Linux 初始化过程页表建立

Linux 初始化过程，会依次建立如下页表映射：

1. 恒等映射：页表基地址 idmap_pg_dir;

2. 粗粒度内核镜像映射：即上篇博文里的“第二次建立页表映射”，页表基地址 init_pg_dir;

3.fixmap 映射：页表基地址为 init_pg_dir, 待 paging_init 之后为 swapper_pg_end;

4. 细粒度内核镜像映射：页表基地址为 swapper_pg_dir;

5. 线性映射：页表基地址为 swapper_pg_dir;

6. 用户空间页表映射：页表基地址 task->mm->pgd;

上篇已经解析了 1 和 2 映射，这篇解析fixmap 映射。

fixmap 映射的由来：

建立了恒等映射和粗粒度内核页表映射，只能保证内核镜像的正常访问，此时尚未建立内存管理子系统，如果想访问 bootloader 传入的 dtb，或者其他 io 设备，还是无法实现的，因此 Linux 提出了 fixmap.

fixmap：将一段 固定虚拟地址 映射到 dtb，以及想要访问的 IO 设备地址(比如串口，用于早期的打印调试)；

1.fixmap 映射

源码参考 arch/arm64/kernel/setup.c 文件：

start_kernel() 
  ->setup_arch()
    ->early_fixmap_init()
    ->early_ioramap_init()
    ->setup_machine_fdt(__fdt_pointer)

early_fixmap_init 函数

void __init early_fixmap_init(void)
{
    pgd_t *pgdp;
    p4d_t *p4dp, p4d;
    pud_t *pudp;
    pmd_t *pmdp;
    unsigned long addr = FIXADDR_START;

    pgdp = pgd_offset_k(addr);  /// 获得 pgd 页表项
    p4dp = p4d_offset(pgdp, addr);   
    p4d = READ_ONCE(*p4dp);
    if (CONFIG_PGTABLE_LEVELS> 3 &&
        !(p4d_none(p4d) || p4d_page_paddr(p4d) == __pa_symbol(bm_pud))) {
        /*
         * We only end up here if the kernel mapping and the fixmap
         * share the top level pgd entry, which should only happen on
         * 16k/4 levels configurations.
         */
        BUG_ON(!IS_ENABLED(CONFIG_ARM64_16K_PAGES));
        pudp = pud_offset_kimg(p4dp, addr);
    } else {if (p4d_none(p4d))
            __p4d_populate(p4dp, __pa_symbol(bm_pud), P4D_TYPE_TABLE);   /// 填充 p4d 表项
        pudp = fixmap_pud(addr);   /// 获得 pud 表项
    }
    if (pud_none(READ_ONCE(*pudp)))
        __pud_populate(pudp, __pa_symbol(bm_pmd), PUD_TYPE_TABLE);      /// 填充 pud 表项
    pmdp = fixmap_pmd(addr);
    __pmd_populate(pmdp, __pa_symbol(bm_pte), PMD_TYPE_TABLE);          /// 填充 pmd 表项

    /*
     * The boot-ioremap range spans multiple pmds, for which
     * we are not prepared:
     */
    BUILD_BUG_ON((__fix_to_virt(FIX_BTMAP_BEGIN) >> PMD_SHIFT)
             != (__fix_to_virt(FIX_BTMAP_END) >> PMD_SHIFT));

    if ((pmdp != fixmap_pmd(fix_to_virt(FIX_BTMAP_BEGIN)))
         || pmdp != fixmap_pmd(fix_to_virt(FIX_BTMAP_END))) {WARN_ON(1);
        pr_warn("pmdp %p != %p, %p\n",
            pmdp, fixmap_pmd(fix_to_virt(FIX_BTMAP_BEGIN)),
            fixmap_pmd(fix_to_virt(FIX_BTMAP_END)));
        pr_warn("fix_to_virt(FIX_BTMAP_BEGIN): %08lx\n",
            fix_to_virt(FIX_BTMAP_BEGIN));
        pr_warn("fix_to_virt(FIX_BTMAP_END):   %08lx\n",
            fix_to_virt(FIX_BTMAP_END));

        pr_warn("FIX_BTMAP_END:       %d\n", FIX_BTMAP_END);
        pr_warn("FIX_BTMAP_BEGIN:     %d\n", FIX_BTMAP_BEGIN);
    }
}

early_ioremap_init 函数

void __init early_ioremap_setup(void)
{
    int i;

    for (i = 0; i < FIX_BTMAPS_SLOTS; i++)
        if (WARN_ON(prev_map[i]))
            break;

    for (i = 0; i < FIX_BTMAPS_SLOTS; i++)
        slot_virt[i] = __fix_to_virt(FIX_BTMAP_BEGIN - NR_FIX_BTMAPS*i);   /// 根据索引，得到虚拟地址
}

上述填充完页表和计算准备虚拟地址，在 early_ioremap 函数，实际建立映射关系；

early_ioremap 函数

static void __init __iomem *
__early_ioremap(resource_size_t phys_addr, unsigned long size, pgprot_t prot)
{
    unsigned long offset;
    resource_size_t last_addr;
    unsigned int nrpages;
    enum fixed_addresses idx;
    int i, slot;

    WARN_ON(system_state>= SYSTEM_RUNNING);

    slot = -1;
    for (i = 0; i < FIX_BTMAPS_SLOTS; i++) {if (!prev_map[i]) {
            slot = i;
            break;
        }
    }

    if (WARN(slot < 0, "%s(%pa, %08lx) not found slot\n",
         __func__, &phys_addr, size))
        return NULL;

    /* Don't allow wraparound or zero size */
    last_addr = phys_addr + size - 1;
    if (WARN_ON(!size || last_addr < phys_addr))
        return NULL;

    prev_size[slot] = size;
    /*
     * Mappings have to be page-aligned
     */
    offset = offset_in_page(phys_addr);
    phys_addr &= PAGE_MASK;
    size = PAGE_ALIGN(last_addr + 1) - phys_addr;

    /*
     * Mappings have to fit in the FIX_BTMAP area.
     */
    nrpages = size >> PAGE_SHIFT;
    if (WARN_ON(nrpages> NR_FIX_BTMAPS))
        return NULL;

    /*
     * Ok, go for it..
     */
    idx = FIX_BTMAP_BEGIN - NR_FIX_BTMAPS*slot;
    while (nrpages> 0) {if (after_paging_init)
            __late_set_fixmap(idx, phys_addr, prot);
        else
            __early_set_fixmap(idx, phys_addr, prot);   /// 建立虚拟地址到 phys_addr 的映射
        phys_addr += PAGE_SIZE;
        --idx;
        --nrpages;
    }
    WARN(early_ioremap_debug, "%s(%pa, %08lx) [%d] => %08lx + %08lx\n",
         __func__, &phys_addr, size, slot, offset, slot_virt[slot]);

    prev_map[slot] = (void __iomem *)(offset + slot_virt[slot]);   /// 返回实际的虚拟地址
    return prev_map[slot];
}

earlycon

在 driver/tty/serail/earlycon.c 文件,earlycon_map 函数实际做 ioremap 映射

static void __iomem * __init earlycon_map(resource_size_t paddr, size_t size)
{
    void __iomem *base;
#ifdef CONFIG_FIX_EARLYCON_MEM
    set_fixmap_io(FIX_EARLYCON_MEM_BASE, paddr & PAGE_MASK);
    base = (void __iomem *)__fix_to_virt(FIX_EARLYCON_MEM_BASE);
    base += paddr & ~PAGE_MASK;
#else
    base = ioremap(paddr, size);
#endif
    if (!base)
        pr_err("%s: Couldn't map %pa\n", __func__, &paddr);

    return base;
}

3.dtb 映射

设备树，通过虚拟地址获取内存信息和板级信息；

setup_machine_fdt(__fdt_pointer);/// 设备树映射

void *__init fixmap_remap_fdt(phys_addr_t dt_phys, int *size, pgprot_t prot)
{const u64 dt_virt_base = __fix_to_virt(FIX_FDT);   /// 获得设备树的虚拟地址 i    
    int offset;
    void *dt_virt;

    /*
     * Check whether the physical FDT address is set and meets the minimum
     * alignment requirement. Since we are relying on MIN_FDT_ALIGN to be
     * at least 8 bytes so that we can always access the magic and size
     * fields of the FDT header after mapping the first chunk, double check
     * here if that is indeed the case.
     */
    BUILD_BUG_ON(MIN_FDT_ALIGN < 8);
    if (!dt_phys || dt_phys % MIN_FDT_ALIGN)
        return NULL;

    /*
     * Make sure that the FDT region can be mapped without the need to
     * allocate additional translation table pages, so that it is safe
     * to call create_mapping_noalloc() this early.
     *
     * On 64k pages, the FDT will be mapped using PTEs, so we need to
     * be in the same PMD as the rest of the fixmap.
     * On 4k pages, we'll use section mappings for the FDT so we only
     * have to be in the same PUD.
     */
    BUILD_BUG_ON(dt_virt_base % SZ_2M);

    BUILD_BUG_ON(__fix_to_virt(FIX_FDT_END) >> SWAPPER_TABLE_SHIFT !=
             __fix_to_virt(FIX_BTMAP_BEGIN) >> SWAPPER_TABLE_SHIFT);

    offset = dt_phys % SWAPPER_BLOCK_SIZE;
    dt_virt = (void *)dt_virt_base + offset;

    /* map the first chunk so we can read the size from the header */
    create_mapping_noalloc(round_down(dt_phys, SWAPPER_BLOCK_SIZE),      /// 建立映射，页表   的已知的，不能临时分配(因为伙伴系统尚未工作)
            dt_virt_base, SWAPPER_BLOCK_SIZE, prot);

    if (fdt_magic(dt_virt) != FDT_MAGIC)
        return NULL;

    *size = fdt_totalsize(dt_virt);
    if (*size> MAX_FDT_SIZE)
        return NULL;

    if (offset + *size> SWAPPER_BLOCK_SIZE)
        create_mapping_noalloc(round_down(dt_phys, SWAPPER_BLOCK_SIZE), dt_virt_base,
                   round_up(offset + *size, SWAPPER_BLOCK_SIZE), prot);

    return dt_virt;
}

建立映射后，即可以通过虚拟地址访问 dtb 内容;
内核通过 dtb 收集了内存布局信息后，会通过 memblock 模块进行管理。最后资源保存在 memblock 的 memory type 数组中；

该部分内容会用在后续的内存子系统建立中(伙伴系统初始化)；