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arm linux 从入口到start_kernel 代码详细分析4

上一篇 / 下一篇  2010-10-07 15:38:06 / 个人分类:Linux移植

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:(可参考arm926EJS TRM):
                                                                                    
                                                                            
    31                          20 19        12 11 10 9 8      5 4 3 2 1 0  
   +------------------------------+------------+-----+-+--------+-+-+-+-+-+ 
   |                              |            |     | |        | | | | | | 
   |       Base Address           |    SBZ     | AP  |0| Domain |1|C|B|1|0| 
   |                              |            |     | |        | | | | | | 
   +------------------------------+------------+-----+-+--------+-+-+-+-+-+ 
                                                                            
                                                                            
   B - Write Buffer Bit                                                     
                                                                            
   C - Cache Bit                                                            
                                                                            
                                                                            
                   +---------------------------------------------------+    
                   |                  Data Cache                       |    
                   +-----------+------------+--------------------------+    
                   | Cache Bit | Buffer Bit |     Page attribute       |    
                   +-----------+------------+--------------------------+    
                   |    0      |     0      | not cached, not buffered |    
                   +-----------+------------+--------------------------+    
                   |    0      |     1      | not cached, buffered     |    
                   +-----------+------------+--------------------------+    
                   |    1      |     0      | cached, writethrough     |    
                   +-----------+------------+--------------------------+    
                   |    1      |     1      | cached, writeback        |    
                   +-----------+------------+--------------------------+    
                                                                            

下面我们来分析 __create_page_tables 函数:

         在 arch/arm/kernel/head.S 中:

- arm linux 从入口到start_kernel 代码分析 -
00206:  .type __create_page_tables, %function
00207: __create_page_tables:
00208:  pgtbl r4    @ page table address
00209:
00210: 
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: 
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: 
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: 
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 \rd, =(__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)


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.
                 
      
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 = .;
        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,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:
                                     
                        _,,_                          _,,_                   
                      -`    `'-.,,.                 -`    `'-.,,.            
                      |           |                 |           |            
                      |           |                 |           |            
                      |           |                 |           |            
                      |           |                 |           |            
                      |           |                 |           |            
                      |           |                 |           |            
                      |           |                 |           |            
                      |           |                 |           |            
                      +-----------+                 |           |            
                      |           |                 |           |            
                      |           |-------------\   |           |            
                      |           |             |   |           |            
                      |  KERNEL   |             |   |           |            
                      |           |             |   |           |            
                      |           |             |   |           |            
                      |           |             |   |           |            
                      |           |             |   |           |            
            +0x8000 ->+-----------+--------\    |   |           |            
                      |           |        |    |   |           |            
                      |    L1     |        |    |   |           |             
                      | Page Table|        |    |   |           |             
                      |           |        |    |   |           |             
            +0x4000 ->+-----------+        |    |   |           |             
                      |           |        |    |   +-----------+             
                      |   Boot    |        |    |   |           |             
                      |  Params   |        |    |   |           |             
                      |           |        |    |   |           |             
    PAGE_OFFSET(3G) ->+-----------+---\    |    |   |           |             
                      |           |   |    |    |   |           |             
                      |           |   |    |    |   |           |             
                      |           |   |    |    |   |           |             
                      |           |   |    |    |   |           |             
                      |           |   |    |    |   |           |             
                      |           |   |    |    |   |           |             
                      |           |   |    |    |   +-----------+             
                      |           |   |    |    |   |           |             
                      |           |   |    |    \-->|           |             
                      |           |   |    |        |           |             
                      |           |   |    |        |  KERNEL   |             
                      |           |   |    |        |           |             
                      |           |   |    |        |           |             
                      +- - - - - -+   |    |        |           |             
                      |   1MB     |   |    |        |           |             
PHYS_OFFSET+0x8000 ->+- - - - - -+--------+------->'-----------+<- +0x8000   
                      |           |   |             |           |             
                      |           |   |             |    L1     |             
                      |           |   |             | Page Table|             
                      |           |   |             |           |             
                      |           |   |             +-----------+<- +0x4000   
                      |           |   |             |           |             
                      |           |   |             |   Boot    |             
                      |           |   |             |  Params   |             
                      |           |   |             |           |             
                      |           |   \------------>+-----------+<- PHYS_OFFSET
                      |           |                 |           |             
                      |           |                 | _,,_      |             
                      |           |                 -`    `'-.,,.             
                  0 --+-----------+                                           
                                                                              
                       VIRT Address                  PHYS Address          
本文来自: (www.91linux.com) 详细出处参考:http://www.91linux.com/html/article/qianrushiyingyong/arm_linux/2010/0909/20474.html


TAG: ARM Arm arm kernel Kernel linux Linux LINUX start 入口

 

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