参阅资料：
《ARM体系结构与编程》
《嵌入式Linux运用开发彻底手册》
Linux_Memory_Address_Mapping
http://www.chinaunix.net/old_jh/4/1021226.html
更多文档拜见：http://pan.baidu.com/s/1mg3DbHQ
本文针对arm linux, 从kernel的第一条指令开端剖析,一向剖析到进入start_kernel()函数.
咱们当时以linux-2.6.19内核版别作为典范来剖析,本文中一切的代码,前面都会加上行号以便于和源码进行对照,
例:
在文件init/main.c中:
00478: asmlinkage void __init start_kernel(void)
前面的”00478:” 标明478行,冒号后边的内容便是源码了.
在剖析代码的过程中,咱们运用缩进来标明各个代码的调用层次.
由于发动部分有一些代码是渠道特定的,尽管大部分的渠道所完结的功用都比较相似,可是为了更好的对code进行阐明,关于渠道相关的代码,咱们挑选at91(ARM926EJS)渠道进行剖析.
别的,本文是以uncompressed kernel开端解说的.关于内核解压缩部分的code,在 arch/arm/boot/compressed中,本文不做评论.
一. 发动条件
一般从体系上电到履行到linux kenel这部分的使命是由boot loader来完结.
关于boot loader的内容,本文就不做过多介绍.
这儿只评论进入到linux kernel的时分的一些约束条件,这一般是boot loader在终究跳转到kernel之前要完结的:
1. CPU有必要处于SVC(supervisor)形式,而且IRQ和FIQ中止都是制止的;
2. MMU(内存办理单元)有必要是封闭的, 此刻虚拟地址对物理地址;
3. 数据cache(Data cache)有必要是封闭的
4. 指令cache(Instruction cache)可所以翻开的,也可所以封闭的,这个没有强制要求;
5. CPU 通用寄存器0 (r0)有必要是 0;
6. CPU 通用寄存器1 (r1)有必要是 ARM Linux machine type (关于machine type, 咱们后边会有解说)
7. CPU 通用寄存器2 (r2) 有必要是 kernel parameter list 的物理地址(parameter list 是由boot loader传递给kernel,用来描绘设备信息特点的列表,具体内容可参阅”Booting ARM Linux”文档).
二. starting kernel
首要，咱们先对几个重要的宏进行阐明(咱们针对有MMU的状况)：
宏 方位 默认值 阐明
KERNEL_RAM_ADDR arch/arm/kernel/head.S +26 0xc8 kernel在RAM中的的虚拟地址
PAGE_OFFSET include/asm-arm/memeory.h +50 0xc0 内核空间的开端虚拟地址
TEXT_OFFSET arch/arm/Makefile +137 0x08 内核相关于存储空间的偏移
TEXTADDR arch/arm/kernel/head.S +49 0xc8 kernel的开端虚拟地址
PHYS_OFFSET include/asm-arm/arch-xxx/memory.h 渠道相关 RAM的开端物理地址
内核的进口是stext,这是在arch/arm/kernel/vmlinux.lds.S中界说的:
11: ENTRY(stext)
关于vmlinux.lds.S,这是ld script文件,此文件的格局和汇编及C程序都不同,本文不对ld script作过多的介绍,只对内核中用到的内容进行解说,关于ld的具体内容能够参阅ld.info
这儿的ENTRY(stext) 标明程序的进口是在符号stext.
而符号stext是在arch/arm/kernel/head.S中界说的:
下面咱们将arm linux boot的首要代码列出来进行一个归纳的介绍,然后,咱们会逐一的进行具体的解说.
在arch/arm/kernel/head.S中 72 – 94 行,是arm linux boot的主代码:
72: ENTRY(stext)
73: msr cpsr_c, #PSR_F_BIT PSR_I_BIT SVC_MODE @ ensure svc mode
74: @ and irqs disabled
75: mrc p15, 0, r9, c0, c0 @ get processor id
76: bl __lookup_processor_type @ r5=procinfo r9=cpuid
77: movs r10, r5 @ invalid processor (r5=0)?
78: beq __error_p @ yes, error p
79: bl __lookup_machine_type @ r5=machinfo
80: movs r8, r5 @ invalid machine (r5=0)?
81: beq __error_a @ yes, error a
82: bl __create_page_tables
83:
84: /*
85: * The following calls CPU specific code in a position independent
86: * manner. See arch/arm/mm/proc-*.S for details. r10 = base of
87: * xxx_proc_info structure selected by __lookup_machine_type
88: * above. On return, the CPU will be ready for the MMU to be
89: * turned on, and r0 will hold the CPU control register value.
90: */
91: ldr r13, __switch_data @ address to jump to after
92: @ mmu has been enabled
93: adr lr, __enable_mmu @ return (PIC) address
94: add pc, r10, #PROCINFO_INITFUNC
其间,73行是确保kernel运转在SVC形式下,而且IRQ和FIRQ中止现已封闭,这样做是很慎重的.
arm linux boot的主线能够归纳为以下几个过程:
1. 确认 processor type (75 – 78行)
2. 确认 machine type (79 – 81行)
3. 创立页表 (82行)
4. 调用渠道特定的__cpu_flush函数 (在struct proc_info_list中) (94 行)
5. 敞开mmu (93行)
6. 切换数据 (91行)
终究跳转到start_kernel (在__switch_data的完毕的时分,调用了 b start_kernel)
下面,咱们依照这个主线,逐渐的剖析Code.
1. 确认 processor type
arch/arm/kernel/head.S中:
75: mrc p15, 0, r9, c0, c0 @ get processor id
76: bl __lookup_processor_type @ r5=procinfo r9=cpuid
77: movs r10, r5 @ invalid processor (r5=0)?
78: beq __error_p @ yes, error p
75行: 经过cp15协处理器的c0寄存器来获得processor id的指令. 关于cp15的具体内容可参阅相关的arm手册
76行: 跳转到__lookup_processor_type.在__lookup_processor_type中,会把processor type 存储在r5中
77,78行: 判别r5中的processor type是否是0,假如是0,阐明是无效的processor type,跳转到__error_p(犯错)
__lookup_processor_type 函数首要是依据从cpu中获得的processor id和体系中的proc_info进行匹配,将匹配到的proc_info_list的基地址存到r5中, 0标明没有找到对应的processor type.
下面咱们剖析__lookup_processor_type函数
arch/arm/kernel/head-common.S中:
00145: .type __lookup_processor_type, %function
00146: __lookup_processor_type:
00147: adr r3, 3f
00148: ldmda r3, {r5 – r7}
00149: sub r3, r3, r7 @ get offset between virt&phys
00150: add r5, r5, r3 @ convert virt addresses to
00151: add r6, r6, r3 @ physical address space
00152: 1: ldmia r5, {r3, r4} @ value, mask
00153: and r4, r4, r9 @ mask wanted bits
00154: teq r3, r4
00155: beq 2f
00156: add r5, r5, #PROC_INFO_SZ @ sizeof(proc_info_list)
00157: cmp r5, r6
00158: blo 1b
00159: mov r5, #0 @ unknown processor
00160: 2: mov pc, lr
00161:
00162: /*
00163: * This provides a C-API version of the above function.
00164: */
00165: ENTRY(lookup_processor_type)
00166: stmfd sp!, {r4 – r7, r9, lr}
00167: mov r9, r0
00168: bl __lookup_processor_type
00169: mov r0, r5
00170: ldmfd sp!, {r4 – r7, r9, pc}
00171:
00172: /*
00173: * Look in include/asm-arm/procinfo.h and arch/arm/kernel/arch.[ch] for
00174: * more information about the __proc_info and __arch_info structures.
00175: */
00176: .long __proc_info_begin
00177: .long __proc_info_end
00178: 3: .long .
00179: .long __arch_info_begin
00180: .long __arch_info_end
145, 146行是函数界说
147行: 取地址指令,这儿的3f是向前symbol称号是3的方位,即第178行,将该地址存入r3.
这儿需求留意的是,adr指令取址,获得的是依据pc的一个地址,要分外留意,这个地址是3f处的”运转时地址”, 由于此刻MMU还没有翻开,也能够了解成物理地址(实地址).(具体内容可参阅arm指令手册)
148行: 由于r3中的地址是178行的方位的地址,因此履行完后: (ldmda标明栈指针递减，即r3递减，内存的地址编号较大的对应寄存器编号较大的)
r5存的是176行符号 __proc_info_begin的地址;
r6存的是177行符号 __proc_info_end的地址;
r7存的是3f处的地址.
这儿需求留意链接地址和运转时地址的差异. r3存储的是运转时地址(物理地址),而r7中存储的是链接地址(虚拟地址).
__proc_info_begin和__proc_info_end是在arch/arm/kernel/vmlinux.lds.S中:
31: __proc_info_begin = .;
32: *(.proc.info.init)
33: __proc_info_end = .;
这儿是声明晰两个变量:__proc_info_begin 和 __proc_info_end,其间等号后边的”.”是location counter(具体内容请参阅ld.info)
这三行的意思是: __proc_info_begin 的方位上,放置一切文件中的 “.proc.info.init” 段的内容,然后紧接着是 __proc_info_end 的方位.
kernel 运用struct proc_info_list来描绘processor type.
在 include/asm-arm/procinfo.h 中:
29: struct proc_info_list {
30: unsigned int cpu_val;
31: unsigned int cpu_mask;
32: unsigned long __cpu_mm_mmu_flags; /* used by head.S */
33: unsigned long __cpu_io_mmu_flags; /* used by head.S */
34: unsigned long __cpu_flush; /* used by head.S */
35: const char *arch_name;
36: const char *elf_name;
37: unsigned int elf_hwcap;
38: const char *cpu_name;
39: struct processor *proc;
40: struct cpu_tlb_fns *tlb;
41: struct cpu_user_fns *user;
42: struct cpu_cache_fns *cache;
43: };
咱们当时以at91为例,其processor是926的.
在arch/arm/mm/proc-arm926.S 中:
00464: .section “.proc.info.init”, #alloc, #execinstr
00465:
00466: .type __arm926_proc_info,#object
00467: __arm926_proc_info:
00468: .long 0x41069260 @ ARM926EJ-S (v5TEJ)
00469: .long 0xff0ffff0
00470: .long PMD_TYPE_SECT \
00471: PMD_SECT_BUFFERABLE \
00472: PMD_SECT_CACHEABLE \
00473: PMD_BIT4 \
00474: PMD_SECT_AP_WRITE \
00475: PMD_SECT_AP_READ
00476: .long PMD_TYPE_SECT \
00477: PMD_BIT4 \
00478: PMD_SECT_AP_WRITE \
00479: PMD_SECT_AP_READ
00480: b __arm926_setup
00481: .long cpu_arch_name
00482: .long cpu_elf_name
00483: .long HWCAP_SWPHWCAP_HALFHWCAP_THUMBHWCAP_FAST_MULTHWCAP_VFPHWCAP_EDSPHWCAP_JAVA
00484: .long cpu_arm926_name
00485: .long arm926_processor_functions
00486: .long v4wbi_tlb_fns
00487: .long v4wb_user_fns
00488: .long arm926_cache_fns
00489: .size __arm926_proc_info, . – __arm926_proc_info
从464行,咱们能够看到 __arm926_proc_info 被放到了”.proc.info.init”段中.
对照struct proc_info_list,咱们能够看到 __cpu_flush的界说是在480行,即__arm926_setup.(咱们将在”4. 调用渠道特定的__cpu_flush函数”一节中具体剖析这部分的内容.)
从以上的内容咱们能够看出: r5中的__proc_info_begin是proc_info_list的开端地址, r6中的__proc_info_end是proc_info_list的完毕地址.
149行: 从上面的剖析咱们能够知道r3中存储的是3f处的物理地址,而r7存储的是3f处的虚拟地址,这一行是核算当时程序运转的物理地址和虚拟地址的差值,将其保存到r3中.
150行: 将r5存储的虚拟地址(__proc_info_begin)转换成物理地址
151行: 将r6存储的虚拟地址(__proc_info_end)转换成物理地址
152行: 对照struct proc_info_list,能够得知,这句是将当时proc_info的cpu_val和cpu_mask别离存r3, r4中
153行: r9中存储了processor id(arch/arm/kernel/head.S中的75行),与r4的cpu_mask进行逻辑与操作,得到咱们需求的值
154行: 将153行中得到的值与r3中的cpu_val进行比较
155行: 假如持平,阐明咱们找到了对应的processor type,跳到160行,回来
156行: (假如不持平) , 将r5指向下一个proc_info,
157行: 和r6比较,查看是否到了__proc_info_end.
158行: 假如没有到__proc_info_end,标明还有proc_info装备,回来152行持续查找
159行: 履行到这儿,阐明一切的proc_info都匹配过了,可是没有找到匹配的,将r5设置成0(unknown processor)
160行: 回来
2. 确认 machine type
arch/arm/kernel/head.S中:
79: bl __lookup_machine_type @ r5=machinfo
80: movs r8, r5 @ invalid machine (r5=0)?
81: beq __error_a @ yes, error a
79行: 跳转到__lookup_machine_type函数,在__lookup_machine_type中,会把struct machine_desc的基地址(machine type)存储在r5中
80,81行: 将r5中的 machine_desc的基地址存储到r8中,并判别r5是否是0,假如是0,阐明是无效的machine type,跳转到__error_a(犯错)
__lookup_machine_type 函数
下面咱们剖析__lookup_machine_type 函数:
arch/arm/kernel/head-common.S中:
00176: .long __proc_info_begin
00177: .long __proc_info_end
00178: 3: .long .
00179: .long __arch_info_begin
00180: .long __arch_info_end
00181:
00182: /*
00183: * Lookup machine architecture in the linker-build list of architectures.
00184: * Note that we cant use the absolute addresses for the __arch_info
00185: * lists since we arent running with the MMU on (and therefore, we are
00186: * not in the correct address space). We have to calculate the offset.
00187: *
00188: * r1 = machine architecture number
00189: * Returns:
00190: * r3, r4, r6 corrupted
00191: * r5 = mach_info pointer in physical address space
00192: */
00193: .type __lookup_machine_type, %function
00194: __lookup_machine_type:
00195: adr r3, 3b
00196: ldmia r3, {r4, r5, r6}
00197: sub r3, r3, r4 @ get offset between virt&phys
00198: add r5, r5, r3 @ convert virt addresses to
00199: add r6, r6, r3 @ physical address space
00200: 1: ldr r3, [r5, #MACHINFO_TYPE] @ get machine type
00201: teq r3, r1 @ matches loader number?
00202: beq 2f @ found
00203: add r5, r5, #SIZEOF_MACHINE_DESC @ next machine_desc
00204: cmp r5, r6
00205: blo 1b
00206: mov r5, #0 @ unknown machine
00207: 2: mov pc, lr
193, 194行: 函数声明
195行: 取地址指令,这儿的3b是向后symbol称号是3的方位,即第178行,将该地址存入r3.
和上面咱们对__lookup_processor_type 函数的剖析相同,r3中寄存的是3b处物理地址.
196行: r3是3b处的地址,因此履行完后:（ldmia 标明栈是递加的，即r3递加，低内存地址对应小号寄存器）
r4存的是 3b处的地址
r5存的是__arch_info_begin 的地址
r6存的是__arch_info_end 的地址
__arch_info_begin 和 __arch_info_end是在 arch/arm/kernel/vmlinux.lds.S中:
34: __arch_info_begin = .;
35: *(.arch.info.init)
36: __arch_info_end = .;
这儿是声明晰两个变量:__arch_info_begin 和 __arch_info_end,其间等号后边的”.”是location counter(具体内容请参阅ld.info)
这三行的意思是: __arch_info_begin 的方位上,放置一切文件中的 “.arch.info.init” 段的内容,然后紧接着是 __arch_info_end 的方位.
kernel 运用struct machine_desc 来描绘 machine type.
在 include/asm-arm/mach/arch.h 中:
17: struct machine_desc {
18: /*
19: * Note! The first four elements are used
20: * by assembler code in head-armv.S
21: */
22: unsigned int nr; /* architecture number */
23: unsigned int phys_io; /* start of physical io */
24: unsigned int io_pg_offst; /* byte offset for io
25: * page tabe entry */
26:
27: const char *name; /* architecture name */
28: unsigned long boot_params; /* tagged list */
29:
30: unsigned int video_start; /* start of video RAM */
31: unsigned int video_end; /* end of video RAM */
32:
33: unsigned int reserve_lp0 :1; /* never has lp0 */
34: unsigned int reserve_lp1 :1; /* never has lp1 */
35: unsigned int reserve_lp2 :1; /* never has lp2 */
36: unsigned int soft_reboot :1; /* soft reboot */
37: void (*fixup)(struct machine_desc *,
38: struct tag *, char **,
39: struct meminfo *);
40: void (*map_io)(void);/* IO mapping function */
41: void (*init_irq)(void);
42: struct sys_timer *timer; /* system tick timer */
43: void (*init_machine)(void);
44: };
45:
46: /*
47: * Set of macros to define architecture features. This is built into
48: * a table by the linker.
49: */
50: #define MACHINE_START(_type,_name) \
51: static const struct machine_desc __mach_desc_##_type \
52: __attribute_used__ \
53: __attribute__((__section__(“.arch.info.init”
)) = { \
54: .nr = MACH_TYPE_##_type, \
55: .name = _name,
56:
57: #define MACHINE_END \
58: };
内核中,一般运用宏MACHINE_START来界说machine type.
关于at91, 在 arch/arm/mach-at91rm9200/board-ek.c 中:
00137: MACHINE_START(AT91RM9200EK, “Atmel AT91RM9200-EK”
00138: /* Maintainer: SAN People/Atmel */
00139: .phys_io = AT91_BASE_SYS,
00140: .io_pg_offst = (AT91_VA_BASE_SYS >> 1
& 0xfffc,
00141: .boot_params = AT91_SDRAM_BASE + 0x100,
00142: .timer = &at91rm9200_timer,
00143: .map_io = ek_map_io,
00144: .init_irq = ek_init_irq,
00145: .init_machine = ek_board_init,
00146: MACHINE_END
197行: r3中存储的是3b处的物理地址,而r4中存储的是3b处的虚拟地址,这儿核算处物理地址和虚拟地址的差值,保存到r3中
198行: 将r5存储的虚拟地址(__arch_info_begin)转换成物理地址
199行: 将r6存储的虚拟地址(__arch_info_end)转换成物理地址
200行: MACHINFO_TYPE 在 arch/arm/kernel/asm-offset.c 101行界说, 这儿是取 struct machine_desc中的nr(architecture number) 到r3中
201行: 将r3中取到的machine type 和 r1中的 machine type(见前面的”发动条件”
进行比较
202行: 假如相同,阐明找到了对应的machine type,跳转到207行的2f处,此刻r5中存储了对应的struct machine_desc的基地址
203行: (不相同), 取下一个machine_desc的地址
204行: 和r6进行比较,查看是否到了__arch_info_end.
205行: 假如不相同,阐明还有machine_desc,回来200行持续查找.
206行: 履行到这儿,阐明一切的machind_desc都查找完了,而且没有找到匹配的, 将r5设置成0(unknown machine).
207行: 回来
3. 创立页表
经过前面的两步,咱们现已确认了processor type 和 machine type.
此刻,一些特定寄存器的值如下所示:
r8 = machine info (struct machine_desc的基地址)
r9 = cpu id (经过cp15协处理器获得的cpu id)
r10 = procinfo (struct proc_info_list的基地址)
创立页表是经过函数 __create_page_tables 来完结的.
这儿,咱们运用的是arm的L1主页表,L1主页表也称为段页表(section page table)
L1 主页表将4 GB 的地址空间分红若干个1 MB的段(section),因此L1页表包括4096个页表项(section entry). 每个页表项是32 bits(4 bytes)
因此L1主页表占用 4096 *4 = 16k的内存空间.
关于ARM926,其L1 section entry的格局为
可参阅arm926EJS TRM):
(一级描绘符的格局 能够参阅《ARM体系结构与编程》P180)
下面咱们来剖析 __create_page_tables 函数:
在 arch/arm/kernel/head.S 中:
00206: .type __create_page_tables, %function
00207: __create_page_tables:
00208: pgtbl r4 @ page table address
00209:
00210: /*
00211: * Clear the 16K level 1 swapper page table
00212: */
00213: mov r0, r4
00214: mov r3, #0
00215: add r6, r0, #0x4
00216: 1: str r3, [r0], #4
00217: str r3, [r0], #4
00218: str r3, [r0], #4
00219: str r3, [r0], #4
00220: teq r0, r6
00221: bne 1b
00:
00223: ldr r7, [r10, #PROCINFO_MM_MMUFLAGS] @ mm_mmuflags
00224:
00225: /*
00226: * Create identity mapping for first MB of kernel to
00227: * cater for the MMU enable. This identity mapping
00228: * will be removed by paging_init(). We use our current program
00229: * counter to determine corresponding section base address.
00230: */
00231: mov r6, pc, lsr #20 @ start of kernel section
00232: orr r3, r7, r6, lsl #20 @ flags + kernel base
00233: str r3, [r4, r6, lsl #2] @ identity mapping
00234:
00235: /*
00236: * Now setup the pagetables for our kernel direct
00237: * mapped region.
00238: */
00239: add r0, r4, #(TEXTADDR & 0xff) >> 18 @ start of kernel
00240: str r3, [r0, #(TEXTADDR & 0x00f00) >> 18]!
00241:
00242: ldr r6, =(_end – PAGE_OFFSET – 1) @ r6 = number of sections
00243: mov r6, r6, lsr #20 @ needed for kernel minus 1
00244:
00245: 1: add r3, r3, #1 << 20
00246: str r3, [r0, #4]!
00247: subs r6, r6, #1
00248: bgt 1b
00249:
00250: /*
00251: * Then map first 1MB of ram in case it contains our boot params.
00252: */
00253: add r0, r4, #PAGE_OFFSET >> 18
00254: orr r6, r7, #PHYS_OFFSET
00255: str r6, [r0]
…
00314: mov pc, lr
00315: .ltorg
206, 207行: 函数声明
208行: 经过宏 pgtbl 将r4设置成页表的基地址(物理地址)
宏pgtbl 在 arch/arm/kernel/head.S 中:
42: .macro pgtbl, rd
43: ldr \rd, =(__virt_to_phys(KERNEL_RAM_ADDR – 0x4))
44: .endm
能够看到,页表是坐落 KERNEL_RAM_ADDR 下面 16k 的方位
宏 __virt_to_phys 是在incude/asm-arm/memory.h 中:
00125: #ifndef __virt_to_phys
00126: #define __virt_to_phys(x) ((x) – PAGE_OFFSET + PHYS_OFFSET)
00127: #define __phys_to_virt(x) ((x) – PHYS_OFFSET + PAGE_OFFSET)
00128: #endif
下面从213行 – 221行, 是将这16k 的页表清0.
213行: r0 = r4, 将页表基地址存在r0中
214行: 将 r3 置成0
215行: r6 = 页表基地址 + 16k, 能够看到这是页表的尾地址
216 – 221 行: 循环,从 r0 到 r6 将这16k页表用0填充.
223行: 获得proc_info_list的__cpu_mm_mmu_flags的值,并存储到 r7中. (宏PROCINFO_MM_MMUFLAGS是在arch/arm/kernel/asm-offset.c中界说，值为8)（能够参阅《嵌入式Linux运用彻底开发手册》P118）（r7的值便是设置这个段描绘符的权限、域字段，）
在arch/arm/mm/proc-arm926.S 中:
00464: .section ".proc.info.init", #alloc, #execinstr
00465:
00466: .type __arm926_proc_info,#object
00467: __arm926_proc_info:
00468: .long 0x41069260 @ ARM926EJ-S (v5TEJ)
00469: .long 0xff0ffff0
00470: .long PMD_TYPE_SECT \
00471: PMD_SECT_BUFFERABLE \
00472: PMD_SECT_CACHEABLE \
00473: PMD_BIT4 \
00474: PMD_SECT_AP_WRITE \
00475: PMD_SECT_AP_READ
00476: .long PMD_TYPE_SECT \
00477: PMD_BIT4 \
00478: PMD_SECT_AP_WRITE \
00479: PMD_SECT_AP_READ
00480: b __arm926_setup
00481: .long cpu_arch_name
00482: .long cpu_elf_name
00483: .long HWCAP_SWPHWCAP_HALFHWCAP_THUMBHWCAP_FAST_MULTHWCAP_VFPHWCAP_EDSPHWCAP_JAVA
00484: .long cpu_arm926_name
00485: .long arm926_processor_functions
00486: .long v4wbi_tlb_fns
00487: .long v4wb_user_fns
00488: .long arm926_cache_fns
00489: .size __arm926_proc_info, . - __arm926_proc_info
231行: 经过pc值的高12位(右移20位),得到kernel的section,并存储到r6中.由于当时是经过运转时地址得到的kernel的section,因此是物理地址.
232行: r3 = r7 (r6 << 20); flags + kernel base,得到页表中需求设置的值.
233行: 设置页表: mem[r4 + r6 * 4] = r3
这儿,由于页表的每一项是32 bits(4 bytes),所以要乘以4(<<2).
上面这三行,设置了kernel的第一个section(物理地址地点的page entry)的页表项
239, 240行: TEXTADDR是内核的开端虚拟地址(0xc8), 这两行是设置kernel开端虚拟地址的页表项(留意,这儿设置的页表项和上面的231 – 233行设置的页表项是不同的 )
履行完后,r0指向kernel的第2个section的虚拟地址地点的页表项.
/* TODO: 这两行的code很古怪,为什么要先取TEXTADDR的高8位(Bit[31:24])0xff,然后再取后边的8位 (Bit[23:20])0x00f00*/
242行: 这一行核算kernel镜像的巨细(bytes).
_end 是在vmlinux.lds.S中162行界说的,符号kernel的完毕方位(虚拟地址):
00158 .bss : {
00159 __bss_start = .; /* BSS */
00160 *(.bss)
00161 *(COMMON)
00162 _end = .;
00163 }
kernel的size = _end – PAGE_OFFSET -1, 这儿 减1的原因是由于 _end 是 location counter,它的地址是kernel镜像后边的一个byte的地址.
243行: 地址右移20位,核算出kernel有多少sections（也便是有多少兆，由于段描绘符每个能够映射1MiB的虚拟地址）,并将成果存到r6中
245 – 248行: 这几行用来填充kernel一切section虚拟地址对应的页表项.
253行: 将r0设置为RAM第一兆虚拟地址的页表项地址(page entry)
254行: r7中存储的是mmu flags, 逻辑或上RAM的开端物理地址,得到RAM第一个MB页表项的值.
255行: 设置RAM的第一个MB虚拟地址的页表.
上面这三行是用来设置RAM中第一兆虚拟地址的页表. 之所以要设置这个页表项的原因是RAM的第一兆内存中或许存储着boot params.
这样,kernel所需求的根本的页表咱们都设置完了, 如下图所示:
下面是linux-2.6.30.4中的arch/arm/kernel/head.S，代码有一些不同，可是作用相同：
1: /*
2: * linux/arch/arm/kernel/head.S
3: *
4: * Copyright (C) 1994-2002 Russell King
5: * Copyright (c) 2003 ARM Limited
6: * All Rights Reserved
7: *
8: * This program is free software; you can redistribute it and/or modify
9: * it under the terms of the GNU General Public License version 2 as
10: * published by the Free Software Foundation.
11: *
12: * Kernel startup code for all 32-bit CPUs
13: */
14: #include
15: #include
16:
17: #include
18: #include
19: #include
20: #include
21: #include
22: #include
23: #include
24:
25: #if (PHYS_OFFSET & 0x001fffff)
26: #error "PHYS_OFFSET must be at an even 2MiB boundary!"
27: #endif
28:
29: #define KERNEL_RAM_VADDR (PAGE_OFFSET + TEXT_OFFSET)
30: #define KERNEL_RAM_PADDR (PHYS_OFFSET + TEXT_OFFSET)
31:
32:
33: /*
34: * swapper_pg_dir is the virtual address of the initial page table.
35: * We place the page tables 16K below KERNEL_RAM_VADDR. Therefore, we must
36: * make sure that KERNEL_RAM_VADDR is correctly set. Currently, we expect
37: * the least significant 16 bits to be 0x8, but we could probably
38: * relax this restriction to KERNEL_RAM_VADDR >= PAGE_OFFSET + 0x4.
39: */
40: #if (KERNEL_RAM_VADDR & 0xffff) != 0x8
41: #error KERNEL_RAM_VADDR must start at 0xXXXX8
42: #endif
43:
44: .globl swapper_pg_dir
45: .equ swapper_pg_dir, KERNEL_RAM_VADDR - 0x4
46:
47: .macro pgtbl, rd
48: ldr \rd, =(KERNEL_RAM_PADDR - 0x4)
49: .endm
50:
51: #ifdef CONFIG_XIP_KERNEL
52: #define KERNEL_START XIP_VIRT_ADDR(CONFIG_XIP_PHYS_ADDR)
53: #define KERNEL_END _edata_loc
54: #else
55: #define KERNEL_START KERNEL_RAM_VADDR
56: #define KERNEL_END _end
57: #endif
58:
59: /*
60: * Kernel startup entry point.
61: *
62: *
63: * This is normally called from the decompressor code. The requirements
64: * are: MMU = off, D-cache = off, I-cache = dont care, r0 = 0,
65: * r1 = machine nr, r2 = atags pointer.
66: *
67: * This code is mostly position independent, so if you link the kernel at
68: * 0xc8, you call this at __pa(0xc8).
69: *
70: * See linux/arch/arm/tools/mach-types for the complete list of machine
71: * numbers for r1.
72: *
73: * Were trying to keep crap to a minimum; DO NOT add any machine specific
74: * crap here - thats what the boot loader (or in extreme, well justified
75: * circumstances, zImage) is for.
76: */
77: .section ".text.head", "ax"
78: ENTRY(stext)
79: msr cpsr_c, #PSR_F_BIT PSR_I_BIT SVC_MODE @ ensure svc mode
80: @ and irqs disabled
81: mrc p15, 0, r9, c0, c0 @ get processor id
82: bl __lookup_processor_type @ r5=procinfo r9=cpuid
83: movs r10, r5 @ invalid processor (r5=0)?
84: beq __error_p @ yes, error p
85: bl __lookup_machine_type @ r5=machinfo
86: movs r8, r5 @ invalid machine (r5=0)?
87: beq __error_a @ yes, error a
88: bl __vet_atags
89: bl __create_page_tables
90:
91: /*
92: * The following calls CPU specific code in a position independent
93: * manner. See arch/arm/mm/proc-*.S for details. r10 = base of
94: * xxx_proc_info structure selected by __lookup_machine_type
95: * above. On return, the CPU will be ready for the MMU to be
96: * turned on, and r0 will hold the CPU control register value.
97: */
98: ldr r13, __switch_data @ address to jump to after
99: @ mmu has been enabled
100: adr lr, __enable_mmu @ return (PIC) address
101: add pc, r10, #PROCINFO_INITFUNC
102: ENDPROC(stext)
103:
104: #if defined(CONFIG_SMP)
105: ENTRY(secondary_startup)
106: /*
107: * Common entry point for secondary CPUs.
108: *
109: * Ensure that were in SVC mode, and IRQs are disabled. Lookup
110: * the processor type - there is no need to check the machine type
: * as it has already been validated by the primary processor.
112: */
113: msr cpsr_c, #PSR_F_BIT PSR_I_BIT SVC_MODE
114: mrc p15, 0, r9, c0, c0 @ get processor id
115: bl __lookup_processor_type
116: movs r10, r5 @ invalid processor?
117: moveq r0, #p @ yes, error p
118: beq __error
119:
120: /*
121: * Use the page tables supplied from __cpu_up.
122: */
123: adr r4, __secondary_data
124: ldmia r4, {r5, r7, r13} @ address to jump to after
125: sub r4, r4, r5 @ mmu has been enabled
126: ldr r4, [r7, r4] @ get secondary_data.pgdir
127: adr lr, __enable_mmu @ return address
128: add pc, r10, #PROCINFO_INITFUNC @ initialise processor
129: @ (return control reg)
130: ENDPROC(secondary_startup)
131:
132: /*
133: * r6 = &secondary_data
134: */
135: ENTRY(__secondary_switched)
136: ldr sp, [r7, #4] @ get secondary_data.stack
137: mov fp, #0
138: b secondary_start_kernel
139: ENDPROC(__secondary_switched)
140:
141: .type __secondary_data, %object
142: __secondary_data:
143: .long .
144: .long secondary_data
145: .long __secondary_switched
146: #endif /* defined(CONFIG_SMP) */
147:
148:
149:
150: /*
151: * Setup common bits before finally enabling the MMU. Essentially
152: * this is just loading the page table pointer and domain access
153: * registers.
154: */
155: __enable_mmu:
156: #ifdef CONFIG_ALIGNMENT_TRAP
157: orr r0, r0, #CR_A
158: #else
159: bic r0, r0, #CR_A
160: #endif
161: #ifdef CONFIG_CPU_DCACHE_DISABLE
162: bic r0, r0, #CR_C
163: #endif
164: #ifdef CONFIG_CPU_BPREDICT_DISABLE
165: bic r0, r0, #CR_Z
166: #endif
167: #ifdef CONFIG_CPU_ICACHE_DISABLE
168: bic r0, r0, #CR_I
169: #endif
170: mov r5, #(domain_val(DOMAIN_USER, DOMAIN_MANAGER) \
171: domain_val(DOMAIN_KERNEL, DOMAIN_MANAGER) \
172: domain_val(DOMAIN_TABLE, DOMAIN_MANAGER) \
173: domain_val(DOMAIN_IO, DOMAIN_CLIENT))
174: mcr p15, 0, r5, c3, c0, 0 @ load domain access register
175: mcr p15, 0, r4, c2, c0, 0 @ load page table pointer
176: b __turn_mmu_on
177: ENDPROC(__enable_mmu)
178:
179: /*
180: * Enable the MMU. This completely changes the structure of the visible
181: * memory space. You will not be able to trace execution through this.
182: * If you have an enquiry about this, *please* check the linux-arm-kernel
183: * mailing list archives BEFORE sending another post to the list.
184: *
185: * r0 = cp#15 control register
186: * r13 = *virtual* address to jump to upon completion
187: *
188: * other registers depend on the function called upon completion
189: */
190: .align 5
191: __turn_mmu_on:
192: mov r0, r0
193: mcr p15, 0, r0, c1, c0, 0 @ write control reg
194: mrc p15, 0, r3, c0, c0, 0 @ read id reg
195: mov r3, r3
196: mov r3, r3
197: mov pc, r13
198: ENDPROC(__turn_mmu_on)
199:
200:
201: /*
202: * Setup the initial page tables. We only setup the barest
203: * amount which are required to get the kernel running, which
204: * generally means mapping in the kernel code.
205: *
206: * r8 = machinfo
207: * r9 = cpuid
208: * r10 = procinfo
209: *
210: * Returns:
211: * r0, r3, r6, r7 corrupted
212: * r4 = physical page table address
213: */
214: __create_page_tables:
215: pgtbl r4 @ page table address
216:
217: /*
218: * Clear the 16K level 1 swapper page table
219: */
220: mov r0, r4
221: mov r3, #0
: add r6, r0, #0x4
223: 1: str r3, [r0], #4
224: str r3, [r0], #4
225: str r3, [r0], #4
226: str r3, [r0], #4
227: teq r0, r6
228: bne 1b
229:
230: ldr r7, [r10, #PROCINFO_MM_MMUFLAGS] @ mm_mmuflags
231:
232: /*
233: * Create identity mapping for first MB of kernel to
234: * cater for the MMU enable. This identity mapping
235: * will be removed by paging_init(). We use our current program
236: * counter to determine corresponding section base address.
237: */
238: mov r6, pc, lsr #20 @ start of kernel section
239: orr r3, r7, r6, lsl #20 @ flags + kernel base
240: str r3, [r4, r6, lsl #2] @ identity mapping
241:
242: /*
243: * Now setup the pagetables for our kernel direct
244: * mapped region.
245: */
246: add r0, r4, #(KERNEL_START & 0xff) >> 18
247: str r3, [r0, #(KERNEL_START & 0x00f00) >> 18]!
248: ldr r6, =(KERNEL_END - 1)
249: add r0, r0, #4
250: add r6, r4, r6, lsr #18
251: 1: cmp r0, r6
252: add r3, r3, #1 << 20
253: strls r3, [r0], #4
254: bls 1b
255:
256: #ifdef CONFIG_XIP_KERNEL
257: /*
258: * Map some ram to cover our .data and .bss areas.
259: */
260: orr r3, r7, #(KERNEL_RAM_PADDR & 0xff)
261: .if (KERNEL_RAM_PADDR & 0x00f00)
262: orr r3, r3, #(KERNEL_RAM_PADDR & 0x00f00)
263: .endif
264: add r0, r4, #(KERNEL_RAM_VADDR & 0xff) >> 18
265: str r3, [r0, #(KERNEL_RAM_VADDR & 0x00f00) >> 18]!
266: ldr r6, =(_end - 1)
267: add r0, r0, #4
268: add r6, r4, r6, lsr #18
269: 1: cmp r0, r6
270: add r3, r3, #1 << 20
271: strls r3, [r0], #4
272: bls 1b
273: #endif
274:
275: /*
276: * Then map first 1MB of ram in case it contains our boot params.
277: */
278: add r0, r4, #PAGE_OFFSET >> 18
279: orr r6, r7, #(PHYS_OFFSET & 0xff)
280: .if (PHYS_OFFSET & 0x00f00)
281: orr r6, r6, #(PHYS_OFFSET & 0x00f00)
282: .endif
283: str r6, [r0]
284:
285: #ifdef CONFIG_DEBUG_LL
286: ldr r7, [r10, #PROCINFO_IO_MMUFLAGS] @ io_mmuflags
287: /*
288: * Map in IO space for serial debugging.
289: * This allows debug messages to be output
290: * via a serial console before paging_init.
291: */
292: ldr r3, [r8, #MACHINFO_PGOFFIO]
293: add r0, r4, r3
294: rsb r3, r3, #0x4 @ PTRS_PER_PGD*sizeof(long)
295: cmp r3, #0x0800 @ limit to 512MB
296: movhi r3, #0x0800
297: add r6, r0, r3
298: ldr r3, [r8, #MACHINFO_PHYSIO]
299: orr r3, r3, r7
300: 1: str r3, [r0], #4
301: add r3, r3, #1 << 20
302: teq r0, r6
303: bne 1b
304: #if defined(CONFIG_ARCH_NETWINDER) defined(CONFIG_ARCH_CATS)
305: /*
306: * If were using the NetWinder or CATS, we also need to map
307: * in the 16550-type serial port for the debug messages
308: */
309: add r0, r4, #0xff >> 18
310: orr r3, r7, #0x7c
311: str r3, [r0]
312: #endif
313: #ifdef CONFIG_ARCH_RPC
314: /*
315: * Map in screen at 0x02 & SCREEN2_BASE
316: * Similar reasons here - for debug. This is
317: * only for Acorn RiscPC architectures.
318: */
319: add r0, r4, #0x02 >> 18
320: orr r3, r7, #0x02
321: str r3, [r0]
322: add r0, r4, #0xd8 >> 18
323: str r3, [r0]
324: #endif
325: #endif
326: mov pc, lr
327: ENDPROC(__create_page_tables)
328: .ltorg
329:
330: #include "head-common.S"
下面仅对__create_page_tables进行简略注释：
1: __create_page_tables:
2: pgtbl r4 @ page table address
3:
4: /*
5: * Clear the 16K level 1 swapper page table
6: */
7: mov r0, r4
8: mov r3, #0
9: add r6, r0, #0x4
10: 1: str r3, [r0], #4
11: str r3, [r0], #4
12: str r3, [r0], #4
13: str r3, [r0], #4
14: teq r0, r6
15: bne 1b
16:
17: ldr r7, [r10, #PROCINFO_MM_MMUFLAGS] @ mm_mmuflags
18:
19: /*
20: * Create identity mapping for first MB of kernel to
21: * cater for the MMU enable. This identity mapping
22: * will be removed by paging_init(). We use our current program
23: * counter to determine corresponding section base address.
24: 下面三句完结：
25: 以tq2440为例：
26:
27: 将虚拟机地址0x30~0x30100映射到物理地址的0x30~0x30100-1
28:
29: */
30: mov r6, pc, lsr #20 @ start of kernel section 此刻pc在0x38邻近，r6=0x300
31: orr r3, r7, r6, lsl #20 @ flags + kernel base 结构段描绘符的内容，为什么是20，拜见《ARM体系结构与编程》
32: str r3, [r4, r6, lsl #2] @ identity mapping 填写页表项，完结映射
33:
34:
35: /*
36: * Now setup the pagetables for our kernel direct
37: * mapped region.
38: KERNEL_START = 0xC8
39: KERNEL_END = _end 在链接脚本中，它的地址是kernel镜像后边的一个byte的地址
40:
41: */
42: add r0, r4, #(KERNEL_START & 0xff) >> 18
43: @为什么是18，由于一级页表每个描绘符4个字节，r4是一个字节一个字节的加
44: str r3, [r0, #(KERNEL_START & 0x00f00) >> 18]!
45: @上面完结的使命：将虚拟地址0xC0~0xC0100-1映射到物理地址的0x30~0x30100-1，由于r3
46: @中仍是前次的值
47:
48: ldr r6, =(KERNEL_END - 1) @能够知道r6是一个虚拟地址，0xC8+解压后的内核巨细-1
49: add r0, r0, #4 @r0指向下一个待填写的页表项
50: add r6, r4, r6, lsr #18 @r6指向终究一个页表项的地址 ls后缀：无符号数小于等于
51: 1: cmp r0, r6
52: add r3, r3, #1 << 20
53: strls r3, [r0], #4
54: bls 1b
55: @经过循环，将内核地点的虚拟地址空间（0xC8+解压内核巨细-1）映射到物理内存
56: @0x38+解压内核巨细-1，接下来，mmu敞开后，就不必考虑是不是方位无关码了。
57:
58:
59: /*
60: * Then map first 1MB of ram in case it contains our boot params.
61: 个人感觉：
62: 关于tq2440将内核加载到间隔物理内存开端地址32KiB的当地时，也便是0x38，下面的代码
63: 不要也能够，由于下面的意图便是将虚拟地址0xC0映射到物理地址的0x30，这个
64: 上面的代码现已完结了。
65:
66: 可是，假如没有将内核加载到间隔物理内存开端地址32KiB的当地，比方加载到0x30300，即间隔
67: 物理内存开端地址3MiB的当地，下面的代码就有必要了，这种状况下，上面的代码仅仅完结了将：
68:
69: 虚拟地址0xC0300~解压内核巨细-1映射到物理内存0x30300~解压内核巨细-1，没有将uboot传给
70: 内核的参数地点的内存区域（一般在间隔物理内存开端地址16KiB范围内）进行映射。下面的代码完结了
71: 这个使命，此刻PAGE_OFFSET=0xc0 PHYS_OFFSET=0x30
72: 完结将虚拟地址0xC0~0xC0100-1映射到物理地址的0x30~0x30100-1
73: */
74: add r0, r4, #PAGE_OFFSET >> 18
75: orr r6, r7, #(PHYS_OFFSET & 0xff)
76: .if (PHYS_OFFSET & 0x00f00)
77: orr r6, r6, #(PHYS_OFFSET & 0x00f00)
78: .endif
79: str r6, [r0]
80:
81: mov pc, lr
82: ENDPROC(__create_page_tables)
83: .ltorg
4. 调用渠道特定的 __cpu_flush 函数
当 __create_page_tables 回来之后
此刻,一些特定寄存器的值如下所示:
r4 = pgtbl (page table 的物理基地址)
r8 = machine info (struct machine_desc的基地址)
r9 = cpu id (经过cp15协处理器获得的cpu id)
r10 = procinfo (struct proc_info_list的基地址)
在咱们需求在敞开mmu之前,做一些有必要的作业:铲除ICache, 铲除 DCache, 铲除 Writebuffer, 铲除TLB等.
这些一般是经过cp15协处理器来完结的,而且是渠道相关的. 这便是 __cpu_flush 需求做的作业.
在 arch/arm/kernel/head.S中
91: ldr r13, __switch_data @ address to jump to after
92: @ mmu has been enabled
93: adr lr, __enable_mmu @ return (PIC) address
94: add pc, r10, #PROCINFO_INITFUNC
第91行: 将r13设置为 __switch_data 的地址
第92行: 将lr设置为 __enable_mmu 的地址
第93行: r10存储的是procinfo的基地址, PROCINFO_INITFUNC是在 arch/arm/kernel/asm-offset.c 中107行界说.
则该即将pc设为 proc_info_list的 __cpu_flush 函数的地址, 即下面跳转到该函数.
关于arm920t来说，PROCINFO_INITFUNC=16，此刻r10+16->b __arm920_setup
1: .section ".proc.info.init", #alloc, #execinstr
2:
3: .type __arm920_proc_info,#object
4: m920_proc_info:
5: .long 0x41009200
6: .long 0xff00fff0
7: .long PMD_TYPE_SECT \
8: PMD_SECT_BUFFERABLE \
9: PMD_SECT_CACHEABLE \
10: PMD_BIT4 \
11: PMD_SECT_AP_WRITE \
12: PMD_SECT_AP_READ
13: .long PMD_TYPE_SECT \
14: PMD_BIT4 \
15: PMD_SECT_AP_WRITE \
16: PMD_SECT_AP_READ
17: b __arm920_setup
18: .long cpu_arch_name
19: .long cpu_elf_name
20: .long HWCAP_SWP HWCAP_HALF HWCAP_THUMB
21: .long cpu_arm920_name
22: .long arm920_processor_functions
23: .long v4wbi_tlb_fns
24: .long v4wb_user_fns
25: def CONFIG_CPU_DCACHE_WRITETHROUGH
26: .long arm920_cache_fns
27: e
28: .long v4wt_cache_fns
29: if
30: .size __arm920_proc_info, . - __arm920_proc_info
在剖析 __lookup_processor_type 的时分,咱们现已知道,关于 ARM926EJS 来说,其__cpu_flush指向的是函数 __arm926_setup
下面咱们来剖析函数 __arm926_setup
在 arch/arm/mm/proc-arm926.S 中:
00391: .type __arm926_setup, #function
00392: __arm926_setup:
00393: mov r0, #0
00394: mcr p15, 0, r0, c7, c7 @ invalidate I,D caches on v4
00395: mcr p15, 0, r0, c7, c10, 4 @ drain write buffer on v4
00396: #ifdef CONFIG_MMU
00397: mcr p15, 0, r0, c8, c7 @ invalidate I,D TLBs on v4
00398: #endif
00399:
00400:
00401: #ifdef CONFIG_CPU_DCACHE_WRITETHROUGH
00402: mov r0, #4 @ disable write-back on caches explicitly
00403: mcr p15, 7, r0, c15, c0, 0
00404: #endif
00405:
00406: adr r5, arm926_crval
00407: ldmia r5, {r5, r6}
00408: mrc p15, 0, r0, c1, c0 @ get control register v4
00409: bic r0, r0, r5
00410: orr r0, r0, r6
00411: #ifdef CONFIG_CPU_CACHE_ROUND_ROBIN
00412: orr r0, r0, #0x4 @ .1.. .... .... ....
00413: #endif
00414: mov pc, lr
00415: .size __arm926_setup, . - __arm926_setup
00416:
00417: /*
00418: * R
00419: * .RVI ZFRS BLDP WCAM
00420: * .011 1 ..11 0101
00421: *
00422: */
00423: .type arm926_crval, #object
00424: arm926_crval:
00425: crval clear=0x07f3f, mmuset=0x03135, ucset=0x01134
第391, 392行: 是函数声明
第393行: 将r0设置为0
第394行: 铲除(invalidate)Instruction Cache 和 Data Cache.
第395行: 铲除(drain) Write Buffer.
第396 - 398行: 假如有装备了MMU,则需求铲除(invalidate)Instruction TLB 和Data TLB
接下来,是对操控寄存器c1进行装备,请参阅 ARM926 TRM.
第401 - 404行: 假如装备了Data Cache运用writethrough方法, 需求关掉write-back.
第406行: 取arm926_crval的地址到r5中, arm926_crval 在第424行
第407行: 这儿咱们需求看一下424和425行,其间用到了宏crval,crval是在 arch/arm/mm/proc-macro.S 中:
53: .macro crval, clear, mmuset, ucset
54: #ifdef CONFIG_MMU
55: .word \clear
56: .word \mmuset
57: #else
58: .word \clear
59: .word \ucset
60: #endif
61: .endm
合作425行,咱们能够看出,首要在arm926_crval的地址处寄存了clear的值,然后接下来的地址寄存了mmuset的值(关于装备了MMU的状况)
所以,在407行中,咱们将clear和mmuset的值别离存到了r5, r6中
第408行: 获得操控寄存器c1的值
第409行: 将r0中的 clear (r5) 对应的位都铲除去
第410行: 设置r0中 mmuset (r6) 对应的位
第411 - 413行: 假如装备了运用 round robin方法,需求设置操控寄存器c1的 Bit[16]
第412行: 取lr的值到pc中.
而lr中的值寄存的是 __enable_mmu 的地址(arch/arm/kernel/head.S 93行),所以,接下来便是跳转到函数 __enable_mmu
5. 敞开mmu
敞开mmu是又函数 __enable_mmu 完结的.
在进入 __enable_mmu 的时分, r0中现已寄存了操控寄存器c1的一些装备(在上一步中进行的设置), 可是并没有真实的翻开mmu,
在 __enable_mmu 中,咱们将翻开mmu.
此刻,一些特定寄存器的值如下所示:
r0 = c1 parameters (用来装备操控寄存器的参数)
r4 = pgtbl (page table 的物理基地址)
r8 = machine info (struct machine_desc的基地址)
r9 = cpu id (经过cp15协处理器获得的cpu id)
r10 = procinfo (struct proc_info_list的基地址)
在 arch/arm/kernel/head.S 中:
00146: .type __enable_mmu, %function
00147: __enable_mmu:
00148: #ifdef CONFIG_ALIGNMENT_TRAP
00149: orr r0, r0, #CR_A
00150: #else
00151: bic r0, r0, #CR_A
00152: #endif
00153: #ifdef CONFIG_CPU_DCACHE_DISABLE
00154: bic r0, r0, #CR_C
00155: #endif
00156: #ifdef CONFIG_CPU_BPREDICT_DISABLE
00157: bic r0, r0, #CR_Z
00158: #endif
00159: #ifdef CONFIG_CPU_%&&&&&%ACHE_DISABLE
00160: bic r0, r0, #CR_I
00161: #endif
00162: mov r5, #(domain_val(DOMAIN_USER, DOMAIN_MANAGER) \
00163: domain_val(DOMAIN_KERNEL, DOMAIN_MANAGER) \
00164: domain_val(DOMAIN_TABLE, DOMAIN_MANAGER) \
00165: domain_val(DOMAIN_IO, DOMAIN_CLIENT))
00166: mcr p15, 0, r5, c3, c0, 0 @ load domain access register
00167: mcr p15, 0, r4, c2, c0, 0 @ load page table pointer
00168: b __turn_mmu_on
00169:
00170: /*
00171: * Enable the MMU. This completely changes the structure of the visible
00172: * memory space. You will not be able to trace execution through this.
00173: * If you have an enquiry about this, *please* check the linux-arm-kernel
00174: * mailing list archives BEFORE sending another post to the list.
00175: *
00176: * r0 = cp#15 control register
00177: * r13 = *virtual* address to jump to upon completion
00178: *
00179: * other registers depend on the function called upon completion
00180: */
00181: .align 5
00182: .type __turn_mmu_on, %function
00183: __turn_mmu_on:
00184: mov r0, r0
00185: mcr p15, 0, r0, c1, c0, 0 @ write control reg
00186: mrc p15, 0, r3, c0, c0, 0 @ read id reg
00187: mov r3, r3
00188: mov r3, r3
00189: mov pc, r13
第146, 147行: 函数声明
第148 - 161行: 依据相应的装备,设置r0中的相应的Bit. (r0 将用来装备操控寄存器c1)
第162 - 165行: 设置 domain 参数r5.(r5 将用来装备domain)
第166行: 装备 domain (具体信息清参阅arm相关手册)
第167行: 装备页表在存储器中的方位(set ttb).这儿页表的基地址是r4, 经过写cp15的c2寄存器来设置页表基地址.
第168行: 跳转到 __turn_mmu_on. 从称号咱们能够猜到,下面是要真实翻开mmu了.
(持续向下看,咱们会发现,__turn_mmu_on就下当时代码的下方,为什么要跳转一下呢? 这是有原因的. go on)
第169 - 180行: 空行和注释. 这儿的注释咱们能够看到, r0是cp15操控寄存器的内容, r13存储了完结后需求跳转的虚拟地址(由于完结后mmu现已翻开了,都是虚拟地址了).
第181行: .algin 5 这句是cache line对齐. 咱们能够看到下面一行便是 __turn_mmu_on, 之所以
第182 - 183行: __turn_mmu_on 的函数声明. 这儿咱们能够看到, __turn_mmu_on 是紧接着上面第168行的跳转指令的,仅仅中心在第181行多了一个cache line对齐.
这么做的原因是: 下面咱们要进行真实的翻开mmu操作了, 咱们要把翻开mmu的操作放到一个独自的cache line上. 而在之前的"发动条件"一节咱们说了,I Cache是能够翻开也能够封闭的,这儿这么做的原因是要确保在I Cache翻开的时分,翻开mmu的操作也能正常履行.
第184行: 这是一个空操作,相当于nop. 在arm中,nop操作经常用指令 mov rd, rd 来完结.
留意: 为什么这儿要有一个nop,我考虑了很长时刻,这儿是我的猜想,或许不是正确的:
由于之前设置了页表基地址(set ttb),到下一行(185行)翻开mmu操作,中心的指令序列是这样的:
set ttb(第167行)
branch(第168行)
nop(第184行)
enable mmu(第185行)
关于arm的五级流水线: fetch - decode - execute - memory - write
他们履行的状况如下图所示:
这儿需求阐明的是,branch操作会在3个cycle中完结,而且会导致从头取指.
从这个图咱们能够看出来,在enable mmu操作取指的时分, set ttb操作刚好完结.
第185行: 写cp15的操控寄存器c1, 这儿是翻开mmu的操作,一起会翻开cache等(依据r0相应的装备)
第186行: 读取id寄存器.
第187 - 188行: 两个nop.
第189行: 取r13到pc中,咱们前面现已看到了, r13中存储的是 __switch_data (在 arch/arm/kernel/head.S 91行),下面会跳到 __switch_data.
第187,188行的两个nop是非常重要的,由于在185行翻开mmu操作之后,要比及3个cycle之后才会收效,这和arm的流水线有联系.
因此,在翻开mmu操作之后的加了两个nop操作.
6. 切换数据
在 arch/arm/kernel/head-common.S 中:
14: .type __switch_data, %object
15: __switch_data:
16: .long __mmap_switched
17: .long __data_loc @ r4
18: .long __data_start @ r5
19: .long __bss_start @ r6
20: .long _end @ r7
21: .long processor_id @ r4
22: .long __machine_arch_type @ r5
23: .long cr_alignment @ r6
24: .long init_thread_union + THREAD_START_SP @ sp
25:
26: /*
27: * The following fragment of code is executed with the MMU on in MMU mode,
28: * and uses absolute addresses; this is not position independent.
29: *
30: * r0 = cp#15 control register
31: * r1 = machine ID
32: * r9 = processor ID
33: */
34: .type __mmap_switched, %function
35: __mmap_switched:
36: adr r3, __switch_data + 4
37:
38: ldmia r3!, {r4, r5, r6, r7}
39: cmp r4, r5 @ Copy data segment if needed
40: 1: cmpne r5, r6
41: ldrne fp, [r4], #4
42: strne fp, [r5], #4
43: bne 1b
44:
45: mov fp, #0 @ Clear BSS (and zero fp)
46: 1: cmp r6, r7
47: strcc fp, [r6],#4
48: bcc 1b
49:
50: ldmia r3, {r4, r5, r6, sp}
51: str r9, [r4] @ Save processor ID
52: str r1, [r5] @ Save machine type
53: bic r4, r0, #CR_A @ Clear A bit
54: stmia r6, {r0, r4} @ Save control register values
55: b start_kernel
第14, 15行: 函数声明
第16 - 24行: 界说了一些地址,例如第16行存储的是 __mmap_switched 的地址, 第17行存储的是 __data_loc 的地址 ......
第34, 35行: 函数 __mmap_switched
第36行: 取 __switch_data + 4的地址到r3. 从上文能够看到这个地址便是第17行的地址.
第37行: 顺次取出从第17行到第20行的地址,存储到r4, r5, r6, r7 中. 而且累加r3的值.当履行完后, r3指向了第21行的方位.
对照上文,咱们能够得知:
r4 - __data_loc
r5 - __data_start
r6 - __bss_start
r7 - _end
这几个符号都是在 arch/arm/kernel/vmlinux.lds.S 中界说的变量:
00102: #ifdef CONFIG_XIP_KERNEL
00103: __data_loc = ALIGN(4); /* location in binary */
00104: . = PAGE_OFFSET + TEXT_OFFSET;
00105: #else
00106: . = ALIGN(THREAD_SIZE);
00107: __data_loc = .;
00108: #endif
00109:
00110: .data : AT(__data_loc) {
00: __data_start = .; /* address in memory */
00112:
00113: /*
00114: * first, the init task union, aligned
00115: * to an 8192 byte boundary.
00116: */
00117: *(.init.task)
......
00158: .bss : {
00159: __bss_start = .; /* BSS */
00160: *(.bss)
00161: *(COMMON)
00162: _end = .;
00163: }
关于这四个变量,咱们简略的介绍一下:
__data_loc 是数据寄存的方位
__data_start 是数据开端的方位
__bss_start 是bss开端的方位
_end 是bss完毕的方位, 也是内核完毕的方位
其间对第110行的指令解说一下: 这儿界说了.data 段,后边的AT(__data_loc) 的意思是这部分的内容是在__data_loc中存储的(要留意,贮存的方位和链接的方位是能够不相同的).
关于 AT 具体的信息请参阅 ld.info
第38行: 比较 __data_loc 和 __data_start
第39 - 43行: 这几行是判别数据存储的方位和数据的开端的方位是否持平,假如不持平,则需求转移数据,从 __data_loc 将数据搬到 __data_start.
其间 __bss_start 是bss的开端的方位,也标志了 data 完毕的方位,因此用其作为判别数据是否转移完结.
第45 - 48行: 是铲除 bss 段的内容,将其都置成0. 这儿运用 _end 来判别 bss 的完毕方位.
第50行: 由于在第38行的时分,r3被更新到指向第21行的方位.因此这儿获得r4, r5, r6, sp的值别离是:
r4 - processor_id
r5 - __machine_arch_type
r6 - cr_alignment
sp - init_thread_union + THREAD_START_SP
processor_id 和 __machine_arch_type 这两个变量是在 arch/arm/kernel/setup.c 中 第62, 63行中界说的.
cr_alignment 是在 arch/arm/kernel/entry-armv.S 中界说的:
00182: .globl cr_alignment
00183: .globl cr_no_alignment
00184: cr_alignment:
00185: .space 4
00186: cr_no_alignment:
00187: .space 4
init_thread_union 是 init进程的基地址. 在 arch/arm/kernel/init_task.c 中:
33: union thread_union init_thread_union
34: __attribute__((__section__(".init.task"))) =
35: { INIT_THREAD_INFO(init_task) };
对照 vmlnux.lds.S 中的 的117行,咱们能够知道init task是寄存在 .data 段的开端8k, 而且是THREAD_SIZE(8k)对齐的
第51行: 将r9中寄存的 processor id (在arch/arm/kernel/head.S 75行) 赋值给变量 processor_id
第52行: 将r1中寄存的 machine id (见"发动条件"一节)赋值给变量 __machine_arch_type
第53行: 铲除r0中的 CR_A 位并将值存到r4中. CR_A 是在 include/asm-arm/system.h 21行界说, 是cp15操控寄存器c1的Bit[1](alignment fault enable/disable)
第54行: 这一行是存储操控寄存器的值.
从上面 arch/arm/kernel/entry-armv.S 的代码咱们能够得知.
这一句是将r0存储到了 cr_alignment 中,将r4存储到了 cr_no_alignment 中.
第55行: 终究跳转到start_kernel
