memory.txt 2.9 KB

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  1. Kernel Memory Layout on ARM Linux
  2. Russell King <rmk@arm.linux.org.uk>
  3. November 17, 2005 (2.6.15)
  4. This document describes the virtual memory layout which the Linux
  5. kernel uses for ARM processors. It indicates which regions are
  6. free for platforms to use, and which are used by generic code.
  7. The ARM CPU is capable of addressing a maximum of 4GB virtual memory
  8. space, and this must be shared between user space processes, the
  9. kernel, and hardware devices.
  10. As the ARM architecture matures, it becomes necessary to reserve
  11. certain regions of VM space for use for new facilities; therefore
  12. this document may reserve more VM space over time.
  13. Start End Use
  14. --------------------------------------------------------------------------
  15. ffff8000 ffffffff copy_user_page / clear_user_page use.
  16. For SA11xx and Xscale, this is used to
  17. setup a minicache mapping.
  18. ffff4000 ffffffff cache aliasing on ARMv6 and later CPUs.
  19. ffff1000 ffff7fff Reserved.
  20. Platforms must not use this address range.
  21. ffff0000 ffff0fff CPU vector page.
  22. The CPU vectors are mapped here if the
  23. CPU supports vector relocation (control
  24. register V bit.)
  25. fffe0000 fffeffff XScale cache flush area. This is used
  26. in proc-xscale.S to flush the whole data
  27. cache. Free for other usage on non-XScale.
  28. fff00000 fffdffff Fixmap mapping region. Addresses provided
  29. by fix_to_virt() will be located here.
  30. ffc00000 ffefffff DMA memory mapping region. Memory returned
  31. by the dma_alloc_xxx functions will be
  32. dynamically mapped here.
  33. ff000000 ffbfffff Reserved for future expansion of DMA
  34. mapping region.
  35. VMALLOC_END feffffff Free for platform use, recommended.
  36. VMALLOC_END must be aligned to a 2MB
  37. boundary.
  38. VMALLOC_START VMALLOC_END-1 vmalloc() / ioremap() space.
  39. Memory returned by vmalloc/ioremap will
  40. be dynamically placed in this region.
  41. VMALLOC_START may be based upon the value
  42. of the high_memory variable.
  43. PAGE_OFFSET high_memory-1 Kernel direct-mapped RAM region.
  44. This maps the platforms RAM, and typically
  45. maps all platform RAM in a 1:1 relationship.
  46. TASK_SIZE PAGE_OFFSET-1 Kernel module space
  47. Kernel modules inserted via insmod are
  48. placed here using dynamic mappings.
  49. 00001000 TASK_SIZE-1 User space mappings
  50. Per-thread mappings are placed here via
  51. the mmap() system call.
  52. 00000000 00000fff CPU vector page / null pointer trap
  53. CPUs which do not support vector remapping
  54. place their vector page here. NULL pointer
  55. dereferences by both the kernel and user
  56. space are also caught via this mapping.
  57. Please note that mappings which collide with the above areas may result
  58. in a non-bootable kernel, or may cause the kernel to (eventually) panic
  59. at run time.
  60. Since future CPUs may impact the kernel mapping layout, user programs
  61. must not access any memory which is not mapped inside their 0x0001000
  62. to TASK_SIZE address range. If they wish to access these areas, they
  63. must set up their own mappings using open() and mmap().