memory.rst 3.5 KB

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  1. =================================
  2. Kernel Memory Layout on ARM Linux
  3. =================================
  4. Russell King <rmk@arm.linux.org.uk>
  5. November 17, 2005 (2.6.15)
  6. This document describes the virtual memory layout which the Linux
  7. kernel uses for ARM processors. It indicates which regions are
  8. free for platforms to use, and which are used by generic code.
  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. As the ARM architecture matures, it becomes necessary to reserve
  13. certain regions of VM space for use for new facilities; therefore
  14. this document may reserve more VM space over time.
  15. =============== =============== ===============================================
  16. Start End Use
  17. =============== =============== ===============================================
  18. ffff8000 ffffffff copy_user_page / clear_user_page use.
  19. For SA11xx and Xscale, this is used to
  20. setup a minicache mapping.
  21. ffff4000 ffffffff cache aliasing on ARMv6 and later CPUs.
  22. ffff1000 ffff7fff Reserved.
  23. Platforms must not use this address range.
  24. ffff0000 ffff0fff CPU vector page.
  25. The CPU vectors are mapped here if the
  26. CPU supports vector relocation (control
  27. register V bit.)
  28. fffe0000 fffeffff XScale cache flush area. This is used
  29. in proc-xscale.S to flush the whole data
  30. cache. (XScale does not have TCM.)
  31. fffe8000 fffeffff DTCM mapping area for platforms with
  32. DTCM mounted inside the CPU.
  33. fffe0000 fffe7fff ITCM mapping area for platforms with
  34. ITCM mounted inside the CPU.
  35. ffc80000 ffefffff Fixmap mapping region. Addresses provided
  36. by fix_to_virt() will be located here.
  37. ffc00000 ffc7ffff Guard region
  38. ff800000 ffbfffff Permanent, fixed read-only mapping of the
  39. firmware provided DT blob
  40. fee00000 feffffff Mapping of PCI I/O space. This is a static
  41. mapping within the vmalloc space.
  42. VMALLOC_START VMALLOC_END-1 vmalloc() / ioremap() space.
  43. Memory returned by vmalloc/ioremap will
  44. be dynamically placed in this region.
  45. Machine specific static mappings are also
  46. located here through iotable_init().
  47. VMALLOC_START is based upon the value
  48. of the high_memory variable, and VMALLOC_END
  49. is equal to 0xff800000.
  50. PAGE_OFFSET high_memory-1 Kernel direct-mapped RAM region.
  51. This maps the platforms RAM, and typically
  52. maps all platform RAM in a 1:1 relationship.
  53. PKMAP_BASE PAGE_OFFSET-1 Permanent kernel mappings
  54. One way of mapping HIGHMEM pages into kernel
  55. space.
  56. MODULES_VADDR MODULES_END-1 Kernel module space
  57. Kernel modules inserted via insmod are
  58. placed here using dynamic mappings.
  59. 00001000 TASK_SIZE-1 User space mappings
  60. Per-thread mappings are placed here via
  61. the mmap() system call.
  62. 00000000 00000fff CPU vector page / null pointer trap
  63. CPUs which do not support vector remapping
  64. place their vector page here. NULL pointer
  65. dereferences by both the kernel and user
  66. space are also caught via this mapping.
  67. =============== =============== ===============================================
  68. Please note that mappings which collide with the above areas may result
  69. in a non-bootable kernel, or may cause the kernel to (eventually) panic
  70. at run time.
  71. Since future CPUs may impact the kernel mapping layout, user programs
  72. must not access any memory which is not mapped inside their 0x0001000
  73. to TASK_SIZE address range. If they wish to access these areas, they
  74. must set up their own mappings using open() and mmap().