arm linux 页表创建

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本文对arm linux页表创建函数进行说明。在http://blog.csdn.net/flaoter/article/details/73381695中对MMU使能之前的临时页表进行了说明，此文是对kernel中正式页表创建过程进行说明。文中使用的kernel版本为4.4。
arm linux使用两级页表，L1是pgd，L2是pte。其中L1页表共2048项，每项占用8bytes，每项对应2M内存，共占用2048*8=16K bytes。L1项指向的page中放有2个L2页表，每个页表256项，每项占用4 bytes，对应4K内存，共2*256*4=2K bytes，如下面代码中的h/w pt。Linux pt是linux管理用的页表。
arch/arm/include/asm/pgtable-2level.h

   pgd             pte
 |        |
 +--------+
 |        |       +------------+ +0
 +- - - - +       | Linux pt 0 |
 |        |       +------------+ +1024
 +--------+ +0    | Linux pt 1 |
 |        |-----> +------------+ +2048
 +- - - - + +4    |  h/w pt 0  |
 |        |-----> +------------+ +3072
 +--------+ +8    |  h/w pt 1  |
 |        |       +------------+ +4096

页表创建通过函数create_mapping实现，在kernel初始化时的paging_init函数中会对memblock进行页表创建。

struct map_desc {
    unsigned long virtual; //虚拟地址
    unsigned long pfn; //page frame
    unsigned long length; //地址的长度
    unsigned int type;
};
struct mem_type {
    pteval_t prot_pte;
    pteval_t prot_pte_s2;
    pmdval_t prot_l1;
    pmdval_t prot_sect;
    unsigned int domain;
};

static void __init create_mapping(struct map_desc *md)
{
    unsigned long addr, length, end;
    phys_addr_t phys;
    const struct mem_type *type;
    pgd_t *pgd;

    if (md->virtual != vectors_base() && md->virtual < TASK_SIZE) { //用户空间返回
        pr_warn("BUG: not creating mapping for 0x%08llx at 0x%08lx in user region\n",
            (long long)__pfn_to_phys((u64)md->pfn), md->virtual);
        return;
    }

    if ((md->type == MT_DEVICE || md->type == MT_ROM) &&
        md->virtual >= PAGE_OFFSET && md->virtual < FIXADDR_START &&
        (md->virtual < VMALLOC_START || md->virtual >= VMALLOC_END)) {   //IO类型内存申请低端内存
        pr_warn("BUG: mapping for 0x%08llx at 0x%08lx out of vmalloc space\n",
            (long long)__pfn_to_phys((u64)md->pfn), md->virtual);
    }

    type = &mem_types[md->type];

    addr = md->virtual & PAGE_MASK;
    phys = __pfn_to_phys(md->pfn);
    length = PAGE_ALIGN(md->length + (md->virtual & ~PAGE_MASK));

    if (type->prot_l1 == 0 && ((addr | phys | length) & ~SECTION_MASK)) {  //非段映射，返回  
        pr_warn("BUG: map for 0x%08llx at 0x%08lx can not be mapped using pages, ignoring.\n",
            (long long)__pfn_to_phys(md->pfn), addr);
        return;
    }

    pgd = pgd_offset_k(addr);             //(1)
    end = addr + length;
    do {
        unsigned long next = pgd_addr_end(addr, end);

        alloc_init_pud(pgd, addr, next, phys, type);  //(2)

        phys += next - addr;
        addr = next;
    } while (pgd++, addr != end);
}

(1) 查找一级页表项

#define PGDIR_SHIFT     21
#define pgd_index(addr)     ((addr) >> PGDIR_SHIFT)
#define pgd_offset(mm, addr)    ((mm)->pgd + pgd_index(addr))
#define pgd_offset_k(addr)  pgd_offset(&init_mm, addr)

在kernel初始化汇编阶段的head.S中可知，一级页表的地址范围是0xC0004000~0xC0008000，即pgd的起始地址是0xC0004000, map_lowmem中addr=0xC0000000, pgd_offset_k(addr)解释为
(pgd_t*)0xC0004000 + (0xC0000000) >> 21

pgtable-2level-types.h

typedef u32 pmdval_t;
typedef pmdval_t pgd_t[2];

pgd_t类型是u32[2]的数组类型，所以(pgd_t*)0xC0004000 + (0xC0000000) >> 21=0xC0007000

(2) 调用alloc_init_pmd

static void __init alloc_init_pmd(pud_t *pud, unsigned long addr,
                      unsigned long end, phys_addr_t phys,
                      const struct mem_type *type)
{
    pmd_t *pmd = pmd_offset(pud, addr);
    unsigned long next;

    do {
        /*
         * With LPAE, we must loop over to map
         * all the pmds for the given range.
         */
        next = pmd_addr_end(addr, end);

        /*
         * Try a section mapping - addr, next and phys must all be
         * aligned to a section boundary.
         */
        if (type->prot_sect &&
                ((addr | next | phys) & ~SECTION_MASK) == 0) {
            __map_init_section(pmd, addr, next, phys, type);  //(a)
        } else {
            alloc_init_pte(pmd, addr, next,
                        __phys_to_pfn(phys), type);   //(b)
        }

        phys += next - addr;

    } while (pmd++, addr = next, addr != end);
}

(a)段映射，

static void __init __map_init_section(pmd_t *pmd, unsigned long addr,
            unsigned long end, phys_addr_t phys,
            const struct mem_type *type)
{
    pmd_t *p = pmd;

#ifndef CONFIG_ARM_LPAE
    /*
     * In classic MMU format, puds and pmds are folded in to
     * the pgds. pmd_offset gives the PGD entry. PGDs refer to a
     * group of L1 entries making up one logical pointer to
     * an L2 table (2MB), where as PMDs refer to the individual
     * L1 entries (1MB). Hence increment to get the correct
     * offset for odd 1MB sections.
     * (See arch/arm/include/asm/pgtable-2level.h)
     */
    if (addr & SECTION_SIZE)
        pmd++;
#endif
    do {
        *pmd = __pmd(phys | type->prot_sect);        //页表填充项是(phys | type->prot_sect)
        phys += SECTION_SIZE;
    } while (pmd++, addr += SECTION_SIZE, addr != end);

    flush_pmd_entry(p);    //刷新到内存
}

typedef u32 pmdval_t;
typedef pmdval_t pmd_t;

pmd_t是u32类型，指针值就是传进来的pgd_t，*pmd填充的内容即页表项就是(phys | type->prot_sect)。此处地址是按1M累加的，传进来的addr是按2M累加的，所以一般情况此函数的do{}while()循环都是执行两次。
下图是我的平台执行map_lowmem后的结果，一共占用了0x74项内容，建立了虚拟地址0xC0000000~0xC7300000到物理地址0x80000000~0x87300000的映射。
0xC0000000 –> 页表项地址0xC0007000 –>页表项内容0x8001141E –>物理地址0x80000000

(b) 二级页表映射

pte_t * __init early_pte_alloc(pmd_t *pmd, unsigned long addr, unsigned long prot)
{
    if (pmd_none(*pmd)) {  //如果L1页表没映射
        pte_t *pte = early_alloc(PTE_HWTABLE_OFF + PTE_HWTABLE_SIZE);  //PTE_HWTABLE_OFF=512*4 bytes,PTE_HWTABLE_SIZE=512*4bystes，共申请4K
        __pmd_populate(pmd, __pa(pte), prot);   //填充L1页表，使L1页表关联到L2页表
    }
    BUG_ON(pmd_bad(*pmd));
    return pte_offset_kernel(pmd, addr);
}

void __init alloc_init_pte(pmd_t *pmd, unsigned long addr,
                  unsigned long end, unsigned long pfn,
                  const struct mem_type *type)
{
    pte_t *pte = early_pte_alloc(pmd, addr, type->prot_l1);  //pte为二级页表指针
    do {
        set_pte_ext(pte, pfn_pte(pfn, __pgprot(type->prot_pte)), 0);  //填充L2页表项
        pfn++;    //L2页表每一项对应4k地址
    } while (pte++, addr += PAGE_SIZE, addr != end);
}

本例中virtual_addr = 0xFFFF0000, virtual_end=0xFFFF1000, pfn=0x000873FC，只进行4K地址的映射，因此只需要填充二级页表的1项内容即可。
二级页表指针pte = 0xC73FF7C0， [0xC73FF7C0] = 0x873FC5DF。建立了虚拟地址0xFFFF0000到物理地址0x873FC000的映射关系。