[ARM] Add ARM virtual memory layout documentation.

Documentation/arm/memory.txt

+		Kernel Memory Layout on ARM Linux
+
+		Russell King <rmk@arm.linux.org.uk>
+			April 27, 2003 (2.5.68)
+
+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.)
+
+ffe00000	fffeffff	Free for platform use, not recommended.
+
+ffc00000	ffdfffff	2MB consistent memory mapping.
+				Memory returned by the consistent_alloc
+				low level function will be dynamically
+				mapped here.
+
+ff000000	ffbfffff	Free for platform use, not recommended.
+
+VMALLOC_END	ff000000	Free for platform use, recommended.
+
+VMALLOC_START	VMALLOC_END	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	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	Kernel module space
+				Kernel modules inserted via insmod are
+				placed here using dynamic mappings.
+
+00001000	TASK_SIZE	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().