[mirror_ubuntu-bionic-kernel.git] / Documentation / arm / memory.txt

		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.

fff00000	fffdffff	Fixmap mapping region.  Addresses provided
				by fix_to_virt() will be located here.

ffc00000	ffefffff	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_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 0xff000000.

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.

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
1da177e4 LT	1	Kernel Memory Layout on ARM Linux
	2
	3	Russell King <rmk@arm.linux.org.uk>
02b30839	4	November 17, 2005 (2.6.15)
1da177e4 LT	5
	6	This document describes the virtual memory layout which the Linux
	7	kernel uses for ARM processors. It indicates which regions are
	8	free for platforms to use, and which are used by generic code.
	9
	10	The ARM CPU is capable of addressing a maximum of 4GB virtual memory
	11	space, and this must be shared between user space processes, the
	12	kernel, and hardware devices.
	13
	14	As the ARM architecture matures, it becomes necessary to reserve
	15	certain regions of VM space for use for new facilities; therefore
	16	this document may reserve more VM space over time.
	17
	18	Start End Use
	19	--------------------------------------------------------------------------
	20	ffff8000 ffffffff copy_user_page / clear_user_page use.
	21	For SA11xx and Xscale, this is used to
	22	setup a minicache mapping.
	23
e624859e LW	24	ffff4000 ffffffff cache aliasing on ARMv6 and later CPUs.
e624859e LW	25
1da177e4 LT	26	ffff1000 ffff7fff Reserved.
	27	Platforms must not use this address range.
	28
	29	ffff0000 ffff0fff CPU vector page.
	30	The CPU vectors are mapped here if the
	31	CPU supports vector relocation (control
	32	register V bit.)
	33
5f0fbf9e NP	34	fffe0000 fffeffff XScale cache flush area. This is used
5f0fbf9e NP	35	in proc-xscale.S to flush the whole data
1dbd30e9 LW	36	cache. (XScale does not have TCM.)
	37
	38	fffe8000 fffeffff DTCM mapping area for platforms with
	39	DTCM mounted inside the CPU.
	40
	41	fffe0000 fffe7fff ITCM mapping area for platforms with
	42	ITCM mounted inside the CPU.
5f0fbf9e NP	43
	44	fff00000 fffdffff Fixmap mapping region. Addresses provided
	45	by fix_to_virt() will be located here.
	46
	47	ffc00000 ffefffff DMA memory mapping region. Memory returned
1da177e4 LT	48	by the dma_alloc_xxx functions will be
	49	dynamically mapped here.
	50
	51	ff000000 ffbfffff Reserved for future expansion of DMA
	52	mapping region.
	53
1da177e4 LT	54	VMALLOC_START VMALLOC_END-1 vmalloc() / ioremap() space.
	55	Memory returned by vmalloc/ioremap will
	56	be dynamically placed in this region.
0536bdf3 NP	57	Machine specific static mappings are also
	58	located here through iotable_init().
	59	VMALLOC_START is based upon the value
	60	of the high_memory variable, and VMALLOC_END
	61	is equal to 0xff000000.
1da177e4 LT	62
	63	PAGE_OFFSET high_memory-1 Kernel direct-mapped RAM region.
	64	This maps the platforms RAM, and typically
	65	maps all platform RAM in a 1:1 relationship.
	66
18fe1cad FB	67	PKMAP_BASE PAGE_OFFSET-1 Permanent kernel mappings
	68	One way of mapping HIGHMEM pages into kernel
	69	space.
	70
	71	MODULES_VADDR MODULES_END-1 Kernel module space
1da177e4 LT	72	Kernel modules inserted via insmod are
	73	placed here using dynamic mappings.
	74
	75	00001000 TASK_SIZE-1 User space mappings
	76	Per-thread mappings are placed here via
	77	the mmap() system call.
	78
	79	00000000 00000fff CPU vector page / null pointer trap
	80	CPUs which do not support vector remapping
	81	place their vector page here. NULL pointer
	82	dereferences by both the kernel and user
	83	space are also caught via this mapping.
	84
	85	Please note that mappings which collide with the above areas may result
	86	in a non-bootable kernel, or may cause the kernel to (eventually) panic
	87	at run time.
	88
	89	Since future CPUs may impact the kernel mapping layout, user programs
	90	must not access any memory which is not mapped inside their 0x0001000
	91	to TASK_SIZE address range. If they wish to access these areas, they
	92	must set up their own mappings using open() and mmap().