源文件是arch/x86/include/asm/fixmap.h
enum fixed_addresses {
#ifdef CONFIG_X86_32
FIX_HOLE,
...
__end_of_permanent_fixed_addresses,
/*
* 256 temporary boot-time mappings, used by early_ioremap(),
* before ioremap() is functional.
*
* If necessary 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
#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,
---
enum fixed_addresses 中每个项代表一个页。通过__fix_to_virt获得地址
#define __fix_to_virt(x) (FIXADDR_TOP - ((x) << PAGE_SHIFT))
比如__fix_to_virt(FIX_BTMAP_BEGIN)，FIXADDR_TOP通常是0xFFFFF000
比如__fix_to_virt(FIX_HOLE)是0xFFFFF000
FIX_BTMAP_END通过
(__end_of_permanent_fixed_addresses ^
(__end_of_permanent_fixed_addresses + TOTAL_FIX_BTMAPS - 1))
& -PTRS_PER_PTE
实现是否沿-PTRS_PER_PTE(0xFFFFFC00)对齐的检测
另一个异或的精彩应用是Buddy算法
http://blog.linuxeden.com/index.php/196616/viewspace-8149.html