Magenta源代码笔记(3) —— 内存管理【转】

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Magenta内核支持虚拟地址的配置，依赖于cpu内的mmu模块。

下面会从以下几个方面对Magenta内核内存管理方面的代码进行分析：

1、mmu初始化，也就是硬件mmu的初始化，以底层寄存器操作为主，汇编

2、pmm初始化，也就是代码中物理内存结构的初始化

3、vmm初始化，也就是代码中虚拟内存结构的初始化


mmu初始化

mmu初始化的代码由汇编完成，其中主要涉及了以下几个结构

TLB：内存映射表，其定义位于c代码中

kernel/arch/arm/arm/mmu.c

[cpp] view plain copy

    uint32_t arm_kernel_translation_table[TT_ENTRY_COUNT] __ALIGNED(16384) __SECTION(".bss.prebss.translation_table");  

以及初始化的内存映射关系，以qemu-virt平台

kernel/platform/qemu-virt/platform.c

[cpp] view plain copy

    struct mmu_initial_mapping mmu_initial_mappings[] = {  
        /* all of memory */  
        {  
            .phys = MEMORY_BASE_PHYS, // 内存物理基地址  
            .virt = KERNEL_BASE, // 内存虚拟基地址  
            .size = MEMORY_APERTURE_SIZE,// 虚拟内存大小  
            .flags = 0,  
            .name = "memory"  
        },  
      
        /* 1GB of peripherals */  
        {  
            .phys = PERIPHERAL_BASE_PHYS, // 外设物理基地址  
            .virt = PERIPHERAL_BASE_VIRT, // 外设虚拟基地址  
            .size = PERIPHERAL_BASE_SIZE, // 虚拟内存大小  
            .flags = MMU_INITIAL_MAPPING_FLAG_DEVICE,  
            .name = "peripherals"  
        },  
      
        /* null entry to terminate the list */  
        { 0 }  
    };  

这两个结构都会在之后的汇编代码中使用。

mmu初始化的汇编代码位于内核的启动文件中，以arm32为例

自己对arm汇编不是很熟悉，在读汇编代码时花费了比较多的时间，希望有错误能指正出来

启动文件中与mmu相关的代码已经提取出来

在其中主要涉及到的操作为以下几个：

1、重置mmu相关寄存器

2、计算物理地址相对虚拟地址的偏移

3、将tlb地址指向空间清零

4、遍历mmu_initial_mappings结构，计算后写入tlb

5、设置mmu相关寄存器

6、跳转至c代码

kernel/arch/arm/arm/start.S

[plain] view plain copy

    #include <asm.h>  
    #include <arch/arm/cores.h>  
    #include <arch/arm/mmu.h>  
    #include <kernel/vm.h>  
      
    .section ".text.boot"  
    .globl _start  
    _start:  
        b   platform_reset  
        b   arm_undefined  
        b   arm_syscall  
        b   arm_prefetch_abort  
        b   arm_data_abort  
        b   arm_reserved  
        b   arm_irq  
        b   arm_fiq  
    #if WITH_SMP  
        b   arm_reset  
    #endif  
      
    .weak platform_reset  
    platform_reset:  
        /* Fall through for the weak symbol */  
      
    // arm复位处理程序  
    .globl arm_reset  
    arm_reset:  
        /* do some early cpu setup */  
        // 读SCTLR寄存器，手册P1711  
        mrc     p15, 0, r12, c1, c0, 0  
        /* i/d cache disable, mmu disabled */  
        // cache位与mmu位置0  
        bic     r12, #(1<<12)  
        bic     r12, #(1<<2 | 1<<0)  
    #if WITH_KERNEL_VM  
        /* enable caches so atomics and spinlocks work */  
        // cache位与mmu位置1  
        orr     r12, r12, #(1<<12)  
        orr     r12, r12, #(1<<2)  
    #endif // WITH_KERNEL_VM  
        // 写SCTLR寄存器  
        mcr     p15, 0, r12, c1, c0, 0  
      
        /* calculate the physical offset from our eventual virtual location */  
        // 计算物理地址相对虚拟地址的偏移，用于之后的转换  
    .Lphys_offset:  
        ldr     r4, =.Lphys_offset  
        adr     r11, .Lphys_offset  
        sub     r11, r11, r4  
      
    ...  
      
    #if ARM_WITH_MMU  
    .Lsetup_mmu:  
      
        /* set up the mmu according to mmu_initial_mappings */  
      
        /* load the base of the translation table and clear the table */  
        // 获取转换表地址  
        ldr     r4, =arm_kernel_translation_table  
        // 获取转换表物理地址  
        add     r4, r4, r11  
            /* r4 = physical address of translation table */  
      
        mov     r5, #0  
        mov     r6, #0  
      
        /* walk through all the entries in the translation table, setting them up */  
        // 遍历转换表结构清零  
    0:  
        str     r5, [r4, r6, lsl #2]  
        add     r6, #1  
        cmp     r6, #4096  
        bne     0b  
      
        /* load the address of the mmu_initial_mappings table and start processing */  
        // 获取初始映射地址  
        ldr     r5, =mmu_initial_mappings  
        // 获取初始映射物理地址  
        add     r5, r5, r11  
            /* r5 = physical address of mmu initial mapping table */  
      
    // 初始映射遍历绑定至转换表  
    // 转换表的绑定 转换表中元素的高12位为物理基地址下标，低20位为mmu相关flag  
    .Linitial_mapping_loop:  
        // 把结构体加载到各个通用寄存器中  
        ldmia   r5!, { r6-r10 }  
            /* r6 = phys, r7 = virt, r8 = size, r9 = flags, r10 = name */  
      
        /* round size up to 1MB alignment */  
        // 上调size对齐1MB  
        ubfx        r10, r6, #0, #20  
        add     r8, r8, r10  
        add     r8, r8, #(1 << 20)  
        sub     r8, r8, #1  
      
        /* mask all the addresses and sizes to 1MB boundaries */  
        // 物理地址 虚拟地址 大小 右移20位 取高12位  
        lsr     r6, #20  /* r6 = physical address / 1MB */  
        lsr     r7, #20  /* r7 = virtual address / 1MB */  
        lsr     r8, #20  /* r8 = size in 1MB chunks */  
      
        /* if size == 0, end of list */  
        // 循环边界判断  
        cmp     r8, #0  
        beq     .Linitial_mapping_done  
      
        /* set up the flags */  
        // 设置mmu相关flag，放置在r10  
        ldr     r10, =MMU_KERNEL_L1_PTE_FLAGS  
        teq     r9, #MMU_INITIAL_MAPPING_FLAG_UNCACHED  
        ldreq   r10, =MMU_INITIAL_MAP_STRONGLY_ORDERED  
        beq     0f  
        teq     r9, #MMU_INITIAL_MAPPING_FLAG_DEVICE  
        ldreq   r10, =MMU_INITIAL_MAP_DEVICE  
            /* r10 = mmu entry flags */  
      
    0:  
        // 计算translation_table元素的值  
        // r10：mmu相关flag r6：物理地址高12位  
        // r12 = r10 | (r6 << 20)  
        // 高20位为物理地址，低12位为mmu相关flag  
        orr     r12, r10, r6, lsl #20  
            /* r12 = phys addr | flags */  
      
        /* store into appropriate translation table entry */  
        // r4：转换表物理基地址 r7：虚拟地址对应的section  
        // r12 -> [r4 + r7 << 2]  
        str     r12, [r4, r7, lsl #2]  
      
        /* loop until we‘re done */  
        // 准备下一个转换表元素的填充  
        add     r6, #1  
        add     r7, #1  
        subs    r8, #1  
        bne     0b  
      
        b       .Linitial_mapping_loop  
      
    .Linitial_mapping_done:  
        ...  
      
        /* set up the mmu */  
        bl      .Lmmu_setup  
    #endif // WITH_KERNEL_VM  
      
        ...  
       // 跳转至c程序  
        bl      lk_main  
        b       .  
      
    #if WITH_KERNEL_VM  
        /* per cpu mmu setup, shared between primary and secondary cpus  
           args:  
           r4 == translation table physical  
           r8 == final translation table physical (if using trampoline)  
        */  
    // 设置mmu相关寄存器  
    // r4：转换表物理基地址  
    // mmu相关寄存器 手册P1724  
    .Lmmu_setup:  
        /* Invalidate TLB */  
        mov     r12, #0  
        mcr     p15, 0, r12, c8, c7, 0  
        isb  
      
        /* Write 0 to TTBCR */  
        // ttbcr写0  
        mcr     p15, 0, r12, c2, c0, 2  
        isb  
      
        /* Set cacheable attributes on translation walk */  
        // 宏MMU_TTBRx_FLAGS为 (1 << 3) | (1 << 6)  
        orr     r12, r4, #MMU_TTBRx_FLAGS  
      
        /* Write ttbr with phys addr of the translation table */  
        // 写入ttbr0  
        mcr     p15, 0, r12, c2, c0, 0  
        isb  
      
        /* Write DACR */  
        // 写DACR cache相关  
        mov     r12, #0x1  
        mcr     p15, 0, r12, c3, c0, 0  
        isb  
      
        /* Read SCTLR into r12 */  
        // 读SCTLR寄存器，手册P1711  
        mrc     p15, 0, r12, c1, c0, 0  
      
        /* Disable TRE/AFE */  
        // 禁用TRE和AFE标志位  
        bic     r12, #(1<<29 | 1<<28)  
      
        /* Turn on the MMU */  
        // MMU使能标志位  
        orr     r12, #0x1  
      
        /* Write back SCTLR */  
        // 写入SCTLR  
        // MMU打开  
        mcr     p15, 0, r12, c1, c0, 0  
        isb  
      
        /* Jump to virtual code address */  
        // 跳转  
        ldr     pc, =1f  
    1:  
        ...  
      
        /* Invalidate TLB */  
        mov     r12, #0  
        mcr     p15, 0, r12, c8, c7, 0  
        isb  
      
        /* assume lr was in physical memory, adjust it before returning */  
        // 计算跳转点的虚拟地址，跳转，之后会调用lk_main  
        sub     lr, r11  
        bx      lr  
    #endif  
      
    ...  

硬件层的内存管理相关的初始化基本完成后，会跳转到c代码

位于kernel/top/main.c

其中有关内存管理的函数调用顺序为：

1、pmm_add_arena 将物理内存加入pmm结构

2、vm_init_preheap 堆初始化前的准备工作（钩子）

3、heap_init 堆的初始化

4、vm_init_postheap 堆初始化后的工作（钩子）

5、arm_mmu_init mmu相关的调整


首先要完成pmm初始化工作

pmm初始化主要分为以下几步：

1、通过fdt库从bootloader中获取物理内存的长度

2、在pmm中加入物理内存

3、标记fdt结构的空间

4、标记bootloader相关的空间

pmm中比较重要的一个结构体，pmm_arena_t代表着一块物理内存的抽象

kernel/include/kernel/vm.h

[cpp] view plain copy

    typedef struct pmm_arena {  
        struct list_node node; // 节点，物理内存链表  
        const char* name; // 名称  
      
        uint flags;  
        uint priority;  
      
        paddr_t base; // 物理内存基地址  
        size_t size; // 物理内存长度  
      
        size_t free_count; // 空闲的页数  
      
        struct vm_page* page_array; // 页结构数组  
        struct list_node free_list; // 节点，该内存中空闲空间的链表  
    } pmm_arena_t;  

接着以qemu-virt的platform为例，分析pmm初始化的过程

kernel/platform/qemu-virt.c

[cpp] view plain copy

    // 全局物理内存结构体  
    static pmm_arena_t arena = {  
        .name = "ram",  
        .base = MEMORY_BASE_PHYS,  
        .size = DEFAULT_MEMORY_SIZE,  
        .flags = PMM_ARENA_FLAG_KMAP,  
    };  
    ...  
    // 该函数为平台的早期初始化，在内核启动时调用  
    void platform_early_init(void)  
    {  
        ...  
        /* look for a flattened device tree just before the kernel */  
        // 获取fdt结构  
        const void *fdt = (void *)KERNEL_BASE;  
        int err = fdt_check_header(fdt);  
        if (err >= 0) {  
            /* walk the nodes, looking for ‘memory‘ and ‘chosen‘ */  
            int depth = 0;  
            int offset = 0;  
            for (;;) {  
                offset = fdt_next_node(fdt, offset, &depth);  
                if (offset < 0)  
                    break;  
      
                /* get the name */  
                const char *name = fdt_get_name(fdt, offset, NULL);  
                if (!name)  
                    continue;  
      
                /* look for the properties we care about */  
                // 从fdt中查找到内存信息  
                if (strcmp(name, "memory") == 0) {  
                    int lenp;  
                    const void *prop_ptr = fdt_getprop(fdt, offset, "reg", &lenp);  
                    if (prop_ptr && lenp == 0x10) {  
                        /* we‘re looking at a memory descriptor */  
                        //uint64_t base = fdt64_to_cpu(*(uint64_t *)prop_ptr);  
                        // 获取内存长度  
                        uint64_t len = fdt64_to_cpu(*((const uint64_t *)prop_ptr + 1));  
      
                        /* trim size on certain platforms */  
    #if ARCH_ARM  
                        // 如果是32位arm，只使用内存前1GB  
                        if (len > 1024*1024*1024U) {  
                            len = 1024*1024*1024; /* only use the first 1GB on ARM32 */  
                            printf("trimming memory to 1GB\n");  
                        }  
    #endif  
                        /* set the size in the pmm arena */  
                        // 保存内存长度  
                        arena.size = len;  
                    }  
                } else if (strcmp(name, "chosen") == 0) {  
                    ...  
                }  
            }  
        }  
      
        /* add the main memory arena */  
        // 将改内存区域加入到pmm中  
        pmm_add_arena(&arena);  
      
        /* reserve the first 64k of ram, which should be holding the fdt */  
        // 标记fdt区域  
        pmm_alloc_range(MEMBASE, 0x10000 / PAGE_SIZE, NULL);  
      
        // 标记bootloader_ramdisk区域  
        platform_preserve_ramdisk();  
        ...  
    }  

内核在接下来初始化堆之前会在内存中构造出出一个VmAspace对象，其代表的是内核空间的抽象

kernel/kernel/vm/vm.cpp

[cpp] view plain copy

    void vm_init_preheap(uint level) {  
        LTRACE_ENTRY;  
      
        // allow the vmm a shot at initializing some of its data structures  
        // 构造代表内核空间的VmAspace对象  
        VmAspace::KernelAspaceInitPreHeap();  
      
        // mark all of the kernel pages in use  
        LTRACEF("marking all kernel pages as used\n");  
        // 标记内核代码所用内存  
        mark_pages_in_use((vaddr_t)&_start, ((uintptr_t)&_end - (uintptr_t)&_start));  
      
        // mark the physical pages used by the boot time allocator  
        // 标记boot time allocator代码所用内存  
        if (boot_alloc_end != boot_alloc_start) {  
            LTRACEF("marking boot alloc used from 0x%lx to 0x%lx\n", boot_alloc_start, boot_alloc_end);  
      
            mark_pages_in_use(boot_alloc_start, boot_alloc_end - boot_alloc_start);  
        }  
    }  

kernel/kernel/vm/vm_aspace.cpp

[cpp] view plain copy

    void VmAspace::KernelAspaceInitPreHeap() {  
        // the singleton kernel address space  
        // 构造一个内核空间单例，因为这个函数只会在启动时调用，所以是这个对象是单例  
        static VmAspace _kernel_aspace(KERNEL_ASPACE_BASE, KERNEL_ASPACE_SIZE, VmAspace::TYPE_KERNEL,  
                                       "kernel");  
        // 初始化  
        auto err = _kernel_aspace.Init();  
        ASSERT(err >= 0);  
      
        // save a pointer to the singleton kernel address space  
        // 保存单例指针  
        VmAspace::kernel_aspace_ = &_kernel_aspace;  
    }  
      
    VmAspace::VmAspace(vaddr_t base, size_t size, uint32_t flags, const char* name)  
        : base_(base), size_(size), flags_(flags) {  
      
        DEBUG_ASSERT(size != 0);  
        DEBUG_ASSERT(base + size - 1 >= base);  
      
        Rename(name);  
      
        LTRACEF("%p ‘%s‘\n", this, name_);  
    }  
      
    status_t VmAspace::Init() {  
        DEBUG_ASSERT(magic_ == MAGIC);  
      
        LTRACEF("%p ‘%s‘\n", this, name_);  
      
        // intialize the architectually specific part  
        // 标记为内核的空间  
        bool is_high_kernel = (flags_ & TYPE_MASK) == TYPE_KERNEL;  
        uint arch_aspace_flags = is_high_kernel ? ARCH_ASPACE_FLAG_KERNEL : 0;  
        // 调用mmu相关的函数  
        return arch_mmu_init_aspace(&arch_aspace_, base_, size_, arch_aspace_flags);  
    }  

kernel/arch/arm/arm/mmu.c

[cpp] view plain copy

    status_t arch_mmu_init_aspace(arch_aspace_t *aspace, vaddr_t base, size_t size, uint flags)  
    {  
        LTRACEF("aspace %p, base 0x%lx, size 0x%zx, flags 0x%x\n", aspace, base, size, flags);  
      
        DEBUG_ASSERT(aspace);  
        DEBUG_ASSERT(aspace->magic != ARCH_ASPACE_MAGIC);  
      
        /* validate that the base + size is sane and doesn‘t wrap */  
        DEBUG_ASSERT(size > PAGE_SIZE);  
        DEBUG_ASSERT(base + size - 1 > base);  
      
        // 初始化内核空间中页的链表  
        list_initialize(&aspace->pt_page_list);  
      
        aspace->magic = ARCH_ASPACE_MAGIC;  
        if (flags & ARCH_ASPACE_FLAG_KERNEL) {  
            // 设置结构内相关参数，其中转换表的物理内存通过vaddr_to_paddr获取  
            // 该函数不详细分析了，实质就是通过转换表进行查询得到的物理地址  
            aspace->base = base;  
            aspace->size = size;  
            aspace->tt_virt = arm_kernel_translation_table;  
            aspace->tt_phys = vaddr_to_paddr(aspace->tt_virt);  
        } else {  
            ...  
        }  
      
        LTRACEF("tt_phys 0x%lx tt_virt %p\n", aspace->tt_phys, aspace->tt_virt);  
      
        return NO_ERROR;  
    }  

到此内核空间的结构初始化完成

接下来进行内核堆的初始化，Magenta内核中提供了两种堆的实现miniheap以及cmpctmalloc，用户可以自己进行配置。

堆的具体实现方法会在之后进行具体的分析

堆的初始化完成以后，会调用相应的钩子函数，该函数的主要的作用如下：

1、在vmm结构中标记内核已使用的虚拟地址

2、根据内核使用的地址的区域，分别设置内存的保护

[cpp] view plain copy

    void vm_init_postheap(uint level) {  
        LTRACE_ENTRY;  
      
        vmm_aspace_t* aspace = vmm_get_kernel_aspace();  
      
        // we expect the kernel to be in a temporary mapping, define permanent  
        // regions for those now  
        struct temp_region {  
            const char* name;  
            vaddr_t base;  
            size_t size;  
            uint arch_mmu_flags;  
        } regions[] = {  
            {  
                .name = "kernel_code",  
                .base = (vaddr_t)&__code_start,  
                .size = ROUNDUP((size_t)&__code_end - (size_t)&__code_start, PAGE_SIZE),  
                .arch_mmu_flags = ARCH_MMU_FLAG_PERM_READ | ARCH_MMU_FLAG_PERM_EXECUTE,  
            },  
            {  
                .name = "kernel_rodata",  
                .base = (vaddr_t)&__rodata_start,  
                .size = ROUNDUP((size_t)&__rodata_end - (size_t)&__rodata_start, PAGE_SIZE),  
                .arch_mmu_flags = ARCH_MMU_FLAG_PERM_READ,  
            },  
            {  
                .name = "kernel_data",  
                .base = (vaddr_t)&__data_start,  
                .size = ROUNDUP((size_t)&__data_end - (size_t)&__data_start, PAGE_SIZE),  
                .arch_mmu_flags = ARCH_MMU_FLAG_PERM_READ | ARCH_MMU_FLAG_PERM_WRITE,  
            },  
            {  
                .name = "kernel_bss",  
                .base = (vaddr_t)&__bss_start,  
                .size = ROUNDUP((size_t)&__bss_end - (size_t)&__bss_start, PAGE_SIZE),  
                .arch_mmu_flags = ARCH_MMU_FLAG_PERM_READ | ARCH_MMU_FLAG_PERM_WRITE,  
            },  
            {  
                .name = "kernel_bootalloc",  
                .base = (vaddr_t)boot_alloc_start,  
                .size = ROUNDUP(boot_alloc_end - boot_alloc_start, PAGE_SIZE),  
                .arch_mmu_flags = ARCH_MMU_FLAG_PERM_READ | ARCH_MMU_FLAG_PERM_WRITE,  
            },  
        };  
      
        for (uint i = 0; i < countof(regions); ++i) {  
            temp_region* region = &regions[i];  
            ASSERT(IS_PAGE_ALIGNED(region->base));  
            status_t status = vmm_reserve_space(aspace, region->name, region->size, region->base);  
            ASSERT(status == NO_ERROR);  
            status = vmm_protect_region(aspace, region->base, region->arch_mmu_flags);  
            ASSERT(status == NO_ERROR);  
        }  
      
        // mmu_initial_mappings should reflect where we are now, use it to construct the actual  
        // mappings.  We will carve out the kernel code/data from any mappings and  
        // unmap any temporary ones.  
        const struct mmu_initial_mapping* map = mmu_initial_mappings;  
        for (map = mmu_initial_mappings; map->size > 0; ++map) {  
            LTRACEF("looking at mapping %p (%s)\n", map, map->name);  
            // Unmap temporary mappings except where they intersect with the  
            // kernel code/data regions.  
            vaddr_t vaddr = map->virt;  
            LTRACEF("vaddr 0x%lx, virt + size 0x%lx\n", vaddr, map->virt + map->size);  
            while (vaddr != map->virt + map->size) {  
                vaddr_t next_kernel_region = map->virt + map->size;  
                vaddr_t next_kernel_region_end = map->virt + map->size;  
      
                // Find the kernel code/data region with the lowest start address  
                // that is within this mapping.  
                for (uint i = 0; i < countof(regions); ++i) {  
                    temp_region* region = &regions[i];  
      
                    if (region->base >= vaddr && region->base < map->virt + map->size &&  
                        region->base < next_kernel_region) {  
      
                        next_kernel_region = region->base;  
                        next_kernel_region_end = region->base + region->size;  
                    }  
                }  
      
                // If vaddr isn‘t the start of a kernel code/data region, then we should make  
                // a mapping between it and the next closest one.  
                if (next_kernel_region != vaddr) {  
                    status_t status =  
                        vmm_reserve_space(aspace, map->name, next_kernel_region - vaddr, vaddr);  
                    ASSERT(status == NO_ERROR);  
      
                    if (map->flags & MMU_INITIAL_MAPPING_TEMPORARY) {  
                        // If the region is part of a temporary mapping, immediately unmap it  
                        LTRACEF("Freeing region [%016lx, %016lx)\n", vaddr, next_kernel_region);  
                        status = vmm_free_region(aspace, vaddr);  
                        ASSERT(status == NO_ERROR);  
                    } else {  
                        // Otherwise, mark it no-exec since it‘s not explicitly code  
                        status = vmm_protect_region(  
                            aspace,  
                            vaddr,  
                            ARCH_MMU_FLAG_PERM_READ | ARCH_MMU_FLAG_PERM_WRITE);  
                        ASSERT(status == NO_ERROR);  
                    }  
                }  
                vaddr = next_kernel_region_end;  
            }  
        }  
    }  

以上代码中涉及到的几个函数，只是做下简单的介绍，不具体分析：

vmm_reserve_space：在vmm中标记一块虚拟内存，这块虚拟内存抽象为VmRegion类，拥有自己的底层mmu相关的配置

vmm_protect_region：对某VmRegion对应的虚拟内存设置内存保护的相关参数


mmu相关的调整
mmu相关的调整，由内核新建的bootstrap2线程进行调用arch_init完成

kernel/arch/arm/arm/arch.c

[cpp] view plain copy

    void arch_init(void)  
    {  
       ...  
    #if ARM_WITH_MMU  
        /* finish intializing the mmu */  
        arm_mmu_init();  
    #endif  
    }  

kernel/arch/arm/arm/mmu.c

[cpp] view plain copy

    void arm_mmu_init(void)  
    {  
        /* unmap the initial mapings that are marked temporary */  
        // 解除具有MMU_INITIAL_MAPPING_TEMPORARY标志的内存映射  
        struct mmu_initial_mapping *map = mmu_initial_mappings;  
        while (map->size > 0) {  
            if (map->flags & MMU_INITIAL_MAPPING_TEMPORARY) {  
                vaddr_t va = map->virt;  
                size_t size = map->size;  
      
                DEBUG_ASSERT(IS_SECTION_ALIGNED(size));  
      
                while (size > 0) {  
                    arm_mmu_unmap_l1_entry(arm_kernel_translation_table, va / SECTION_SIZE);  
                    va += MB;  
                    size -= MB;  
                }  
            }  
            map++;  
        }  
        arm_after_invalidate_tlb_barrier();  
      
    #if KERNEL_ASPACE_BASE != 0  
        /* bounce the ttbr over to ttbr1 and leave 0 unmapped */  
        // 重新设置mmu相关的寄存器，禁用ttbcr0，将原先ttbr0的映射移动到ttbr1  
        // ttbr1为内核空间使用的寄存器  
        uint32_t n = __builtin_clz(KERNEL_ASPACE_BASE) + 1;  
        DEBUG_ASSERT(n <= 7);  
      
        uint32_t ttbcr = (1<<4) | n; /* disable TTBCR0 and set the split between TTBR0 and TTBR1 */  
      
        arm_write_ttbr1(arm_read_ttbr0());  
        ISB;  
        arm_write_ttbcr(ttbcr);  
        ISB;  
        arm_write_ttbr0(0);  
        ISB;  
    #endif  
    }  


至此Magenta内核有关内存管理的初始化完成。