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memory.txt

Guest on 20th July 2021 02:58:21 PM

  1.                 Kernel Memory Layout on ARM Linux
  2.  
  3.                 Russell King <rmk@arm.linux.org.uk>
  4.                    
  5. This document describes the virtual memory layout which the Linux
  6. kernel uses for ARM processors.  It indicates which regions are
  7. free for platforms to use, and which are used by generic code.
  8.  
  9. The ARM CPU is capable of addressing a maximum of 4GB virtual memory
  10. space, and this must be shared between user space processes, the
  11. kernel, and hardware devices.
  12.  
  13. As the ARM architecture matures, it becomes necessary to reserve
  14. certain regions of VM space for use for new facilities; therefore
  15. this document may reserve more VM space over time.
  16.  
  17. Start           End             Use
  18. --------------------------------------------------------------------------
  19. ffff8000        ffffffff        copy_user_page / clear_user_page use.
  20.                                 For SA11xx and Xscale, this is used to
  21.                                 setup a minicache mapping.
  22.  
  23. ffff4000        ffffffff        cache aliasing on ARMv6 and later CPUs.
  24.  
  25. ffff1000        ffff7fff        Reserved.
  26.                                 Platforms must not use this address range.
  27.  
  28. ffff0000        ffff0fff        CPU vector page.
  29.                                 The CPU vectors are mapped here if the
  30.                                 CPU supports vector relocation (control
  31.                                 register V bit.)
  32.  
  33. fffe0000        fffeffff        XScale cache flush area.  This is used
  34.                                 in proc-xscale.S to flush the whole data
  35.                                 cache. (XScale does not have TCM.)
  36.  
  37. fffe8000        fffeffff        DTCM mapping area for platforms with
  38.                                 DTCM mounted inside the CPU.
  39.  
  40. fffe0000        fffe7fff        ITCM mapping area for platforms with
  41.                                 ITCM mounted inside the CPU.
  42.  
  43. fff00000        fffdffff        Fixmap mapping region.  Addresses provided
  44.                                 by fix_to_virt() will be located here.
  45.  
  46. ffc00000        ffefffff        DMA memory mapping region.  Memory returned
  47.                                 by the dma_alloc_xxx functions will be
  48.                                 dynamically mapped here.
  49.  
  50. ff000000        ffbfffff        Reserved for future expansion of DMA
  51.                                 mapping region.
  52.  
  53. VMALLOC_START   VMALLOC_END-1   vmalloc() / ioremap() space.
  54.                                 Memory returned by vmalloc/ioremap will
  55.                                 be dynamically placed in this region.
  56.                                 Machine specific static mappings are also
  57.                                 located here through iotable_init().
  58.                                 VMALLOC_START is based upon the value
  59.                                 of the high_memory variable, and VMALLOC_END
  60.                                 is equal to 0xff000000.
  61.  
  62. PAGE_OFFSET     high_memory-1   Kernel direct-mapped RAM region.
  63.                                 This maps the platforms RAM, and typically
  64.                                 maps all platform RAM in a 1:1 relationship.
  65.  
  66. PKMAP_BASE      PAGE_OFFSET-1   Permanent kernel mappings
  67.                                 One way of mapping HIGHMEM pages into kernel
  68.                                 space.
  69.  
  70. MODULES_VADDR   MODULES_END-1   Kernel module space
  71.                                 Kernel modules inserted via insmod are
  72.                                 placed here using dynamic mappings.
  73.  
  74. 00001000        TASK_SIZE-1     User space mappings
  75.                                 Per-thread mappings are placed here via
  76.                                 the mmap() system call.
  77.  
  78. 00000000        00000fff        CPU vector page / null pointer trap
  79.                                 CPUs which do not support vector remapping
  80.                                 place their vector page here.  NULL pointer
  81.                                 dereferences by both the kernel and user
  82.                                 space are also caught via this mapping.
  83.  
  84. Please note that mappings which collide with the above areas may result
  85. in a non-bootable kernel, or may cause the kernel to (eventually) panic
  86. at run time.
  87.  
  88. Since future CPUs may impact the kernel mapping layout, user programs
  89. must not access any memory which is not mapped inside their 0x0001000
  90. to TASK_SIZE address range.  If they wish to access these areas, they
  91. must set up their own mappings using open() and mmap().

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