Kernel Memory Layout on ARM Linux

  Russell King <rmk@arm.linux.org.uk>
   May 21, 2004 (2.6.6)

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_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().