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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
Nicolas Pitre437b6802014-05-30 20:58:27 +010044ffc00000 ffdfffff Fixmap mapping region. Addresses provided
Nicolas Pitre5f0fbf92008-09-16 13:05:53 -040045 by fix_to_virt() will be located here.
46
Rob Herringc2794432012-02-29 18:10:58 -060047fee00000 feffffff Mapping of PCI I/O space. This is a static
48 mapping within the vmalloc space.
49
Linus Torvalds1da177e2005-04-16 15:20:36 -070050VMALLOC_START VMALLOC_END-1 vmalloc() / ioremap() space.
51 Memory returned by vmalloc/ioremap will
52 be dynamically placed in this region.
Nicolas Pitre0536bdf2011-08-25 00:35:59 -040053 Machine specific static mappings are also
54 located here through iotable_init().
55 VMALLOC_START is based upon the value
56 of the high_memory variable, and VMALLOC_END
57 is equal to 0xff000000.
Linus Torvalds1da177e2005-04-16 15:20:36 -070058
59PAGE_OFFSET high_memory-1 Kernel direct-mapped RAM region.
60 This maps the platforms RAM, and typically
61 maps all platform RAM in a 1:1 relationship.
62
Fenkart/Bostandzhyan18fe1ca2010-02-07 21:48:38 +010063PKMAP_BASE PAGE_OFFSET-1 Permanent kernel mappings
64 One way of mapping HIGHMEM pages into kernel
65 space.
66
67MODULES_VADDR MODULES_END-1 Kernel module space
Linus Torvalds1da177e2005-04-16 15:20:36 -070068 Kernel modules inserted via insmod are
69 placed here using dynamic mappings.
70
7100001000 TASK_SIZE-1 User space mappings
72 Per-thread mappings are placed here via
73 the mmap() system call.
74
7500000000 00000fff CPU vector page / null pointer trap
76 CPUs which do not support vector remapping
77 place their vector page here. NULL pointer
78 dereferences by both the kernel and user
79 space are also caught via this mapping.
80
81Please note that mappings which collide with the above areas may result
82in a non-bootable kernel, or may cause the kernel to (eventually) panic
83at run time.
84
85Since future CPUs may impact the kernel mapping layout, user programs
86must not access any memory which is not mapped inside their 0x0001000
87to TASK_SIZE address range. If they wish to access these areas, they
88must set up their own mappings using open() and mmap().