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:
00206: .type__create_page_tables, %function
00207: __create_page_tables:
00208: pgtblr4@ page table address
00209:
00210:
00213: movr0, r4
00214: movr3, #0
00215: addr6, r0, #0x4000
00216: 1:strr3, [r0], #4
00217: strr3, [r0], #4
00218: strr3, [r0], #4
00219: strr3, [r0], #4
00220: teqr0, r6
00221: bne1b
00222:
00223: ldrr7, [r10, #PROCINFO_MM_MMUFLAGS] @ mm_mmuflags
00224:
00225:
00231: movr6, pc, lsr #20@ start of kernel section
00232: orrr3, r7, r6, lsl #20@ flags + kernel base
00233: strr3, [r4, r6, lsl #2]@ identity mapping
00234:
00235:
00239: addr0, r4, #(TEXTADDR & 0xff000000) >> 18@ start of kernel
00240: strr3, [r0, #(TEXTADDR & 0x00f00000) >> 18]!
00241:
00242: ldrr6, =(_end - PAGE_OFFSET - 1)@ r6 = number of sections
00243: movr6, r6, lsr #20@ needed for kernel minus 1
00244:
00245: 1:addr3, r3, #1 < 20
00246: strr3, [r0, #4]!
00247: subsr6, r6, #1
00248: bgt1b
00249:
00250:
00253: addr0, r4, #PAGE_OFFSET >> 18
00254: orrr6, r7, #PHYS_OFFSET
00255: strr6, [r0]
...
00314:movpc, 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:.macropgtbl, 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
