUFLAGS] @ mm_mmuflags @load mmuflags
29
30
34 adr r0, __enable_mmu_loc
35 ldmia r0, {r3, r5, r6}
36 sub r0, r0, r3 @ virt->phys offset 重新计算得到偏移地址，应当是负值，r0位置无关地址，r3链接地址
37 add r5, r5, r0 @ __enable_mmu 重新得到实际物理地址
38 add r6, r6, r0 @ __enable_mmu_end
39 mov r5, r5, lsr #SECTION_SHIFT @1M对齐
40 mov r6, r6, lsr #SECTION_SHIFT
41
42 1: orr r3, r7, r5, lsl #SECTION_SHIFT @ flags + kernel base 从enable_mmu地址开始映射，1M为单位，直接映射
@r5为enable_mmu物理地址
43 str r3, [r4, r5, lsl #PMD_ORDER] @ identity mapping 括号内可得物理地址表项，物理地址对应表项映射回相应地址
44 cmp r5, r6 @ 直到 mmu_end
45 addlo r5, r5, #1 @ next section
46 blo 1b
47
48
52 mov r3, pc @当前pc存入r3
53 mov r3, r3, lsr #SECTION_SHIFT @1M对齐
54 orr r3, r7, r3, lsl #SECTION_SHIFT @kernel映射，0xc...虚拟地址映射到当前pc物理地址
55 add r0, r4, #(KERNEL_START & 0xff000000) >> (SECTION_SHIFT - PMD_ORDER)
56 str r3, [r0, #((KERNEL_START & 0x00f00000) >> SECTION_SHIFT) << PMD_ORDER]!
57 ldr r6, =(KERNEL_END - 1)
58 add r0, r0, #1 << PMD_ORDER
59 add r6, r4, r6, lsr #(SECTION_SHIFT - PMD_ORDER)
60 1: cmp r0, r6
61 add r3, r3, #1 << SECTION_SHIFT
62 strls r3, [r0], #1 << PMD_ORDER
1 .ltorg
2 .align
3 __enable_mmu_loc:
4 .long .
5 .long __enable_mmu
6 .long __enable_mmu_end
1
5 mov r0, r2, lsr #SECTION_SHIFT
6 movs r0, r0, lsl #SECTION_SHIFT @清空了r0 1m
7 moveq r0, r8 @如果是0，也就是没有指定boot params则放入r8，即内存其实地址0x3------
8 sub r3, r0, r8 @得到r0与r8的差，如果没指定则差为0，如果指定了得出个偏移
9 add r3, r3, #PAGE_OFFSET @差值+内存基地址，得到实际参数地址
10 add r3, r4, r3, lsr #(SECTION_SHIFT - PMD_ORDER) @做映射，直接映射，无偏移
11 orr r6, r7, r0
12 str r6, [r3]
3.0选择性将enablemmu和boot params双映射，并不是以前机械的前8M双映射
1 mov pc, lr
回到
1
8 ldr r13, =__mmap_switched @ address to jump to after
9 @ mmu has been enabled
10 adr lr, BSYM(1f) @ return (PIC) address
11 mov r8, r4 @ set TTBR1 to swapper_pg_dir
12 ARM( add pc, r10, #PROCINFO_INITFUNC )
13 THUMB( add r12, r10, #PROCINFO_INITFUNC )
14 THUMB( mov pc, r12 )
15 1: b __enable_mmu
进入enable_mmu
1
13 __enable_mmu:
14 #if defined(CONFIG_ALIGNMENT_TRAP) && __LINUX_ARM_ARCH__ < 6
15 orr r0, r0, #CR_A
16 #else
17 bic r0, r0, #CR_A
18 #endif
19 #ifdef CONFIG_CPU_DCACHE_DISABLE
20 bic r0, r0, #CR_C
21 #endif
22 #ifdef CONFIG_CPU_BPREDICT_DISABLE
23 bic r0, r0, #CR_Z
24 #endif
25 #ifdef CONFIG_CPU_ICACHE_DISABLE
26 bic r0, r0, #CR_I
27 #endif
28 mov r5, #(domain_val(DOMAIN_USER, DOMAIN_MANAGER) | \
29 domain_val(DOMAIN_KERNEL, DOMAIN_MANAGER) | \
30 domain_val(DOMAIN_TABLE, DOMAIN_MANAGER) | \
31 domain_val(DOMAIN_IO, DOMAIN_CLIENT))
32 mcr p15, 0, r5, c3, c0, 0 @ load domain access register
33 mcr p15, 0, r4, c2, c0, 0 @ load page table pointer
34 b __turn_mmu_on
35 ENDPROC(__enable_mmu)
1
15 .align 5
16 __turn_mmu_on:
17 mov r0, r0
18 mcr p15, 0, r0, c1, c0, 0 @ write control reg
19 mrc p15, 0, r3, c0, c0, 0 @ read id reg
20 mov r3, r3
21 mov r3, r13 //跳回r13，=__mmap_switched
22 mov pc, r3
23 __enable_mmu_end:
24 ENDPROC(__turn_mmu_on)
#arch/arm/kernel/head-common.s
1
10 __INIT
11 __mmap_switched:
12 adr r3, __mmap_switched_data
13
14 ldmia r3!, {r4, r5, r6, r7}
15 cmp r4, r5 @ Copy data segment if needed
16 1: cmpne r5, r6
17 ldrne fp, [r4], #4
18 strne fp, [r5], #4
19 bne 1b
20
21 mov fp, #0 @ Clear BSS (and zero fp)
22 1: cmp r6, r7
23 strcc fp, [r6],#4
24 bcc 1b
25
26 ARM( ldmia r3, {r4, r5, r6, r7, sp})
27 THUMB( ldmia r3, {r4, r5, r6, r7} )
28 THUMB( ldr sp, [r3, #16] )
29 str r9, [r4] @ Save processor ID
30 str r1, [r5] @ Save machine type
31 str r2, [r6] @ Save atags pointer
32 bic r4, r0, #CR_A @ Clear 'A' bit
33 stmia r7, {r0, r4} @ Save control register values
34 b start_kernel
35 ENDPROC(__mmap_switched)
绝对地址操作，copy data，bss清空，跳入start_kernel，进入c环境。
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