[c]
struct mem_type {
        unsigned int prot_pte;
        unsigned int prot_l1;
        unsigned int prot_sect;
        unsigned int domain;
};
[/c] 

其中处成员含义如下：
prot_pte代表页表项的访问控制权，pte即第二级映射表项（页表项）。
prot_l1代表段表项的访问控制位，l1即第一级映射表项（段表项/主页表项）。
prot_sect代表主页表（注意，不是主页表项）的访问控制位和内存域。
domain代表所属的内存域。
对于ARM处理器，Linux定义了一个类型为struct mem_type的局部静态数组(arch/arm/mm/mmu.c)。根据不同的映射类型，定义了不同的访问权限。

   [c]
static struct mem_type mem_types[] = {
 [MT_DEVICE] = { 
 .prot_pte = PROT_PTE_DEVICE | L_PTE_MT_DEV_SHARED |
 L_PTE_SHARED,
 .prot_l1 = PMD_TYPE_TABLE,
 .prot_sect = PROT_SECT_DEVICE | PMD_SECT_S,
 .domain = DOMAIN_IO,
 },
 [MT_DEVICE_NONSHARED] = { 
 .prot_pte = PROT_PTE_DEVICE | L_PTE_MT_DEV_NONSHARED,
 .prot_l1 = PMD_TYPE_TABLE,
 .prot_sect = PROT_SECT_DEVICE,
 .domain = DOMAIN_IO,
 },
 [MT_DEVICE_CACHED] = { 
 .prot_pte = PROT_PTE_DEVICE | L_PTE_MT_DEV_CACHED,
 .prot_l1 = PMD_TYPE_TABLE,
 .prot_sect = PROT_SECT_DEVICE | PMD_SECT_WB,
 .domain = DOMAIN_IO,
 },
 [MT_DEVICE_WC] = { 
 .prot_pte = PROT_PTE_DEVICE | L_PTE_MT_DEV_WC,
 .prot_l1 = PMD_TYPE_TABLE,
 .prot_sect = PROT_SECT_DEVICE,
 .domain = DOMAIN_IO,
 },
 [MT_UNCACHED] = {
 .prot_pte = PROT_PTE_DEVICE,
 .prot_l1 = PMD_TYPE_TABLE,
 .prot_sect = PMD_TYPE_SECT | PMD_SECT_XN,
 .domain = DOMAIN_IO,
 },
 [MT_CACHECLEAN] = {
 .prot_sect = PMD_TYPE_SECT | PMD_SECT_XN,
 .domain = DOMAIN_KERNEL,
 },
 [MT_MINICLEAN] = {
 .prot_sect = PMD_TYPE_SECT | PMD_SECT_XN | PMD_SECT_MINICACHE,
 .domain = DOMAIN_KERNEL,
 },
 [MT_LOW_VECTORS] = {
 .prot_pte = L_PTE_PRESENT | L_PTE_YOUNG | L_PTE_DIRTY |
 L_PTE_EXEC,
 .prot_l1 = PMD_TYPE_TABLE,
 .domain = DOMAIN_USER,
 },
 [MT_HIGH_VECTORS] = {
 .prot_pte = L_PTE_PRESENT | L_PTE_YOUNG | L_PTE_DIRTY |
 L_PTE_USER | L_PTE_EXEC,
 .prot_l1 = PMD_TYPE_TABLE,
 .domain = DOMAIN_USER,
 },
 [MT_MEMORY] = {
 .prot_sect = PMD_TYPE_SECT | PMD_SECT_AP_WRITE,
 .domain = DOMAIN_KERNEL,
 },
 [MT_ROM] = {
 .prot_sect = PMD_TYPE_SECT,
 .domain = DOMAIN_KERNEL,
 },
 [MT_MEMORY_NONCACHED] = {
 .prot_sect = PMD_TYPE_SECT | PMD_SECT_AP_WRITE,
 .domain = DOMAIN_KERNEL,
 },
};
[/c] 

系统中定义了多个映射类型，最常用的是MT_MEMORY，它对应RAM;MT_DEVICE则对应了其他I/O设备，应用于ioremap;MT_ROM对应于ROM;MT_LOW_VECTORS对应0地址开始的向量;MT_HIGH_VECTORS对应高地址开始的向量，它有vector_base宏决定。

   [c]
arch/arm/include/asm/io.h

#define MT_DEVICE               0
#define MT_DEVICE_NONSHARED     1
#define MT_DEVICE_CACHED        2
#define MT_DEVICE_WC            3

arch/arm/include/asm/mach/map.h

#define MT_UNCACHED             4
#define MT_CACHECLEAN           5
#define MT_MINICLEAN            6
#define MT_LOW_VECTORS          7
#define MT_HIGH_VECTORS         8
#define MT_MEMORY               9
#define MT_ROM                  10
[/c] 

尽管ARM定义了16种不同的域，但是Linux只使用其中的三种：D0 ~ D2 (arch/arm/include/asm/domain.h)

   [c]
#define DOMAIN_KERNEL   0
#define DOMAIN_TABLE    0
#define DOMAIN_USER     1
#define DOMAIN_IO       2
[/c] 

内存空间和三种域的对应关系如下：

ARM处理器为每一个域定义了四种不两只的访问类型(0b00 ~ 0x11)，Linux使用其中的三种(0b10不用)，宏定义如下：
arch/arm/include/asm/domain.h

   [c]
#define DOMAIN_NOACCESS 0
#define DOMAIN_CLIENT   1
#define DOMAIN_MANAGER  3
[/c] 

Linux在系统引导设置MMU时初始化c3寄存器来实现对内存域的访问控制。其中对DOMAIN_USER，DOMAIN_KERNEL和DOMAIN_TABLE均设置DOMAIN_MANAGER权限;对DOMAIN_IO设置DOMAIN_CLIENT权限。
arch/arm/include/asm/domain.h

   [c]
#define domain_val(dom,type)    ((type) << (2*(dom)))

arch/arm/kernel/head.S
    ......
    mov     r5, #(domain_val(DOMAIN_USER, DOMAIN_MANAGER) | \
                  domain_val(DOMAIN_KERNEL, DOMAIN_MANAGER) | \
                  domain_val(DOMAIN_TABLE, DOMAIN_MANAGER) | \
                  domain_val(DOMAIN_IO, DOMAIN_CLIENT))
    mcr     p15, 0, r5, c3, c0, 0           @ load domain access register
    mcr     p15, 0, r4, c2, c0, 0           @ load page table pointer
    b       __turn_mmu_on
ENDPROC(__enable_mmu)
[/c] 

在系统的引导过程中对这3个域的访问控制位并不是一成不变的，它提供了一个名为modify_domain的宏来修改域访问控制位。系统在setup_arch中调用early_trap_init后，DOMAIN_USER的权限位将被设置成DOMAIN_CLIENT。
arch/arm/include/asm/domain.h

   [c]
#define set_domain(x)                                   \
        do {                                            \
        __asm__ __volatile__(                           \
        "mcr    p15, 0, %0, c3, c0      @ set domain"   \
          : : "r" (x));                                 \
        isb();                                          \
        } while (0)

#define modify_domain(dom,type)                                 \
        do {                                                    \
        struct thread_info *thread = current_thread_info();     \
        unsigned int domain = thread->cpu_domain;               \
        domain &= ~domain_val(dom, DOMAIN_MANAGER);             \
        thread->cpu_domain = domain | domain_val(dom, type);    \
        set_domain(thread->cpu_domain);                         \
        } while (0)
[/c]