1 Kernel Memory Layout on ARM Linux
2
3 Russell King <rmk@arm.linux.org.uk>
4 November 17, 2005 (2.6.15)
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
24 ffff1000 ffff7fff Reserved.
25 Platforms must not use this address range.
26
27 ffff0000 ffff0fff CPU vector page.
28 The CPU vectors are mapped here if the
29 CPU supports vector relocation (control
30 register V bit.)
31
32 ffc00000 fffeffff DMA memory mapping region. Memory returned
33 by the dma_alloc_xxx functions will be
34 dynamically mapped here.
35
36 ff000000 ffbfffff Reserved for future expansion of DMA
37 mapping region.
38
39 VMALLOC_END feffffff Free for platform use, recommended.
40 VMALLOC_END must be aligned to a 2MB
41 boundary.
42
43 VMALLOC_START VMALLOC_END-1 vmalloc() / ioremap() space.
44 Memory returned by vmalloc/ioremap will
45 be dynamically placed in this region.
46 VMALLOC_START may be based upon the value
47 of the high_memory variable.
48
49 PAGE_OFFSET high_memory-1 Kernel direct-mapped RAM region.
50 This maps the platforms RAM, and typically
51 maps all platform RAM in a 1:1 relationship.
52
53 TASK_SIZE PAGE_OFFSET-1 Kernel module space
54 Kernel modules inserted via insmod are
55 placed here using dynamic mappings.
56
57 00001000 TASK_SIZE-1 User space mappings
58 Per-thread mappings are placed here via
59 the mmap() system call.
60
61 00000000 00000fff CPU vector page / null pointer trap
62 CPUs which do not support vector remapping
63 place their vector page here. NULL pointer
64 dereferences by both the kernel and user
65 space are also caught via this mapping.
66
67 Please note that mappings which collide with the above areas may result
68 in a non-bootable kernel, or may cause the kernel to (eventually) panic
69 at run time.
70
71 Since future CPUs may impact the kernel mapping layout, user programs
72 must not access any memory which is not mapped inside their 0x0001000
73 to TASK_SIZE address range. If they wish to access these areas, they
74 must set up their own mappings using open() and mmap().
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