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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
24ffff4000 ffffffff cache aliasing on ARMv6 and later CPUs.
25
26ffff1000 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
34fffe0000 fffeffff XScale cache flush area. This is used
35 in proc-xscale.S to flush the whole data
36 cache. Free for other usage on non-XScale.
37
38fff00000 fffdffff Fixmap mapping region. Addresses provided
39 by fix_to_virt() will be located here.
40
41ffc00000 ffefffff DMA memory mapping region. Memory returned
42 by the dma_alloc_xxx functions will be
43 dynamically mapped here.
44
45ff000000 ffbfffff Reserved for future expansion of DMA
46 mapping region.
47
48VMALLOC_END feffffff Free for platform use, recommended.
49 VMALLOC_END must be aligned to a 2MB
50 boundary.
51
52VMALLOC_START VMALLOC_END-1 vmalloc() / ioremap() space.
53 Memory returned by vmalloc/ioremap will
54 be dynamically placed in this region.
55 VMALLOC_START may be based upon the value
56 of the high_memory variable.
57
58PAGE_OFFSET high_memory-1 Kernel direct-mapped RAM region.
59 This maps the platforms RAM, and typically
60 maps all platform RAM in a 1:1 relationship.
61
62PKMAP_BASE PAGE_OFFSET-1 Permanent kernel mappings
63 One way of mapping HIGHMEM pages into kernel
64 space.
65
66MODULES_VADDR MODULES_END-1 Kernel module space
67 Kernel modules inserted via insmod are
68 placed here using dynamic mappings.
69
7000001000 TASK_SIZE-1 User space mappings
71 Per-thread mappings are placed here via
72 the mmap() system call.
73
7400000000 00000fff CPU vector page / null pointer trap
75 CPUs which do not support vector remapping
76 place their vector page here. NULL pointer
77 dereferences by both the kernel and user
78 space are also caught via this mapping.
79
80Please note that mappings which collide with the above areas may result
81in a non-bootable kernel, or may cause the kernel to (eventually) panic
82at run time.
83
84Since future CPUs may impact the kernel mapping layout, user programs
85must not access any memory which is not mapped inside their 0x0001000
86to TASK_SIZE address range. If they wish to access these areas, they
87must set up their own mappings using open() and mmap().