__create_page_tables ========================== create_pag_tables主要完成以下工作 - 无效init_pg区域的cacheline - 清零init_pg内存区域 - 在idmap_pg区域为kernel创建恒等映射(物理地址与虚拟地址一致)，由于只在开始MMU时使用，因此只会为部分代码创建映射 - 在init_pg区域为kernel创建映射 - 再次无效init_pg和idmap_pg区域对应的cacheline .. note:: 在ARM64架构中，汇编代码初始化阶段会创建两次地址映射。第一次是为了打开MMU操作的准备，因为在打开MMU之前当前代码运行在物理地址之上，而打开 MMU之后代码与逆行在虚拟地址之上。为了从物理地址转换到虚拟地址的平滑过渡，ARM推荐VA和PA相等的一段映射(例如虚拟地址0xffff8000通过页表查询映射的物理地址也是0xffff8000) 这段映射在Linux中称为identity mapping, 第二次是kernel Image映射 执行完create_page_tables，将得到如下的地址空间布局 .. image:: res/kernel_mm_map.png 当kernel image加载到物理内存后，为.idmap.text段创建了idmap映射，其中.idmap.text属于.text段的一部分。idmap_pg页表空间位于.data段和.text段之间.为整个kernel iamge创建了Init映射， 其中init_pg页表位于.data段 .. note:: 为idmap.text创建恒等映射，实际就是创建idmap页表，通过填充pgd, pud, pmd页表项完成。其中pgd, pud为页表描述符，指向下级页表，pmd为块描述符，pmd指向idmap_text区域 =============================== ================================================================================================================= 主要宏 说明 ------------------------------- ----------------------------------------------------------------------------------------------------------------- KIMAGE_VADDR kernel起始虚拟地址，_text地址 PAGE_OFFSET Linear Mapping起始虚拟地址 PAGE_END Linear Mapping结束虚拟地址 __PHYS_OFFSET kernel起始虚拟地址，_text地址 KERNEL_START kernel起始虚拟地址，_text地址 =============================== ================================================================================================================= 建立页初始化的过程 :: __create_page_tables: mov x28, lr //保存LR,通过ret返回 /* * Invalidate the init page tables to avoid potential dirty cache lines * being evicted. Other page tables are allocated in rodata as part of * the kernel image, and thus are clean to the PoC per the boot * protocol. */ adrp x0, init_pg_dir // adrp x1, init_pg_end // sub x1, x1, x0 // bl __inval_dcache_area //将init_pg_end和init_pg_dir之间的区域对应的cacheline设定为无效 关于 ``init_pg_dir`` 和 ``init_pg_end`` 相关定义如下, 计算出kernel映射需要多少个page :: //arch/arm64/kernel/vmlinnux.lds.S . = ALIGN(PAGE_SIZE); init_pg_dir = .; . += INIT_DIR_SIZE; init_pg_end = .; #define INIT_DIR_SIZE (PAGE_SIZE * EARLY_PAGES(KIMAGE_VADDR, _end)) #define EARLY_PAGES(vstart, vend) ( 1 /* PGDIR page */ \ + EARLY_PGDS((vstart), (vend)) /* each PGDIR needs a next level page table */ \ + EARLY_PUDS((vstart), (vend)) /* each PUD needs a next level page table */ \ + EARLY_PMDS((vstart), (vend))) /* each PMD needs a next level page table */ #define EARLY_PGDS(vstart, vend) (EARLY_ENTRIES(vstart, vend, PGDIR_SHIFT)) #define EARLY_PUDS(vstart, vend) (0) #define EARLY_PMDS(vstart, vend) (EARLY_ENTRIES(vstart, vend, SWAPPER_TABLE_SHIFT)) #define EARLY_ENTRIES(vstart, vend, shift) (((vend) >> (shift)) \ - ((vstart) >> (shift)) + 1 + EARLY_KASLR) - #define PAGE_SIZE (_AC(1, UL) << PAGE_SHIFT) #define PGDIR_SHIFT ARM64_HW_PGTABLE_LEVEL_SHIFT(4 - CONFIG_PGTABLE_LEVELS) #define ARM64_HW_PGTABLE_LEVEL_SHIFT(n) ((PAGE_SHIFT - 3) * (4 - (n)) + 3) #define SWAPPER_PGTABLE_LEVELS (CONFIG_PGTABLE_LEVELS - 1) #define IDMAP_PGTABLE_LEVELS (ARM64_HW_PGTABLE_LEVELS(PHYS_MASK_SHIFT) - 1) #define PAGE_SHIFT CONFIG_ARM64_PAGE_SHIFT #define CONFIG_ARM64_PAGE_SHIFT 12 清除init page table :: /* * Clear the init page tables. */ adrp x0, init_pg_dir adrp x1, init_pg_end sub x1, x1, x0 1: stp xzr, xzr, [x0], #16 //将初始化页表地址清零 stp xzr, xzr, [x0], #16 stp xzr, xzr, [x0], #16 stp xzr, xzr, [x0], #16 subs x1, x1, #64 b.ne 1b mov x7, SWAPPER_MM_MMUFLAGS x7中保存了SWAPPER_MM_MMUFLAGS，相关定义如下 :: #define SWAPPER_MM_MMUFLAGS (PMD_ATTRINDX(MT_NORMAL) | SWAPPER_PMD_FLAGS) /* * AttrIndx[2:0] encoding (mapping attributes defined in the MAIR* registers). */ #define PMD_ATTRINDX(t) (_AT(pmdval_t, (t)) << 2) /* * Initial memory map attributes. */ #define SWAPPER_PMD_FLAGS (PMD_TYPE_SECT | PMD_SECT_AF | PMD_SECT_S) /*最低位为01，根据页表描述符为块描述符*/ #define PMD_TYPE_SECT (_AT(pmdval_t, 1) << 0) 创建identity映射 :: /* * Create the identity mapping. */ adrp x0, idmap_pg_dir //x0保存了id_map区域页表存放的起始地址idmap_pg_dir adrp x3, __idmap_text_start //x3保存了idmap_test_start的物理地址，它就是需要创建恒等映射的起始地 ``idmap_pg_dir`` 定义如下 :: #arch/arm64/kernel/vmlinux.lds.S idmap_pg_dir = .; . += IDMAP_DIR_SIZE; idmap_pg_end = .; #define IDMAP_DIR_SIZE (IDMAP_PGTABLE_LEVELS * PAGE_SIZE) #define IDMAP_PGTABLE_LEVELS (ARM64_HW_PGTABLE_LEVELS(PHYS_MASK_SHIFT) - 1) :: mov x5, #VA_BITS_MIN //获取总线位宽 adr_l x6, vabits_actual //获取vabit_actual变量地址 str x5, [x6] //将总线位宽写入到vabits_actual中 dmb sy // 内存屏障指令，等待上述指令完成 dc ivac, x6 //将x6指定的虚拟地址的数据缓存清除 /* * VA_BITS may be too small to allow for an ID mapping to be created * that covers system RAM if that is located sufficiently high in the * physical address space. So for the ID map, use an extended virtual * range in that case, and configure an additional translation level * if needed. * * Calculate the maximum allowed value for TCR_EL1.T0SZ so that the * entire ID map region can be mapped. As T0SZ == (64 - #bits used), * this number conveniently equals the number of leading zeroes in * the physical address of __idmap_text_end. */ adrp x5, __idmap_text_end //获取内核代码终止地址 clz x5, x5 //地址前导0个数，并赋值给x5 cmp x5, TCR_T0SZ(VA_BITS) //虚拟地址的最大值前导0个数和最高物理地址比较 //如果物理地址的前导0多，说明地址够用，不用扩展 b.ge 1f // .. then skip VA range extension adr_l x6, idmap_t0sz //获取idmap_t0sz变量地址 str x5, [x6] // dmb sy dc ivac, x6 // Invalidate potentially stale cache line /* * If VA_BITS == 48, we don't have to configure an additional * translation level, but the top-level table has more entries. */ mov x4, #1 << (PHYS_MASK_SHIFT - PGDIR_SHIFT) str_l x4, idmap_ptrs_per_pgd, x5 1: ldr_l x4, idmap_ptrs_per_pgd //获取idmap_ptrs_per_pgd地址 mov x5, x3 //x3中保存着内核代码起始地址，赋值为x5 adr_l x6, __idmap_text_end //获取内核代码终止地址 //map_memory是一个宏, x0页表位置，x1下一级页表项位置, x3需要映射的开始地址，x6需要映射的结束地址 //x7 映射最后一级页表项的flag, x3映射的物理地址 x4: pgd项个数 //此处为idmap test创建恒等映射, idmap.text段在head.S中申明 map_memory x0, x1, x3, x6, x7, x3, x4, x10, x11, x12, x13, x14 .. note:: 通过查看System.map可以知道idmap_text段包含以下内容 .. image:: res/id_map_text.png :: /* * Map the kernel image (starting with PHYS_OFFSET). */ adrp x0, init_pg_dir //x0存放页表的起始地址 mov_q x5, KIMAGE_VADDR + TEXT_OFFSET //x5存放kernel开始映射的虚拟地址 add x5, x5, x23 // add KASLR displacement mov x4, PTRS_PER_PGD //pgd页表项个数 adrp x6, _end //x6保存kernel结束映射的物理地址 adrp x3, _text //x3保存kernel的物理地址 sub x6, x6, x3 // _end - _text add x6, x6, x5 //计算得到kernel结束映射的虚拟地址并保存在x6中 //为kernel image创建页表 map_memory x0, x1, x5, x6, x7, x3, x4, x10, x11, x12, x13, x14 /* * Since the page tables have been populated with non-cacheable * accesses (MMU disabled), invalidate the idmap and swapper page * tables again to remove any speculatively loaded cache lines. */ adrp x0, idmap_pg_dir adrp x1, init_pg_end sub x1, x1, x0 dmb sy bl __inval_dcache_area //将init_pg和idmap_pg区域对应的cacheline ret x28 ENDPROC(__create_page_tables)