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1 Kernel Memory Layout on ARM Linux
2
3 Russell King <rmk@arm.linux.org.uk>
4 November 17, 2005 (2.6.15)
5
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
24ffff1000 ffff7fff Reserved.
25 Platforms must not use this address range.
26
27ffff0000 ffff0fff CPU vector page.
28 The CPU vectors are mapped here if the
29 CPU supports vector relocation (control
30 register V bit.)
31
32fffe0000 fffeffff XScale cache flush area. This is used
33 in proc-xscale.S to flush the whole data
34 cache. Free for other usage on non-XScale.
35
36fff00000 fffdffff Fixmap mapping region. Addresses provided
37 by fix_to_virt() will be located here.
38
39ffc00000 ffefffff DMA memory mapping region. Memory returned
40 by the dma_alloc_xxx functions will be
41 dynamically mapped here.
42
43ff000000 ffbfffff Reserved for future expansion of DMA
44 mapping region.
45
46VMALLOC_END feffffff Free for platform use, recommended.
47 VMALLOC_END must be aligned to a 2MB
48 boundary.
49
50VMALLOC_START VMALLOC_END-1 vmalloc() / ioremap() space.
51 Memory returned by vmalloc/ioremap will
52 be dynamically placed in this region.
53 VMALLOC_START may be based upon the value
54 of the high_memory variable.
55
56PAGE_OFFSET high_memory-1 Kernel direct-mapped RAM region.
57 This maps the platforms RAM, and typically
58 maps all platform RAM in a 1:1 relationship.
59
60TASK_SIZE PAGE_OFFSET-1 Kernel module space
61 Kernel modules inserted via insmod are
62 placed here using dynamic mappings.
63
6400001000 TASK_SIZE-1 User space mappings
65 Per-thread mappings are placed here via
66 the mmap() system call.
67
6800000000 00000fff CPU vector page / null pointer trap
69 CPUs which do not support vector remapping
70 place their vector page here. NULL pointer
71 dereferences by both the kernel and user
72 space are also caught via this mapping.
73
74Please note that mappings which collide with the above areas may result
75in a non-bootable kernel, or may cause the kernel to (eventually) panic
76at run time.
77
78Since future CPUs may impact the kernel mapping layout, user programs
79must not access any memory which is not mapped inside their 0x0001000
80to TASK_SIZE address range. If they wish to access these areas, they
81must set up their own mappings using open() and mmap().