fixmap

PLKA :

Basically, a fix-mapped linear address is a constant linear address like 0xffffc000 whose corresponding physical address does not have to be the linear address minus 0xc000000, but rather a physical address set in an arbitrary way. Thus, each fixmapped linear address maps one page frame of the physical memory. As we’ll see in later chapters, the kernel uses fix-mapped linear addresses instead of pointer variables that never change their value.

Each fix-mapped linear address is represented by a small integer index defined in the enum fixed_addresses data structure:

To associate a physical address with a fix-mapped linear address, the kernel uses the set_fixmap(idx,phys) and set_fixmap_nocache(idx,phys) macros. Both of them initialize the Page Table entry corresponding to the fix_to_virt(idx) linear address with the physical address phys; however, the second function also sets the PCD flag of the Page Table entry, thus disabling the hardware cache when accessing the data in the page frame (see the section “Hardware Cache” earlier in this chapter). Conversely, clear_fixmap(idx) removes the linking between a fix-mapped linear address idx and the physical address.

就是这个意思了，将固定的虚拟地址空间划分出来，来映射物理地址

• 物理地址不固定。LAPIC 的 MMIO 物理地址由固件（ACPI MADT / MP table）在启动时上报（register_lapic_address 的 mp_lapic_addr，典型默认值是 0xFEE00000），编译期无法知道。
• 但内核代码想要一个编译期常量地址来访问它。APIC_BASE 展开为 fix_to_virt(FIX_APIC_BASE)，是个常数，于是 native_apic_mem_read() 就是对常量地址做 readl：
  • 不需要任何全局指针变量，也就没有”变量必须先初始化才能用”的时序问题——这在启动早期（ioremap/vmalloc 还没就绪、甚至汇编阶段）尤其重要；
  • 生成的代码更直接，访存不需要先 load 一个指针；
  • 这些页是全局映射，TLB 项不会因进程切换被刷掉（enum fixed_addresses 的注释里也写了这一点）。

调用 set_fixmap 来构建映射。

关键数据结构

/*
 * Here we define all the compile-time 'special' virtual
 * addresses. The point is to have a constant address at
 * compile time, but to set the physical address only
 * in the boot process.
 * for x86_32: We allocate these special addresses
 * from the end of virtual memory (0xfffff000) backwards.
 * Also this lets us do fail-safe vmalloc(), we
 * can guarantee that these special addresses and
 * vmalloc()-ed addresses never overlap.
 *
 * These 'compile-time allocated' memory buffers are
 * fixed-size 4k pages (or larger if used with an increment
 * higher than 1). Use set_fixmap(idx,phys) to associate
 * physical memory with fixmap indices.
 *
 * TLB entries of such buffers will not be flushed across
 * task switches.
 */
enum fixed_addresses {
#ifdef CONFIG_X86_32
	FIX_HOLE,
#else
#ifdef CONFIG_X86_VSYSCALL_EMULATION
	VSYSCALL_PAGE = (FIXADDR_TOP - VSYSCALL_ADDR) >> PAGE_SHIFT,
#endif
#endif
	FIX_DBGP_BASE,
	FIX_EARLYCON_MEM_BASE,
#ifdef CONFIG_PROVIDE_OHCI1394_DMA_INIT
	FIX_OHCI1394_BASE,
#endif
#ifdef CONFIG_X86_LOCAL_APIC
	FIX_APIC_BASE,	/* local (CPU) APIC) -- required for SMP or not */
#endif
#ifdef CONFIG_X86_IO_APIC
	FIX_IO_APIC_BASE_0,
	FIX_IO_APIC_BASE_END = FIX_IO_APIC_BASE_0 + MAX_IO_APICS - 1,
#endif
#ifdef CONFIG_KMAP_LOCAL
	FIX_KMAP_BEGIN,	/* reserved pte's for temporary kernel mappings */
	FIX_KMAP_END = FIX_KMAP_BEGIN + (KM_MAX_IDX * NR_CPUS) - 1,
#ifdef CONFIG_PCI_MMCONFIG
	FIX_PCIE_MCFG,
#endif
#endif
#ifdef CONFIG_PARAVIRT_XXL
	FIX_PARAVIRT_BOOTMAP,
#endif

#ifdef CONFIG_ACPI_APEI_GHES
	/* Used for GHES mapping from assorted contexts */
	FIX_APEI_GHES_IRQ,
	FIX_APEI_GHES_NMI,
#endif

	__end_of_permanent_fixed_addresses,

	/*
	 * 512 temporary boot-time mappings, used by early_ioremap(),
	 * before ioremap() is functional.
	 *
	 * If necessary we round it up to the next 512 pages boundary so
	 * that we can have a single pmd entry and a single pte table:
	 */
#define NR_FIX_BTMAPS		64
#define FIX_BTMAPS_SLOTS	8
#define TOTAL_FIX_BTMAPS	(NR_FIX_BTMAPS * FIX_BTMAPS_SLOTS)
	FIX_BTMAP_END =
	 (__end_of_permanent_fixed_addresses ^
	  (__end_of_permanent_fixed_addresses + TOTAL_FIX_BTMAPS - 1)) &
	 -PTRS_PER_PTE
	 ? __end_of_permanent_fixed_addresses + TOTAL_FIX_BTMAPS -
	   (__end_of_permanent_fixed_addresses & (TOTAL_FIX_BTMAPS - 1))
	 : __end_of_permanent_fixed_addresses,
	FIX_BTMAP_BEGIN = FIX_BTMAP_END + TOTAL_FIX_BTMAPS - 1,
#ifdef CONFIG_X86_32
	FIX_WP_TEST,
#endif
#ifdef CONFIG_INTEL_TXT
	FIX_TBOOT_BASE,
#endif
	__end_of_fixed_addresses
};


/*
 * 'index to address' translation. If anyone tries to use the idx
 * directly without translation, we catch the bug with a NULL-deference
 * kernel oops. Illegal ranges of incoming indices are caught too.
 */
static __always_inline unsigned long fix_to_virt(const unsigned int idx)
{
	BUILD_BUG_ON(idx >= __end_of_fixed_addresses);
	return __fix_to_virt(idx);
}

static inline unsigned long virt_to_fix(const unsigned long vaddr)
{
	BUG_ON(vaddr >= FIXADDR_TOP || vaddr < FIXADDR_START);
	return __virt_to_fix(vaddr);
}

经典案例

APIC

#define APIC_BASE (fix_to_virt(FIX_APIC_BASE))
#define APIC_BASE_MSR		0x800
#define APIC_X2APIC_ID_MSR	0x802
#define XAPIC_ENABLE		BIT(11)
#define X2APIC_ENABLE		BIT(10)

void __init register_lapic_address(unsigned long address)
{
	/* This should only happen once */
	WARN_ON_ONCE(mp_lapic_addr);
	mp_lapic_addr = address;

	if (!x2apic_mode)
		apic_set_fixmap(true);
}

// arch/x86/kernel/apic/apic.c:2084，register_lapic_address 和 native_apic_mem_read 至今未变，
// 只是打印宏从 apic_printk(APIC_VERBOSE, ...) 换成了 apic_pr_verbose(...)
static __init void apic_set_fixmap(bool read_apic)
{
	set_fixmap_nocache(FIX_APIC_BASE, mp_lapic_addr);
	apic_mmio_base = APIC_BASE;
	apic_pr_verbose("Mapped APIC to %16lx (%16lx)\n",
			apic_mmio_base, mp_lapic_addr);
	if (read_apic)
		apic_read_boot_cpu_id(false);
}

static inline u32 native_apic_mem_read(u32 reg)
{
	return readl((void __iomem *)(APIC_BASE + reg));
}

// arch/x86/mm/pgtable.c:585（旧版在 arch/x86/mm/ioremap.c，已搬家，实现不变）
void native_set_fixmap(unsigned /* enum fixed_addresses */ idx,
		       phys_addr_t phys, pgprot_t flags)
{
	/* Sanitize 'prot' against any unsupported bits: */
	pgprot_val(flags) &= __default_kernel_pte_mask;

	__native_set_fixmap(idx, pfn_pte(phys >> PAGE_SHIFT, flags));
}

###

kmap 需要一个”预留的虚拟地址池”，而 fixmap 恰好就是内核提供的这种池子。拆开说：

kmap 要解决的问题

要访问一个 struct page，必须有一个虚拟地址，其 PTE 指向该页的物理页帧。问题是：用哪个虚拟地址？

• 32 位 highmem 机器上，高端内存页没有线性映射，天然需要临时建映射；\
• 但这个”临时映射的虚拟地址”不能随便挑——必须是编译期就预留好的，否则会和 vmalloc、直接映射区等撞车。

fixmap 正好就是这个预留机制

回顾一下 fixmap 的定义：一段编译期保留的虚拟地址区域，每个槽位的虚拟地址是常量，物理地址运行时才绑定。 kmap 只是 fixmap 的一个”客户”——它从 fixmap 里划走一整段槽位（FIX_KMAP_BEGIN ~ FIX_KMAP_END）当作自己的临时映射地址池。所以二者的关系是：

fixmap          = 机制：编译期保留的常量虚拟地址槽位 + 运行时绑定物理页
kmap_local_page = 客户：借用其中 FIX_KMAP_* 一段槽位，加上自己的槽位分配纪律

当前内核的实际流程（mm/highmem.c 的 __kmap_local_pfn_prot()，mm-fixmap.md 里引用的就是它的槽位）：

1. migrate_disable(); preempt_disable(); —— 槽位是 per-CPU 的，不能跑到别的 CPU 上；
2. idx = arch_kmap_local_map_idx(kmap_local_idx_push(), pfn); —— 像栈一样压入一个嵌套编号（per-task 的 kmap_ctrl.idx），这样中断里嵌套 kmap 也 不会踩到别人的槽；
3. vaddr = __fix_to_virt(FIX_KMAP_BEGIN + idx); —— 虚拟地址就是 fixmap 槽位号算出来的常量；
4. kmap_get_pte(vaddr, idx) 找到覆盖该 fixmap 地址的 PTE，写入 pfn_pte(pfn, prot)，刷 TLB，返回 vaddr。

kunmap_local 反过来：清掉那个 PTE、弹出 idx。

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