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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

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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