Virtual Kernel Memory Layout on ARM Linux

  Kernel Memory Layout on ARM Linux

Russell King 
     November 17, 2005 (2.6.15)

This document describes the virtual memory layout which the Linux
kernel uses for ARM processors.  It indicates which regions are
free for platforms to use, and which are used by generic code.

The ARM CPU is capable of addressing a maximum of 4GB virtual memory
space, and this must be shared between user space processes, the
kernel, and hardware devices.

As the ARM architecture matures, it becomes necessary to reserve
certain regions of VM space for use for new facilities; therefore
this document may reserve more VM space over time.

Start  End  Use
--------------------------------------------------------------------------
ffff8000 ffffffff copy_user_page / clear_user_page use.
  For SA11xx and Xscale, this is used to
  setup a minicache mapping.

ffff1000 ffff7fff Reserved.
  Platforms must not use this address range.

ffff0000 ffff0fff CPU vector page.
  The CPU vectors are mapped here if the
  CPU supports vector relocation (control
  register V bit.)

ffc00000 fffeffff DMA memory mapping region.  Memory returned
  by the dma_alloc_xxx functions will be
  dynamically mapped here.

ff000000 ffbfffff Reserved for future expansion of DMA
  mapping region.

VMALLOC_END feffffff Free for platform use, recommended.
  VMALLOC_END must be aligned to a 2MB
  boundary.

VMALLOC_START VMALLOC_END-1 vmalloc() / ioremap() space.
  Memory returned by vmalloc/ioremap will
  be dynamically placed in this region.
  VMALLOC_START may be based upon the value
  of the high_memory variable.

PAGE_OFFSET high_memory-1 Kernel direct-mapped RAM region.
  This maps the platforms RAM, and typically
  maps all platform RAM in a 1:1 relationship.

TASK_SIZE PAGE_OFFSET-1 Kernel module space
  Kernel modules inserted via insmod are
  placed here using dynamic mappings.

00001000 TASK_SIZE-1 User space mappings
  Per-thread mappings are placed here via
  the mmap() system call.

00000000 00000fff CPU vector page / null pointer trap
  CPUs which do not support vector remapping
  place their vector page here.  NULL pointer
  dereferences by both the kernel and user
  space are also caught via this mapping.

Please note that mappings which collide with the above areas may result
in a non-bootable kernel, or may cause the kernel to (eventually) panic
at run time.

Since future CPUs may impact the kernel mapping layout, user programs
must not access any memory which is not mapped inside their 0x0001000
to TASK_SIZE address range.  If they wish to access these areas, they
must set up their own mappings using open() and mmap().
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