arm linux kernel 从入口到start_kernel 的代码分析

2019-07-12 23:04发布

参考资料: 《ARM体系结构与编程》 《嵌入式Linux应用开发完全手册》 Linux_Memory_Address_Mapping http://www.chinaunix.net/old_jh/4/1021226.html 更多文档参见:http://pan.baidu.com/s/1mg3DbHQ   本文针对arm linux, 从kernel的第一条指令开始分析,一直分析到进入start_kernel()函数.
我们当前以linux-2.6.19内核版本作为范例来分析,本文中所有的代码,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:" 表示478行,冒号后面的内容就是源码了.
在分析代码的过程中,我们使用缩进来表示各个代码的调用层次.
由于启动部分有一些代码是平台特定的,虽然大部分的平台所实现的功能都比较类似,但是为了更好的对code进行说明,对于平台相关的代码,我们选择at91(ARM926EJS)平台进行分析.
另外,本文是以uncompressed kernel开始讲解的.对于内核解压缩部分的code,在 arch/arm/boot/compressed中,本文不做讨论.

一. 启动条件

          通常从系统上电到执行到linux kenel这部分的任务是由boot loader来完成.
        关于boot loader的内容,本文就不做过多介绍.
        这里只讨论进入到linux kernel的时候的一些限制条件,这一般是boot loader在最后跳转到kernel之前要完成的:
        1. CPU必须处于SVC(supervisor)模式,并且IRQ和FIQ中断都是禁止的;
        2. MMU(内存管理单元)必须是关闭的, 此时虚拟地址对物理地址;
        3. 数据cache(Data cache)必须是关闭的
        4. 指令cache(Instruction cache)可以是打开的,也可以是关闭的,这个没有强制要求;
        5. CPU 通用寄存器0 (r0)必须是 0;
        6. CPU 通用寄存器1 (r1)必须是 ARM Linux machine type (关于machine type, 我们后面会有讲解)
        7. CPU 通用寄存器2 (r2) 必须是 kernel parameter list 的物理地址(parameter list 是由boot loader传递给kernel,用来描述设备信息属性的列表,详细内容可参考"Booting ARM Linux"文档).

二. starting kernel

  首先,我们先对几个重要的宏进行说明(我们针对有MMU的情况):
     宏                                               位置                                       默认值             说明
KERNEL_RAM_ADDR      arch/arm/kernel/head.S +26              0xc0008000      kernel在RAM中的的虚拟地址
PAGE_OFFSET                include/asm-arm/memeory.h +50     0xc0000000      内核空间的起始虚拟地址
TEXT_OFFSET                 arch/arm/Makefile +137                    0x00008000      内核相对于存储空间的偏移
TEXTADDR                     arch/arm/kernel/head.S +49              0xc0008000      kernel的起始虚拟地址
PHYS_OFFSET                include/asm-arm/arch-xxx/memory.h   平台相关          RAM的起始物理地址
        内核的入口是stext,这是在arch/arm/kernel/vmlinux.lds.S中定义的:
        00011: ENTRY(stext)
        对于vmlinux.lds.S,这是ld script文件,此文件的格式和汇编及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讲解,关于ld的详细内容可以参考ld.info
        这里的ENTRY(stext) 表示程序的入口是在符号stext.
        而符号stext是在arch/arm/kernel/head.S中定义的:
        下面我们将arm linux boot的主要代码列出来进行一个概括的介绍,然后,我们会逐个的进行详细的讲解.
        在arch/arm/kernel/head.S中 72 - 94 行,是arm linux boot的主代码:
00072: ENTRY(stext)                                                        
00073:         msr        cpsr_c, #PSR_F_BIT | PSR_I_BIT | SVC_MODE @ ensure svc mode
00074:                                                 @ and irqs disabled        
00075:         mrc        p15, 0, r9, c0, c0                @ get processor id         
00076:         bl        __lookup_processor_type                @ r5=procinfo r9=cpuid     
00077:         movs        r10, r5                                @ invalid processor (r5=0)?
00078:         beq        __error_p                        @ yes, error 'p'           
00079:         bl        __lookup_machine_type                @ r5=machinfo              
00080:         movs        r8, r5                                @ invalid machine (r5=0)?  
00081:         beq        __error_a                        @ yes, error 'a'           
00082:         bl        __create_page_tables                                       
00083:                                                                     
00084:         /*                                                                 
00085:          * The following calls CPU specific code in a position independent
00086:          * manner.  See arch/arm/mm/proc-*.S for details.  r10 = base of   
00087:          * xxx_proc_info structure selected by __lookup_machine_type      
00088:          * above.  On return, the CPU will be ready for the MMU to be      
00089:          * turned on, and r0 will hold the CPU control register value.     
00090:          */                                                               
00091:         ldr        r13, __switch_data                @ address to jump to after
00092:                                                 @ mmu has been enabled     
00093:         adr        lr, __enable_mmu                @ return (PIC) address     
00094:         add        pc, r10, #PROCINFO_INITFUNC                                
其中,73行是确保kernel运行在SVC模式下,并且IRQ和FIRQ中断已经关闭,这样做是很谨慎的.
arm linux boot的主线可以概括为以下几个步骤:
        1. 确定 processor type                        (75 - 78行)
        2. 确定 machine type                        (79 - 81行)
        3. 创建页表                                (82行)     
        4. 调用平台特定的__cpu_flush函数        (在struct proc_info_list中)        (94 行)                           
        5. 开启mmu                                (93行)
        6. 切换数据                                 (91行)
        最终跳转到start_kernel                        (在__switch_data的结束的时候,调用了 b        start_kernel)
下面,我们按照这个主线,逐步的分析Code.

1. 确定 processor type

      arch/arm/kernel/head.S中:
00075:         mrc        p15, 0, r9, c0, c0                @ get processor id         
00076:         bl        __lookup_processor_type                @ r5=procinfo r9=cpuid     
00077:         movs        r10, r5                                @ invalid processor (r5=0)?
00078:         beq        __error_p                        @ yes, error 'p'           
75行: 通过cp15协处理器的c0寄存器来获得processor id的指令. 关于cp15的详细内容可参考相关的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: 判断r5中的processor type是否是0,如果是0,说明是无效的processor type,跳转到__error_p(出错)
__lookup_processor_type 函数主要是根据从cpu中获得的processor id和系统中的proc_info进行匹配,将匹配到的proc_info_list的基地址存到r5中, 0表示没有找到对应的processor type.
下面我们分析__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行是函数定义
147行: 取地址指令,这里的3f是向前symbol名称是3的位置,即第178行,将该地址存入r3.
        这里需要注意的是,adr指令取址,获得的是基于pc的一个地址,要格外注意,这个地址是3f处的"运行时地址", 由于此时MMU还没有打开,也可以理解成物理地址(实地址).(详细内容可参考arm指令手册)
148行: 因为r3中的地址是178行的位置的地址,因而执行完后: (ldmda表示栈指针递减,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.
        这里需要注意链接地址和运行时地址的区别. r3存储的是运行时地址(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:
        00031:                __proc_info_begin = .;
        00032:                        *(.proc.info.init)
        00033:                __proc_info_end = .;
        这里是声明了两个变量:__proc_info_begin 和 __proc_info_end,其中等号后面的"."是location counter(详细内容请参考ld.info)
        这三行的意思是: __proc_info_begin 的位置上,放置所有文件中的 ".proc.info.init" 段的内容,然后紧接着是 __proc_info_end 的位置.
        kernel 使用struct proc_info_list来描述processor type.
                在 include/asm-arm/procinfo.h 中:
        00029: struct proc_info_list {
        00030:         unsigned int                cpu_val;
        00031:         unsigned int                cpu_mask;
        00032:         unsigned long                __cpu_mm_mmu_flags;        /* used by head.S */
        00033:         unsigned long                __cpu_io_mmu_flags;        /* used by head.S */
        00034:         unsigned long                __cpu_flush;                /* used by head.S */
        00035:         const char                *arch_name;
        00036:         const char                *elf_name;
        00037:         unsigned int                elf_hwcap;
        00038:         const char                *cpu_name;
        00039:         struct processor        *proc;
        00040:         struct cpu_tlb_fns        *tlb;
        00041:         struct cpu_user_fns        *user;
        00042:         struct cpu_cache_fns        *cache;
        00043: };
        我们当前以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_SWP|HWCAP_HALF|HWCAP_THUMB|HWCAP_FAST_MULT|HWCAP_VFP|HWCAP_EDSP|HWCAP_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,我们可以看到 __arm926_proc_info 被放到了".proc.info.init"段中.
        对照struct proc_info_list,我们可以看到 __cpu_flush的定义是在480行,即__arm926_setup.(我们将在"4. 调用平台特定的__cpu_flush函数"一节中详细分析这部分的内容.)
从以上的内容我们可以看出: r5中的__proc_info_begin是proc_info_list的起始地址, r6中的__proc_info_end是proc_info_list的结束地址.
149行: 从上面的分析我们可以知道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,这一行是计算当前程序运行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,可以得知,这句是将当前proc_info的cpu_val和cpu_mask分别存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,得到我们需要的值
154行: 将153行中得到的值与r3中的cpu_val进行比较
155行: 如果相等,说明我们找到了对应的processor type,跳到160行,返回
156行: (如果不相等) , 将r5指向下一个proc_info,
157行: 和r6比较,检查是否到了__proc_info_end.
158行: 如果没有到__proc_info_end,表明还有proc_info配置,返回152行继续查找
159行: 执行到这里,说明所有的proc_info都匹配过了,但是没有找到匹配的,将r5设置成0(unknown processor)
160行: 返回
 

2. 确定 machine type

      arch/arm/kernel/head.S中:
00079:         bl        __lookup_machine_type                @ r5=machinfo              
00080:         movs        r8, r5                                @ invalid machine (r5=0)?  
00081:         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,并判断r5是否是0,如果是0,说明是无效的machine type,跳转到__error_a(出错)
__lookup_machine_type 函数
下面我们分析__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 can't use the absolute addresses for the __arch_info
00185:  * lists since we aren't 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,这里的3b是向后symbol名称是3的位置,即第178行,将该地址存入r3.
        和上面我们对__lookup_processor_type 函数的分析相同,r3中存放的是3b处物理地址.
196行: r3是3b处的地址,因而执行完后:(ldmia 表示栈是递增的,即r3递增,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:
        00034:                __arch_info_begin = .;
        00035:                        *(.arch.info.init)
        00036:                __arch_info_end = .;
        这里是声明了两个变量:__arch_info_begin 和 __arch_info_end,其中等号后面的"."是location counter(详细内容请参考ld.info)
        这三行的意思是: __arch_info_begin 的位置上,放置所有文件中的 ".arch.info.init" 段的内容,然后紧接着是 __arch_info_end 的位置.
        kernel 使用struct machine_desc 来描述 machine type.
        在 include/asm-arm/mach/arch.h 中:
        00017: struct machine_desc {
        00018:         /*
        00019:          * Note! The first four elements are used
        00020:          * by assembler code in head-armv.S
        00021:          */
        00022:         unsigned int                nr;                /* architecture number        */
        00023:         unsigned int                phys_io;        /* start of physical io        */
        00024:         unsigned int                io_pg_offst;        /* byte offset for io
        00025:                                                  * page tabe entry        */
        00026:
        00027:         const char                *name;                /* architecture name        */
        00028:         unsigned long                boot_params;        /* tagged list                */
        00029:
        00030:         unsigned int                video_start;        /* start of video RAM        */
        00031:         unsigned int                video_end;        /* end of video RAM        */
        00032:
        00033:         unsigned int                reserve_lp0 :1;        /* never has lp0        */
        00034:         unsigned int                reserve_lp1 :1;        /* never has lp1        */
        00035:         unsigned int                reserve_lp2 :1;        /* never has lp2        */
        00036:         unsigned int                soft_reboot :1;        /* soft reboot                */
        00037:         void                        (*fixup)(struct machine_desc *,
        00038:                                          struct tag *, char **,
        00039:                                          struct meminfo *);
        00040:         void                        (*map_io)(void);/* IO mapping function        */
        00041:         void                        (*init_irq)(void);
        00042:         struct sys_timer        *timer;                /* system tick timer        */
        00043:         void                        (*init_machine)(void);
        00044: };
        00045:
        00046: /*
        00047:  * Set of macros to define architecture features.  This is built into
        00048:  * a table by the linker.
        00049:  */
        00050: #define MACHINE_START(_type,_name)                       
        00051: static const struct machine_desc __mach_desc_##_type       
        00052:  __attribute_used__                                       
        00053:  __attribute__((__section__(".arch.info.init"
icon_wink)) = {       
        00054:         .nr                = MACH_TYPE_##_type,               
        00055:         .name                = _name,
        00056:
        00057: #define MACHINE_END                               
        00058: };        
        内核中,一般使用宏MACHINE_START来定义machine type.
        对于at91, 在 arch/arm/mach-at91rm9200/board-ek.c 中:
        00137: MACHINE_START(AT91RM9200EK, "Atmel AT91RM9200-EK"
icon_wink[1]
        00138:         /* Maintainer: SAN People/Atmel */
        00139:         .phys_io        = AT91_BASE_SYS,
        00140:         .io_pg_offst        = (AT91_VA_BASE_SYS >> 1
icon_cool & 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,这里计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行定义, 这里是取 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"启动条件"
icon_wink[2]进行比较
202行: 如果相同,说明找到了对应的machine type,跳转到207行的2f处,此时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,检查是否到了__arch_info_end.
205行: 如果不相同,说明还有machine_desc,返回200行继续查找.
206行: 执行到这里,说明所有的machind_desc都查找完了,并且没有找到匹配的, 将r5设置成0(unknown machine).
207行: 返回
 

3. 创建页表

  通过前面的两步,我们已经确定了processor type 和 machine type.
此时,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             (通过cp15协处理器获得的cpu id)
r10 = procinfo          (struct proc_info_list的基地址)
创建页表是通过函数 __create_page_tables 来实现的.
这里,我们使用的是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 的地址空间分成若干个1 MB的段(section),因此L1页表包含4096个页表项(section entry). 每个页表项是32 bits(4 bytes)
因而L1主页表占用 4096 *4 = 16k的内存空间.
        对于ARM926,其L1 section entry的格式为
icon_sad可参考arm926EJS TRM): (一级描述符的格式  可以参考《ARM体系结构与编程》P180)
image
下面我们来分析 __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, #0x4000
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
00222:
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 & 0xff000000) >> 18        @ start of kernel
00240:         str        r3, [r0, #(TEXTADDR & 0x00f00000) >> 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: 通过宏 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:
        00042:        .macro        pgtbl, rd
        00043:        ldr        d, =(__virt_to_phys(KERNEL_RAM_ADDR - 0x4000))
        00044:        .endm
        可以看到,页表是位于 KERNEL_RAM_ADDR 下面 16k 的位置
        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, 可以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中定义,值为8)(可以参考《嵌入式Linux应用完全开发手册》P118)(r7的值就是设置这个段描述符的权限、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_SWP|HWCAP_HALF|HWCAP_THUMB|HWCAP_FAST_MULT|HWCAP_VFP|HWCAP_EDSP|HWCAP_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 image
231行: 通过pc值的高12位(右移20位),得到kernel的section,并存储到r6中.因为当前是通过运行时地址得到的kernel的section,因而是物理地址.
232行: r3 = r7 | (r6 << 20); flags + kernel base,得到页表中需要设置的值.
233行: 设置页表: mem[r4 + r6 * 4] = r3
        这里,因为页表的每一项是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(物理地址所在的page entry)的页表项
239, 240行: TEXTADDR是内核的起始虚拟地址(0xc0008000), 这两行是设置kernel起始虚拟地址的页表项(注意,这里设置的页表项和上面的231 - 233行设置的页表项是不同的 )
        执行完后,r0指向kernel的第2个section的虚拟地址所在的页表项.
        /* TODO: 这两行的code很奇怪,为什么要先取TEXTADDR的高8位(Bit[31:24])0xff000000,然后再取后面的8位 (Bit[23:20])0x00f00000*/           
242行: 这一行计算kernel镜像的大小(bytes).
        _end 是在vmlinux.lds.S中162行定义的,标记kernel的结束位置(虚拟地址):
        00158                .bss : {
        00159                __bss_start = .;        /* BSS                                */
        00160                *(.bss)
        00161                *(COMMON)
        00162                _end = .;
        00163        }
        kernel的size =  _end - PAGE_OFFSET -1, 这里 减1的原因是因为 _end 是 location counter,它的地址是kernel镜像后面的一个byte的地址.
243行: 地址右移20位,计算出kernel有多少sections(也就是有多少兆,因为段描述符每个可以映射1MiB的虚拟地址),并将结果存到r6中
245 - 248行: 这几行用来填充kernel所有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的起始物理地址,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的第一兆内存中可能存储着boot params.
这样,kernel所需要的基本的页表我们都设置完了, 如下图所示:
20080730_f4ddb40ebe53bbb8039esF6YSUk3UEIq
  下面是linux-2.6.30.4中的arch/arm/kernel/head.S,代码有一些不同,但是效果一样: 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 <asm/assembler.h> 18: #include <asm/domain.h> 19: #include <asm/ptrace.h> 20: #include <asm/asm-offsets.h> 21: #include <asm/memory.h> 22: #include <asm/thread_info.h> 23: #include <asm/system.h> 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 0x8000, but we could probably 38: * relax this restriction to KERNEL_RAM_VADDR >= PAGE_OFFSET + 0x4000. 39: */ 40: #if (KERNEL_RAM_VADDR & 0xffff) != 0x8000 41: #error KERNEL_RAM_VADDR must start at 0xXXXX8000 42: #endif 43:  44: .globl swapper_pg_dir 45: .equ swapper_pg_dir, KERNEL_RAM_VADDR - 0x4000 46:  47: .macro pgtbl, rd 48: ldr d, =(KERNEL_RAM_PADDR - 0x4000) 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: * 0xc0008000, you call this at __pa(0xc0008000). 69: * 70: * See linux/arch/arm/tools/mach-types for the complete list of machine 71: * numbers for r1. 72: * 73: * We're trying to keep crap to a minimum; DO NOT add any machine specific 74: * crap here - that's 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 we're in SVC mode, and IRQs are disabled. Lookup 110: * the processor type - there is no need to check the machine type 111: * 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