linux内核高端内存管理之固定内存区与映射

前面总结了高端内存中永久内核映射和临时内核映射。linux高端内存中的临时内存区为固定内存区的一部分，下面是Linux内存布局图

对于固定内存在linux内核中有下面描述

enum fixed_addresses {
#ifdef CONFIG_X86_32
	FIX_HOLE,
	FIX_VDSO,
#else
	VSYSCALL_LAST_PAGE,
	VSYSCALL_FIRST_PAGE = VSYSCALL_LAST_PAGE
			    + ((VSYSCALL_END-VSYSCALL_START) >> PAGE_SHIFT) - 1,
	VSYSCALL_HPET,
#endif
	FIX_DBGP_BASE,
	FIX_EARLYCON_MEM_BASE,
#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_X86_VISWS_APIC
	FIX_CO_CPU,	/* Cobalt timer */
	FIX_CO_APIC,	/* Cobalt APIC Redirection Table */
	FIX_LI_PCIA,	/* Lithium PCI Bridge A */
	FIX_LI_PCIB,	/* Lithium PCI Bridge B */
#endif
#ifdef CONFIG_X86_F00F_BUG
	FIX_F00F_IDT,	/* Virtual mapping for IDT */
#endif
#ifdef CONFIG_X86_CYCLONE_TIMER
	FIX_CYCLONE_TIMER, /*cyclone timer register*/
#endif
#ifdef CONFIG_X86_32
	FIX_KMAP_BEGIN,	/* reserved pte's for temporary kernel mappings */
	FIX_KMAP_END = FIX_KMAP_BEGIN+(KM_TYPE_NR*NR_CPUS)-1,
#ifdef CONFIG_PCI_MMCONFIG
	FIX_PCIE_MCFG,
#endif
#endif
#ifdef CONFIG_PARAVIRT
	FIX_PARAVIRT_BOOTMAP,
#endif
	FIX_TEXT_POKE1,	/* reserve 2 pages for text_poke() */
	FIX_TEXT_POKE0, /* first page is last, because allocation is backward */
	__end_of_permanent_fixed_addresses,
	/*
	 * 256 temporary boot-time mappings, used by early_ioremap(),
	 * before ioremap() is functional.
	 *
	 * We round it up to the next 256 pages boundary so that we
	 * can have a single pgd entry and a single pte table:
	 */
#define NR_FIX_BTMAPS		64
#define FIX_BTMAPS_SLOTS	4
	FIX_BTMAP_END = __end_of_permanent_fixed_addresses + 256 -
			(__end_of_permanent_fixed_addresses & 255),
	FIX_BTMAP_BEGIN = FIX_BTMAP_END + NR_FIX_BTMAPS*FIX_BTMAPS_SLOTS - 1,
#ifdef CONFIG_PROVIDE_OHCI1394_DMA_INIT
	FIX_OHCI1394_BASE,
#endif
#ifdef CONFIG_X86_32
	FIX_WP_TEST,
#endif
#ifdef CONFIG_INTEL_TXT
	FIX_TBOOT_BASE,
#endif
	__end_of_fixed_addresses
};

固定映射

ioremap的作用是将IO和BIOS以及物理地址空间映射到在896M至1G的128M的地址空间内，使得kernel能够访问该空间并进行相应的读写操作。

start_kernel()->setup_arch()->early_ioremap_init()

void __init early_ioremap_init(void)
{
	pmd_t *pmd;
	int i;

	if (early_ioremap_debug)
		printk(KERN_INFO "early_ioremap_init()\n");
	/*将fixed_address里的索引的虚拟地址放入slot_virt
	，从代码里面可以看出，放入slot_virt中得虚拟地址为1M*/
	for (i = 0; i < FIX_BTMAPS_SLOTS; i++)
		slot_virt[i] = __fix_to_virt(FIX_BTMAP_BEGIN - NR_FIX_BTMAPS*i);

	/*得到固定映射区的pmd
	，此pmd为虚拟地址转换为物理地址的pmd*/
	pmd = early_ioremap_pmd(fix_to_virt(FIX_BTMAP_BEGIN));
	memset(bm_pte, 0, sizeof(bm_pte));
	/*将bm_pte页表设置为固定映射区开始地址的pmd的第一个页表；*/
	pmd_populate_kernel(&init_mm, pmd, bm_pte);

	/*
	 * The boot-ioremap range spans multiple pmds, for which
	 * we are not prepared:
	 */
	 /*系统要求所有的ioremap映射在一个pmd上，超出这个pmd将警告*/
	if (pmd != early_ioremap_pmd(fix_to_virt(FIX_BTMAP_END))) {
		WARN_ON(1);
		printk(KERN_WARNING "pmd %p != %p\n",
		       pmd, early_ioremap_pmd(fix_to_virt(FIX_BTMAP_END)));
		printk(KERN_WARNING "fix_to_virt(FIX_BTMAP_BEGIN): %08lx\n",
			fix_to_virt(FIX_BTMAP_BEGIN));
		printk(KERN_WARNING "fix_to_virt(FIX_BTMAP_END):   %08lx\n",
			fix_to_virt(FIX_BTMAP_END));

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

static unsigned long slot_virt[FIX_BTMAPS_SLOTS] __initdata;

#define __fix_to_virt(x)	(FIXADDR_TOP - ((x) << PAGE_SHIFT))

其中FIXADDR_TOP为4G-4K。

对于ioremap的使用需要通过early_memremap和early_iounmap进行。由于对应于ioremap的内存空间是有限的，所以对于ioremap空间的使用遵照使用结束马上释放的原则。这就是说early_memremap和early_iounmap必须配对使用并且访问结束必须马上执行unmap。

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 idx0, idx;
	int i, slot;

	WARN_ON(system_state != SYSTEM_BOOTING);

	slot = -1;
	/*pre_map[]是一个索引与slot_virt[]一一对应，这段for
	的含义在于找到一个没有被使用过的slot_virt[i]的页面，
	该slot_virt[i]所指向的虚拟页面地址就是将会和实际物
	理地址phys_addr相绑定的虚拟地址。*/
	for (i = 0; i < FIX_BTMAPS_SLOTS; i++) {
		if (!prev_map[i]) {
			slot = i;
			break;
		}
	}

	if (slot < 0) {
		printk(KERN_INFO "early_iomap(%08llx, %08lx) not found slot\n",
			 (u64)phys_addr, size);
		WARN_ON(1);
		return NULL;
	}

	if (early_ioremap_debug) {
		printk(KERN_INFO "early_ioremap(%08llx, %08lx) [%d] => ",
		       (u64)phys_addr, size, slot);
		dump_stack();
	}

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

	prev_size[slot] = size;
	/*
	 * Mappings have to be page-aligned
	 */
	offset = phys_addr & ~PAGE_MASK;/*offset是页内的偏移*/
	phys_addr &= PAGE_MASK;/*现在phys_addr就是起始页面的地址*/
	/*现在size就是指出了到底占据了多少个页面的大小*/
	size = PAGE_ALIGN(last_addr + 1) - phys_addr;

	/*
	 * Mappings have to fit in the FIX_BTMAP area.
	 */
	 /*到底我们需要多少页面*/
	nrpages = size >> PAGE_SHIFT;
	if (nrpages > NR_FIX_BTMAPS) {
		WARN_ON(1);
		return NULL;
	}

	/*
	 * Ok, go for it..
	 */
	 /*找到空闲slot所对应的fixed_address中的索引号*/
	idx0 = FIX_BTMAP_BEGIN - NR_FIX_BTMAPS*slot;
	idx = idx0;
	while (nrpages > 0) {
		/*在bm_ptes中将指定的idx索引的页表项填充为对应的物理地址使得bm_pte[idx]指向正确的物理页面地址*/
		early_set_fixmap(idx, phys_addr, prot);
		phys_addr += PAGE_SIZE;
		--idx;
		--nrpages;
	}
	if (early_ioremap_debug)
		printk(KERN_CONT "%08lx + %08lx\n", offset, slot_virt[slot]);
	/*返回phys_addr所指向的虚拟地址*/
	prev_map[slot] = (void __iomem *)(offset + slot_virt[slot]);
	return prev_map[slot];
}