Fix-Mapped Linear Address

Architecture: ARM64
[Intro]

Fixmap是固定一直指到physical addr的特定位址

Kernel會一次配一塊4K的virtual memory mapping 到physical addr

[Usage of Fix-Mapped Linear Address]

Kernel linear address的第四個GB中至少會有一塊128MB的memory mapping到physical address

這一塊memory 可以讓kernel implement 

(1) noncontiguous memory allocation (2) fix-mapped linear address

[Fix-Mapped Linear Address VS. Physical Address]

Fix-Mapped Linear Address 是一個constant address. 例如 0xffffc000

每一個Fix-Mapped Linear Address maps到一個page frame(4K) 的physical memory

Fix-Mapped Linear Address 跟 linear address that map the first 896MB of RAM 很像

但Fix-Mapped Linear Address可以mapping到任何physical address

[Data Structure enum fixed_addresses]

每一個fix-mapped linear address都有一個專用的integer index defined 在 enum fixed_addresses中

  enum fixed_addresses {
          FIX_HOLE,
  
          /*
           * Reserve a virtual window for the FDT that is 2 MB larger than the
           * maximum supported size, and put it at the top of the fixmap region.
           * The additional space ensures that any FDT that does not exceed
           * MAX_FDT_SIZE can be mapped regardless of whether it crosses any
           * 2 MB alignment boundaries.
           *
           * Keep this at the top so it remains 2 MB aligned.
           */
  #define FIX_FDT_SIZE            (MAX_FDT_SIZE + SZ_2M)
          FIX_FDT_END,
          FIX_FDT = FIX_FDT_END + FIX_FDT_SIZE / PAGE_SIZE - 1,
  
          FIX_EARLYCON_MEM_BASE,
          FIX_TEXT_POKE0,
          __end_of_permanent_fixed_addresses,
  
          /*
           * Temporary boot-time mappings, used by early_ioremap(),
           * before ioremap() is functional.
           */
  #define NR_FIX_BTMAPS           (SZ_256K / PAGE_SIZE)
  #define FIX_BTMAPS_SLOTS        7
  #define TOTAL_FIX_BTMAPS        (NR_FIX_BTMAPS * FIX_BTMAPS_SLOTS)
  
          FIX_BTMAP_END = __end_of_permanent_fixed_addresses,
          FIX_BTMAP_BEGIN = FIX_BTMAP_END + TOTAL_FIX_BTMAPS - 1,
  
          /*
           * Used for kernel page table creation, so unmapped memory may be used
           * for tables.
           */
          FIX_PTE,
          FIX_PMD,
          FIX_PUD,
          FIX_PGD,
  
          __end_of_fixed_addresses
  };

[How to Obtain the Linear Address Set of a Fix-Mapped Linear Address]

 #ifndef __ASSEMBLY__
  /*
   * '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);
  }

fix_to_virt -> __fix_to_virt: 能夠根據index找到 constant linear address
如下, PAGE_SHIFT = 12
所以每一個index 對應到的linear address是從FIXADDR_TOP開始減 4K 的倍數

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

目前畫出的memory layout不一定正確, 若有誤會再修正

[Reference]
Fixmap:
http://palliatory66.rssing.com/chan-60693167/latest.php

ARM64 memory
http://blog.csdn.net/qianlong4526888/article/details/9058221

Linux doc about arm64 memory
http://lxr.free-electrons.com/source/Documentation/arm64/memory.txt?v=4.4

Linux kernel memory management
https://0xax.gitbooks.io/linux-insides/content/mm/linux-mm-2.html

Windows7 memory layout
http://www.codemachine.com/article_x64kvas.html