Magenta源代码笔记(3) —— 内存管理【转】

转自：http://blog.csdn.net/boymax2/article/details/52550197

版权声明：本文为博主原创文章，未经博主允许不得转载。
 
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：
 
、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
 
、pmm初始化，也就是代码中物理内存结构的初始化
 
、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() __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 = ,
            .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 */
        {  }
    };  
 
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：
 
、重置mmu相关寄存器
 
、计算物理地址相对虚拟地址的偏移
 
、将tlb地址指向空间清零
 
、遍历mmu_initial_mappings结构，计算后写入tlb
 
、设置mmu相关寄存器
 
、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, , r12, c1, c0,
        /* i/d cache disable, mmu disabled */
        // cache位与mmu位置0
        bic     r12, #(<<)
        bic     r12, #(<< | <<)
    #if WITH_KERNEL_VM
        /* enable caches so atomics and spinlocks work */
        // cache位与mmu位置1
        orr     r12, r12, #(<<)
        orr     r12, r12, #(<<)
    #endif // WITH_KERNEL_VM
        // 写SCTLR寄存器
        mcr     p15, , r12, c1, c0,   
 
        /* 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, #
        mov     r6, #  
 
        /* walk through all the entries in the translation table, setting them up */
        // 遍历转换表结构清零
    :
        str     r5, [r4, r6, lsl #]
        add     r6, #
        cmp     r6, #
        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, #, #
        add     r8, r8, r10
        add     r8, r8, #( << )
        sub     r8, r8, #  
 
        /* 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, #  /* r6 = physical address / 1MB */
        lsr     r7, #  /* r7 = virtual address / 1MB */
        lsr     r8, #  /* r8 = size in 1MB chunks */  
 
        /* if size == 0, end of list */
        // 循环边界判断
        cmp     r8, #
        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 */  
 
    :
        // 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 #
            /* 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 #]  
 
        /* loop until we're done */
        // 准备下一个转换表元素的填充
        add     r6, #
        add     r7, #
        subs    r8, #
        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, #
        mcr     p15, , r12, c8, c7,
        isb  
 
        /* Write 0 to TTBCR */
        // ttbcr写0
        mcr     p15, , r12, c2, c0,
        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, , r12, c2, c0,
        isb  
 
        /* Write DACR */
        // 写DACR cache相关
        mov     r12, #0x1
        mcr     p15, , r12, c3, c0,
        isb  
 
        /* Read SCTLR into r12 */
        // 读SCTLR寄存器，手册P1711
        mrc     p15, , r12, c1, c0,   
 
        /* Disable TRE/AFE */
        // 禁用TRE和AFE标志位
        bic     r12, #(<< | <<)  
 
        /* Turn on the MMU */
        // MMU使能标志位
        orr     r12, #0x1  
 
        /* Write back SCTLR */
        // 写入SCTLR
        // MMU打开
        mcr     p15, , r12, c1, c0,
        isb  
 
        /* Jump to virtual code address */
        // 跳转
        ldr     pc, =1f
    :
        ...  
 
        /* Invalidate TLB */
        mov     r12, #
        mcr     p15, , r12, c8, c7,
        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：
 
、pmm_add_arena 将物理内存加入pmm结构
 
、vm_init_preheap 堆初始化前的准备工作（钩子）
 
、heap_init 堆的初始化
 
、vm_init_postheap 堆初始化后的工作（钩子）
 
、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：
 
、通过fdt库从bootloader中获取物理内存的长度
 
、在pmm中加入物理内存
 
、标记fdt结构的空间
 
、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 >= ) {
            /* walk the nodes, looking for 'memory' and 'chosen' */
            int depth = ;
            int offset = ;
            for (;;) {
                offset = fdt_next_node(fdt, offset, &depth);
                if (offset < )
                    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") == ) {
                    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 + ));  
 
                        /* trim size on certain platforms */
    #if ARCH_ARM
                        // 如果是32位arm，只使用内存前1GB
                        if (len > **1024U) {
                            len = **; /* 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") == ) {
                    ...
                }
            }
        }  
 
        /* 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 >= );  
 
        // 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 != );
        DEBUG_ASSERT(base + size -  >= 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 : ;
        // 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 -  > 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：
 
、在vmm结构中标记内核已使用的虚拟地址
 
、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 = ; 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 > ; ++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 = ; 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 > ) {
            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 > ) {
                    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) + ;
        DEBUG_ASSERT(n <= );  
 
        uint32_t ttbcr = (<<) | 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();
        ISB;
    #endif
    }  
 
至此Magenta内核有关内存管理的初始化完成。