__create_page_tables

/* * Setup the initial page tables.  We only setup the barest * amount which are required to get the kernel running, which * generally means mapping in the kernel code. * * r8 = phys_offset, r9 = cpuid, r10 = procinfo * * Returns: *  r0, r3, r5-r7 corrupted *  r4 = page table (see ARCH_PGD_SHIFT in asm/memory.h) */ @ __create_page_tables函数初始化了一个非常简单页表，仅映射了使内核能够正常启动的代码空间， @ 更加细致的工作将会在后续阶段完善。__create_page_tables:pgtblr4, r8@ page table address,[r4] = 0x20000000([r8])+0x8000-0x4000@ 该地址是页表的起始地址，在kernel的入口地址（0x20008000）之前16KB。@ 第一步、将这16KB空间清零/* * Clear the swapper page table */movr0, r4 @ [r0] = 0x20004000movr3, #0 @ [r3] = 0addr6, r0, #PG_DIR_SIZE @ [r6] = 0x200080001:strr3, [r0], #4 @ loop clear from 0x20004000 to 0x20008000strr3, [r0], #4strr3, [r0], #4strr3, [r0], #4teqr0, r6bne1b#ifdef CONFIG_ARM_LPAE/* * Build the PGD(Page Global Directory) table (first level) to point to the PMD(Page Middle Directory) table. A PGD * entry is 64-bit wide. */movr0, r4addr3, r4, #0x1000@ first PMD table addressorrr3, r3, #3@ PGD block typemovr6, #4@ PTRS_PER_PGDmovr7, #1 << (55 - 32)@ L_PGD_SWAPPER1:#ifdef CONFIG_CPU_ENDIAN_BE8strr7, [r0], #4@ set top PGD entry bitsstrr3, [r0], #4@ set bottom PGD entry bits#elsestrr3, [r0], #4@ set bottom PGD entry bitsstrr7, [r0], #4@ set top PGD entry bits#endifaddr3, r3, #0x1000@ next PMD tablesubsr6, r6, #1bne1baddr4, r4, #0x1000@ point to the PMD tables#ifdef CONFIG_CPU_ENDIAN_BE8addr4, r4, #4@ we only write the bottom word#endif#endif第二步、 __enable_mmu页表的建立/*__enable_mmu函数使能mmu后，CPU发出的地址是虚拟地址，程序正常运行需要映射得到物理地址，为了保障正常地配置mmu，这里需要对这段代码1:1的绝对映射，映射范围__turn_mmu_on至__turn_mmu_on_end。正常使能mmu后，不需要这段特定的映射了，在后续C代码启动阶段时被paging_init()函数删除。*/ldrr7, [r10, #PROCINFO_MM_MMUFLAGS] @ mm_mmuflags,[r7] = 10001110000001110b=0x11c0e/* * Create identity mapping to cater for __enable_mmu. * This identity mapping will be removed by paging_init(). */adrr0, __turn_mmu_on_locldmiar0, {r3, r5, r6}subr0, r0, r3@ virt->phys offsetaddr5, r5, r0@ phys __turn_mmu_onaddr6, r6, r0@ phys __turn_mmu_on_endmovr5, r5, lsr #SECTION_SHIFT @ >>20 1Mmovr6, r6, lsr #SECTION_SHIFT @ >>201:orrr3, r7, r5, lsl #SECTION_SHIFT@ flags + kernel basestrr3, [r4, r5, lsl #PMD_ORDER]@ identity mapping@ [r4 + r5 << 2] = [r3],将描述符写到转换表中，4bytes对齐，每个符占4bytescmpr5, r6addlor5, r5, #1@ next sectionblo1b@ 第三步、根据当前内核所划分的虚拟地址空间，建立内核页表项，Linux默认内核在最高1G，因此内核起始虚拟地址一般为0xc0000000/* * Map our RAM from the start to the end of the kernel .bss section. */addr0, r4, #PAGE_OFFSET >> (SECTION_SHIFT - PMD_ORDER) @>>20 + <<2 = >>18,内核空间起始地址的描述符@ 截取PAGE_OFFSET的高14位与表头地址相加，作为基址送入寄存器r0；@ 这里之所以截取高14位，是由于section页表，可记录4K个表项，正好取虚拟地址的高12位作为页表的索引，@ 每个表项占用4字节，则索引号需乘4，为14位。[r0]对应转换表的起始一项ldrr6, =(_end - 1) @ bss section结束地址-1orrr3, r8, r7 @ [r7] = mm_mmuflags,[r8] = phys_offset = 0x20000000addr6, r4, r6, lsr #(SECTION_SHIFT - PMD_ORDER) @ 需要map的bss end address,[r6]对应转换表的最后一项@ 由此可以算得一级描述符的条目为([r6]-[r0])>>2个1:strr3, [r0], #1 << PMD_ORDER @ 将一个描述符（4bytes）写到对应转换表项后，转换表项后移4bytes，这是个1:1映射addr3, r3, #1 << SECTION_SHIFT @ 下1Mcmpr0, r6bls1b#ifdef CONFIG_XIP_KERNEL/* * Map the kernel image separately as it is not located in RAM. */@ 这里只是将内核存储空间重新映射 #define XIP_START XIP_VIRT_ADDR(CONFIG_XIP_PHYS_ADDR)movr3, pcmovr3, r3, lsr #SECTION_SHIFTorrr3, r7, r3, lsl #SECTION_SHIFTaddr0, r4,  #(XIP_START & 0xff000000) >> (SECTION_SHIFT - PMD_ORDER)strr3, [r0, #((XIP_START & 0x00f00000) >> SECTION_SHIFT) << PMD_ORDER]! @ [r0] = [r0] + #((XIP_START & 0x00f00000) >> SECTION_SHIFT) << PMD_ORDERldrr6, =(_edata_loc - 1)addr0, r0, #1 << PMD_ORDERaddr6, r4, r6, lsr #(SECTION_SHIFT - PMD_ORDER)1:cmpr0, r6addr3, r3, #1 << SECTION_SHIFTstrlsr3, [r0], #1 << PMD_ORDERbls1b#endif/* * Then map boot params address in r2 if specified. * We map 2 sections in case the ATAGs/DTB crosses a section boundary. */@ 第四步、填写boot传来的内核启动参数所在段的映射，一般为内存开始时的1MB段，这个段一般已经映射过了@ 对boot 参数的映射,boot参数存在r2中，需要map 2个sections以防该参数处于一个段的边界 movr0, r2, lsr #SECTION_SHIFT @ [r0] = [r2] >> 20movsr0, r0, lsl #SECTION_SHIFT @ [r0]就是[r2]的高12bit，1M对齐subner3, r0, r8 @ [r3] = [r0] - phys_addraddner3, r3, #PAGE_OFFSET @ 对应的虚拟地址addner3, r4, r3, lsr #(SECTION_SHIFT - PMD_ORDER) @ 转换表项 orrner6, r7, r0 @ 描述符项strner6, [r3], #1 << PMD_ORDER @ 下一个setion的描述符也更新addner6, r6, #1 << SECTION_SHIFTstrner6, [r3]@ 如果哪一步等于0就说明已经map过了#if defined(CONFIG_ARM_LPAE) && defined(CONFIG_CPU_ENDIAN_BE8)subr4, r4, #4@ Fixup page table pointer@ for 64-bit descriptors#endif#ifdef CONFIG_DEBUG_LL#if !defined(CONFIG_DEBUG_ICEDCC) && !defined(CONFIG_DEBUG_SEMIHOSTING)/* * Map in IO space for serial debugging. * This allows debug messages to be output * via a serial console before paging_init. */addruart r7, r3, r0movr3, r3, lsr #SECTION_SHIFTmovr3, r3, lsl #PMD_ORDERaddr0, r4, r3movr3, r7, lsr #SECTION_SHIFTldrr7, [r10, #PROCINFO_IO_MMUFLAGS] @ io_mmuflagsorrr3, r7, r3, lsl #SECTION_SHIFT#ifdef CONFIG_ARM_LPAEmovr7, #1 << (54 - 32)@ XN#ifdef CONFIG_CPU_ENDIAN_BE8strr7, [r0], #4strr3, [r0], #4#elsestrr3, [r0], #4strr7, [r0], #4#endif#elseorrr3, r3, #PMD_SECT_XNstrr3, [r0], #4#endif#else /* CONFIG_DEBUG_ICEDCC || CONFIG_DEBUG_SEMIHOSTING *//* we don't need any serial debugging mappings */ldrr7, [r10, #PROCINFO_IO_MMUFLAGS] @ io_mmuflags#endif#if defined(CONFIG_ARCH_NETWINDER) || defined(CONFIG_ARCH_CATS)/* * If we're using the NetWinder or CATS, we also need to map * in the 16550-type serial port for the debug messages */addr0, r4, #0xff000000 >> (SECTION_SHIFT - PMD_ORDER)orrr3, r7, #0x7c000000strr3, [r0]#endif#ifdef CONFIG_ARCH_RPC/* * Map in screen at 0x02000000 & SCREEN2_BASE * Similar reasons here - for debug.  This is * only for Acorn RiscPC architectures. */addr0, r4, #0x02000000 >> (SECTION_SHIFT - PMD_ORDER)orrr3, r7, #0x02000000strr3, [r0]addr0, r4, #0xd8000000 >> (SECTION_SHIFT - PMD_ORDER)strr3, [r0]#endif#endif#ifdef CONFIG_ARM_LPAEsubr4, r4, #0x1000@ point to the PGD tablemovr4, r4, lsr #ARCH_PGD_SHIFT#endifmovpc, lrENDPROC(__create_page_tables).ltorg.align__turn_mmu_on_loc:.long..long__turn_mmu_on.long__turn_mmu_on_end