2、 虚拟映射初始化
在文件kernel/base/vm/los_vm_boot.c中的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，代码如下。⑴处函数使TLB失效，清理虚实映射缓存数据，涉及些cp15寄存器和汇编，后续再分析。⑵处函数切换到临时TTB。⑶处设置内核地址空间的映射。下面分别详细这些函数代码。
VOID OsInitMappingStartUp(VOID)
{
⑴ OsArmInvalidateTlbBarrier();⑵ OsSwitchTmpTTB();⑶ OsSetKSectionAttr(KERNEL_VMM_BASE, 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复制到申请的内存区域。⑴处获取内核地址空间。L1页表由4096个页表项组成，每个4 bytes，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。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));
}