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Linus Torvalds1da177e2005-04-16 15:20:36 -07001 Kernel Memory Layout on ARM Linux
2
3 Russell King <rmk@arm.linux.org.uk>
Russell King02b30832005-11-17 22:43:30 +00004 November 17, 2005 (2.6.15)
Linus Torvalds1da177e2005-04-16 15:20:36 -07005
6This document describes the virtual memory layout which the Linux
7kernel uses for ARM processors. It indicates which regions are
8free for platforms to use, and which are used by generic code.
9
10The ARM CPU is capable of addressing a maximum of 4GB virtual memory
11space, and this must be shared between user space processes, the
12kernel, and hardware devices.
13
14As the ARM architecture matures, it becomes necessary to reserve
15certain regions of VM space for use for new facilities; therefore
16this document may reserve more VM space over time.
17
18Start End Use
19--------------------------------------------------------------------------
20ffff8000 ffffffff copy_user_page / clear_user_page use.
21 For SA11xx and Xscale, this is used to
22 setup a minicache mapping.
23
Linus Walleije6248592009-07-27 22:11:59 +010024ffff4000 ffffffff cache aliasing on ARMv6 and later CPUs.
25
Linus Torvalds1da177e2005-04-16 15:20:36 -070026ffff1000 ffff7fff Reserved.
27 Platforms must not use this address range.
28
29ffff0000 ffff0fff CPU vector page.
30 The CPU vectors are mapped here if the
31 CPU supports vector relocation (control
32 register V bit.)
33
Nicolas Pitre5f0fbf92008-09-16 13:05:53 -040034fffe0000 fffeffff XScale cache flush area. This is used
35 in proc-xscale.S to flush the whole data
Linus Walleij1dbd30e2010-07-12 21:53:28 +010036 cache. (XScale does not have TCM.)
37
38fffe8000 fffeffff DTCM mapping area for platforms with
39 DTCM mounted inside the CPU.
40
41fffe0000 fffe7fff ITCM mapping area for platforms with
42 ITCM mounted inside the CPU.
Nicolas Pitre5f0fbf92008-09-16 13:05:53 -040043
44fff00000 fffdffff Fixmap mapping region. Addresses provided
45 by fix_to_virt() will be located here.
46
47ffc00000 ffefffff DMA memory mapping region. Memory returned
Linus Torvalds1da177e2005-04-16 15:20:36 -070048 by the dma_alloc_xxx functions will be
49 dynamically mapped here.
50
51ff000000 ffbfffff Reserved for future expansion of DMA
52 mapping region.
53
Linus Torvalds1da177e2005-04-16 15:20:36 -070054VMALLOC_START VMALLOC_END-1 vmalloc() / ioremap() space.
55 Memory returned by vmalloc/ioremap will
56 be dynamically placed in this region.
Nicolas Pitre0536bdf2011-08-25 00:35:59 -040057 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.
Linus Torvalds1da177e2005-04-16 15:20:36 -070062
63PAGE_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
Fenkart/Bostandzhyan18fe1ca2010-02-07 21:48:38 +010067PKMAP_BASE PAGE_OFFSET-1 Permanent kernel mappings
68 One way of mapping HIGHMEM pages into kernel
69 space.
70
71MODULES_VADDR MODULES_END-1 Kernel module space
Linus Torvalds1da177e2005-04-16 15:20:36 -070072 Kernel modules inserted via insmod are
73 placed here using dynamic mappings.
74
7500001000 TASK_SIZE-1 User space mappings
76 Per-thread mappings are placed here via
77 the mmap() system call.
78
7900000000 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
85Please note that mappings which collide with the above areas may result
86in a non-bootable kernel, or may cause the kernel to (eventually) panic
87at run time.
88
89Since future CPUs may impact the kernel mapping layout, user programs
90must not access any memory which is not mapped inside their 0x0001000
91to TASK_SIZE address range. If they wish to access these areas, they
92must set up their own mappings using open() and mmap().