鸿蒙轻内核A核源码分析系列之虚实映射（2）虚实映射初始化
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2、 虚拟映射初始化
在文件kernel/base/vm/los_vm_boot.c中的码分系统内存初始化函数OsSysMemInit()会调用虚实映射初始化函数OsInitMappingStartUp()。该函数代码定义在文件arch/arm/arm/src/los_arch_mmu.c，析系虚实虚实代码如下。映射映射
⑴处函数使TLB失效，初始清理虚实映射缓存数据，鸿蒙核A核源化涉及些cp15寄存器和汇编，轻内后续再分析。码分
⑵处函数切换到临时TTB。析系虚实虚实
⑶处设置内核地址空间的映射映射映射。下面分别详细这些函数代码。初始
VOID OsInitMappingStartUp(VOID) { ⑴ OsArmInvalidateTlbBarrier(); ⑵ OsSwitchTmpTTB(); ⑶ OsSetKSectionAttr(KERNEL_VMM_BASE,鸿蒙核A核源化 FALSE); OsSetKSectionAttr(UNCACHED_VMM_BASE, TRUE); OsKSectionNewAttrEnable(); }2.1 函数OsSwitchTmpTTB
函数OsSwitchTmpTTB申请16KiB的内存存放L1页表项数据，把页表项数据从g_firstPageTable复制到申请的轻内内存区域。⑴处获取内核地址空间。码分L1页表由4096个页表项组成，每个4KiB，共需要16KiB大小。所以⑵处代码按16KiB对齐申请16KiB大小的内存区域存放L1页表项。⑶处设置内核虚拟内存地址空间的转换表基地址TTB。⑷处把g_firstPageTable页表数据复制到内核地址空间的转换表区域。如果复制失败，则直接使用g_firstPageTable。⑸处设置内核虚拟地址空间的TTB转换地址对应的源码下载物理内存地址，然后调用函数OsArmWriteTtbr0写入MMU寄存器。
STATIC VOID OsSwitchTmpTTB(VOID) { PTE_T *tmpTtbase = NULL; errno_t err; ⑴ LosVmSpace *kSpace = LOS_GetKVmSpace(); /* ttbr address should be 16KByte align */ ⑵ tmpTtbase = LOS_MemAllocAlign(m_aucSysMem0, MMU_DESCRIPTOR_L1_SMALL_ENTRY_NUMBERS, MMU_DESCRIPTOR_L1_SMALL_ENTRY_NUMBERS); if (tmpTtbase == NULL) { VM_ERR("memory alloc failed"); return; } ⑶ kSpace->archMmu.virtTtb = tmpTtbase; ⑷ err = memcpy_s(kSpace->archMmu.virtTtb, MMU_DESCRIPTOR_L1_SMALL_ENTRY_NUMBERS, g_firstPageTable, MMU_DESCRIPTOR_L1_SMALL_ENTRY_NUMBERS); if (err != EOK) { (VOID)LOS_MemFree(m_aucSysMem0, tmpTtbase); kSpace->archMmu.virtTtb = (VADDR_T *)g_firstPageTable; VM_ERR("memcpy failed, errno: %d", err); return; } ⑸ kSpace->archMmu.physTtb = LOS_PaddrQuery(kSpace->archMmu.virtTtb); OsArmWriteTtbr0(kSpace->archMmu.physTtb | MMU_TTBRx_FLAGS); ISB; }2.2 函数OsSetKSectionAttr
内部函数OsSetKSectionAttr用于设置内核虚拟地址空间的区间属性，分别针对内核虚拟地址空间的内核区间[KERNEL_ASPACE_BASE,KERNEL_ASPACE_BASE+KERNEL_ASPACE_SIZE]和未缓存区间[UNCACHED_VMM_BASE,UNCACHED_VMM_BASE+UNCACHED_VMM_SIZE]进行设置。内核虚拟地址空间是固定映射到物理内存的，内核地址空间的映射包含代码段、数据段、堆栈区间映射，如下示意图所示：
⑴处计算相对内核虚拟地址空间基地址KERNEL_VMM_BASE的偏移大小。⑵处先计算相对偏移值的text、rodata、data_bss段的虚拟内存地址，然后创建这些段的虚实映射关系数组mmuKernelMappings。⑶处设置内核虚拟地址区间的虚拟转换基地址TTB和物理转换基地址TTB。然后解除虚拟地址virtAddr的虚实映射，解除映射的长度就是代码段、只读数据段、数据BSS段这些内存段的长度。云服务器提供商⑷处按指定的标签flags对text代码段之前的内存区间进行虚实映射。⑸处映射text代码段、rodata只读数据段、data_bss数据段的内存区间，并调用函数LOS_VmSpaceReserve在进程空间中预定地址区间。⑹是BSS段后面的heap区、stack区的映射，映射虚拟地址空间的内存堆栈区间到对应的物理内存区间。
STATIC VOID OsSetKSectionAttr(UINTPTR virtAddr, BOOL uncached) { ⑴ UINT32 offset = virtAddr - KERNEL_VMM_BASE; /* every section should be page aligned */ ⑵ UINTPTR textStart = (UINTPTR)&__text_start + offset; UINTPTR textEnd = (UINTPTR)&__text_end + offset; UINTPTR rodataStart = (UINTPTR)&__rodata_start + offset; UINTPTR rodataEnd = (UINTPTR)&__rodata_end + offset; UINTPTR ramDataStart = (UINTPTR)&__ram_data_start + offset; UINTPTR bssEnd = (UINTPTR)&__bss_end + offset; UINT32 bssEndBoundary = ROUNDUP(bssEnd, MB); LosArchMmuInitMapping mmuKernelMappings[] = { { .phys = SYS_MEM_BASE + textStart - virtAddr, .virt = textStart, .size = ROUNDUP(textEnd - textStart, MMU_DESCRIPTOR_L2_SMALL_SIZE), .flags = VM_MAP_REGION_FLAG_PERM_READ | VM_MAP_REGION_FLAG_PERM_EXECUTE, .name = "kernel_text" }, { .phys = SYS_MEM_BASE + rodataStart - virtAddr, .virt = rodataStart, .size = ROUNDUP(rodataEnd - rodataStart, MMU_DESCRIPTOR_L2_SMALL_SIZE), .flags = VM_MAP_REGION_FLAG_PERM_READ, .name = "kernel_rodata" }, { .phys = SYS_MEM_BASE + ramDataStart - virtAddr, .virt = ramDataStart, .size = ROUNDUP(bssEndBoundary - ramDataStart, MMU_DESCRIPTOR_L2_SMALL_SIZE), .flags = VM_MAP_REGION_FLAG_PERM_READ | VM_MAP_REGION_FLAG_PERM_WRITE, .name = "kernel_data_bss" } }; LosVmSpace *kSpace = LOS_GetKVmSpace(); status_t status; UINT32 length; int i; LosArchMmuInitMapping *kernelMap = NULL; UINT32 kmallocLength; UINT32 flags; /* use second-level mapping of default READ and WRITE */ ⑶ kSpace->archMmu.virtTtb = (PTE_T *)g_firstPageTable; kSpace->archMmu.physTtb = LOS_PaddrQuery(kSpace->archMmu.virtTtb); status = LOS_ArchMmuUnmap(&kSpace->archMmu, virtAddr, (bssEndBoundary - virtAddr) >> MMU_DESCRIPTOR_L2_SMALL_SHIFT); if (status != ((bssEndBoundary - virtAddr) >> MMU_DESCRIPTOR_L2_SMALL_SHIFT)) { VM_ERR("unmap failed, status: %d", status); return; } flags = VM_MAP_REGION_FLAG_PERM_READ | VM_MAP_REGION_FLAG_PERM_WRITE | VM_MAP_REGION_FLAG_PERM_EXECUTE; if (uncached) { flags |= VM_MAP_REGION_FLAG_UNCACHED; } ⑷ status = LOS_ArchMmuMap(&kSpace->archMmu, virtAddr, SYS_MEM_BASE, (textStart - virtAddr) >> MMU_DESCRIPTOR_L2_SMALL_SHIFT, flags); if (status != ((textStart - virtAddr) >> MMU_DESCRIPTOR_L2_SMALL_SHIFT)) { VM_ERR("mmap failed, status: %d", status); return; } ⑸ length = sizeof(mmuKernelMappings) / sizeof(LosArchMmuInitMapping); for (i = 0; i < length; i++) { kernelMap = &mmuKernelMappings[i]; if (uncached) { kernelMap->flags |= VM_MAP_REGION_FLAG_UNCACHED; } status = LOS_ArchMmuMap(&kSpace->archMmu, kernelMap->virt, kernelMap->phys, kernelMap->size >> MMU_DESCRIPTOR_L2_SMALL_SHIFT, kernelMap->flags); if (status != (kernelMap->size >> MMU_DESCRIPTOR_L2_SMALL_SHIFT)) { VM_ERR("mmap failed, status: %d", status); return; } LOS_VmSpaceReserve(kSpace, kernelMap->size, kernelMap->virt); } ⑹ kmallocLength = virtAddr + SYS_MEM_SIZE_DEFAULT - bssEndBoundary; flags = VM_MAP_REGION_FLAG_PERM_READ | VM_MAP_REGION_FLAG_PERM_WRITE; if (uncached) { flags |= VM_MAP_REGION_FLAG_UNCACHED; } status = LOS_ArchMmuMap(&kSpace->archMmu, bssEndBoundary, SYS_MEM_BASE + bssEndBoundary - virtAddr, kmallocLength >> MMU_DESCRIPTOR_L2_SMALL_SHIFT, flags); if (status != (kmallocLength >> MMU_DESCRIPTOR_L2_SMALL_SHIFT)) { VM_ERR("mmap failed, status: %d", status); return; } LOS_VmSpaceReserve(kSpace, kmallocLength, bssEndBoundary); }2.3 函数OsKSectionNewAttrEnable
函数OsKSectionNewAttrEnable设置虚实地址的转换表基地址TTB并清楚TLB缓存。⑴处获取内核虚拟进程空间，⑵处设置进程空间MMU的虚拟地址转换表基地址TTB，然后查询到物理内存地址并设置物理内存地址转换表基地址。⑶处从CP15 C2寄存器读取TTB地址，取高20位。⑷处将内核物理内存页表基地址写入CP15 c2 TTB寄存器。⑸处清空TLB缓冲区，然后释放内存。涉及到了MMU寄存器，后续系列会专门详细讲解。
STATIC VOID OsKSectionNewAttrEnable(VOID) { ⑴ LosVmSpace *kSpace = LOS_GetKVmSpace(); paddr_t oldTtPhyBase; ⑵ kSpace->archMmu.virtTtb = (PTE_T *)g_firstPageTable; kSpace->archMmu.physTtb = LOS_PaddrQuery(kSpace->archMmu.virtTtb); /* we need free tmp ttbase */ ⑶ oldTtPhyBase = OsArmReadTtbr0(); oldTtPhyBase = oldTtPhyBase & MMU_DESCRIPTOR_L2_SMALL_FRAME; ⑷ OsArmWriteTtbr0(kSpace->archMmu.physTtb | MMU_TTBRx_FLAGS); ISB; /* we changed page table entry, so we need to clean TLB here */ ⑸ OsCleanTLB(); (VOID)LOS_MemFree(m_aucSysMem0, (VOID *)(UINTPTR)(oldTtPhyBase - SYS_MEM_BASE + KERNEL_VMM_BASE)); }想了解更多内容，请访问：
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