[people/ms/linux.git] / Documentation / arm / memory.rst

=================================
Kernel Memory Layout on ARM Linux
=================================

		Russell King <rmk@arm.linux.org.uk>

		     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.

ffff4000	ffffffff	cache aliasing on ARMv6 and later CPUs.

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.)

fffe0000	fffeffff	XScale cache flush area.  This is used
				in proc-xscale.S to flush the whole data
				cache. (XScale does not have TCM.)

fffe8000	fffeffff	DTCM mapping area for platforms with
				DTCM mounted inside the CPU.

fffe0000	fffe7fff	ITCM mapping area for platforms with
				ITCM mounted inside the CPU.

ffc80000	ffefffff	Fixmap mapping region.  Addresses provided
				by fix_to_virt() will be located here.

ffc00000	ffc7ffff	Guard region

ff800000	ffbfffff	Permanent, fixed read-only mapping of the
				firmware provided DT blob

fee00000	feffffff	Mapping of PCI I/O space. This is a static
				mapping within the vmalloc space.

VMALLOC_START	VMALLOC_END-1	vmalloc() / ioremap() space.
				Memory returned by vmalloc/ioremap will
				be dynamically placed in this region.
				Machine specific static mappings are also
				located here through iotable_init().
				VMALLOC_START is based upon the value
				of the high_memory variable, and VMALLOC_END
				is equal to 0xff800000.

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.

PKMAP_BASE	PAGE_OFFSET-1	Permanent kernel mappings
				One way of mapping HIGHMEM pages into kernel
				space.

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

TASK_SIZE	MODULES_VADDR-1	KASAn shadow memory when KASan is in use.
				The range from MODULES_VADDR to the top
				of the memory is shadowed here with 1 bit
				per byte of memory.

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().
Commit	Line	Data
dc7a12bd MCC	1	=================================
	2	Kernel Memory Layout on ARM Linux
	3	=================================
1da177e4 LT	4
1da177e4 LT	5	Russell King <rmk@arm.linux.org.uk>
dc7a12bd	6
02b30839	7	November 17, 2005 (2.6.15)
1da177e4 LT	8
	9	This document describes the virtual memory layout which the Linux
	10	kernel uses for ARM processors. It indicates which regions are
	11	free for platforms to use, and which are used by generic code.
	12
	13	The ARM CPU is capable of addressing a maximum of 4GB virtual memory
	14	space, and this must be shared between user space processes, the
	15	kernel, and hardware devices.
	16
	17	As the ARM architecture matures, it becomes necessary to reserve
	18	certain regions of VM space for use for new facilities; therefore
	19	this document may reserve more VM space over time.
	20
dc7a12bd	21	=============== =============== ===============================================
1da177e4	22	Start End Use
dc7a12bd	23	=============== =============== ===============================================
1da177e4 LT	24	ffff8000 ffffffff copy_user_page / clear_user_page use.
	25	For SA11xx and Xscale, this is used to
	26	setup a minicache mapping.
	27
e624859e LW	28	ffff4000 ffffffff cache aliasing on ARMv6 and later CPUs.
e624859e LW	29
1da177e4 LT	30	ffff1000 ffff7fff Reserved.
	31	Platforms must not use this address range.
	32
	33	ffff0000 ffff0fff CPU vector page.
	34	The CPU vectors are mapped here if the
	35	CPU supports vector relocation (control
	36	register V bit.)
	37
5f0fbf9e NP	38	fffe0000 fffeffff XScale cache flush area. This is used
5f0fbf9e NP	39	in proc-xscale.S to flush the whole data
1dbd30e9 LW	40	cache. (XScale does not have TCM.)
	41
	42	fffe8000 fffeffff DTCM mapping area for platforms with
	43	DTCM mounted inside the CPU.
	44
	45	fffe0000 fffe7fff ITCM mapping area for platforms with
	46	ITCM mounted inside the CPU.
5f0fbf9e	47
7a1be318	48	ffc80000 ffefffff Fixmap mapping region. Addresses provided
5f0fbf9e NP	49	by fix_to_virt() will be located here.
5f0fbf9e NP	50
7a1be318 AB	51	ffc00000 ffc7ffff Guard region
	52
	53	ff800000 ffbfffff Permanent, fixed read-only mapping of the
	54	firmware provided DT blob
	55
c2794437 RH	56	fee00000 feffffff Mapping of PCI I/O space. This is a static
	57	mapping within the vmalloc space.
	58
1da177e4 LT	59	VMALLOC_START VMALLOC_END-1 vmalloc() / ioremap() space.
	60	Memory returned by vmalloc/ioremap will
	61	be dynamically placed in this region.
0536bdf3 NP	62	Machine specific static mappings are also
	63	located here through iotable_init().
	64	VMALLOC_START is based upon the value
	65	of the high_memory variable, and VMALLOC_END
6ff09660	66	is equal to 0xff800000.
1da177e4 LT	67
	68	PAGE_OFFSET high_memory-1 Kernel direct-mapped RAM region.
	69	This maps the platforms RAM, and typically
	70	maps all platform RAM in a 1:1 relationship.
	71
18fe1cad FB	72	PKMAP_BASE PAGE_OFFSET-1 Permanent kernel mappings
	73	One way of mapping HIGHMEM pages into kernel
	74	space.
	75
	76	MODULES_VADDR MODULES_END-1 Kernel module space
1da177e4 LT	77	Kernel modules inserted via insmod are
	78	placed here using dynamic mappings.
	79
c12366ba LW	80	TASK_SIZE MODULES_VADDR-1 KASAn shadow memory when KASan is in use.
	81	The range from MODULES_VADDR to the top
	82	of the memory is shadowed here with 1 bit
	83	per byte of memory.
	84
1da177e4 LT	85	00001000 TASK_SIZE-1 User space mappings
	86	Per-thread mappings are placed here via
	87	the mmap() system call.
	88
	89	00000000 00000fff CPU vector page / null pointer trap
	90	CPUs which do not support vector remapping
	91	place their vector page here. NULL pointer
	92	dereferences by both the kernel and user
	93	space are also caught via this mapping.
dc7a12bd	94	=============== =============== ===============================================
1da177e4 LT	95
	96	Please note that mappings which collide with the above areas may result
	97	in a non-bootable kernel, or may cause the kernel to (eventually) panic
	98	at run time.
	99
	100	Since future CPUs may impact the kernel mapping layout, user programs
	101	must not access any memory which is not mapped inside their 0x0001000
	102	to TASK_SIZE address range. If they wish to access these areas, they
	103	must set up their own mappings using open() and mmap().