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1da177e4 LT |
1 | Kernel Memory Layout on ARM Linux |
2 | ||
3 | Russell King <rmk@arm.linux.org.uk> | |
02b30839 | 4 | November 17, 2005 (2.6.15) |
1da177e4 LT |
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 | ||
e624859e LW |
24 | ffff4000 ffffffff cache aliasing on ARMv6 and later CPUs. |
25 | ||
1da177e4 LT |
26 | ffff1000 ffff7fff Reserved. |
27 | Platforms must not use this address range. | |
28 | ||
29 | ffff0000 ffff0fff CPU vector page. | |
30 | The CPU vectors are mapped here if the | |
31 | CPU supports vector relocation (control | |
32 | register V bit.) | |
33 | ||
5f0fbf9e NP |
34 | fffe0000 fffeffff XScale cache flush area. This is used |
35 | in proc-xscale.S to flush the whole data | |
1dbd30e9 LW |
36 | cache. (XScale does not have TCM.) |
37 | ||
38 | fffe8000 fffeffff DTCM mapping area for platforms with | |
39 | DTCM mounted inside the CPU. | |
40 | ||
41 | fffe0000 fffe7fff ITCM mapping area for platforms with | |
42 | ITCM mounted inside the CPU. | |
5f0fbf9e NP |
43 | |
44 | fff00000 fffdffff Fixmap mapping region. Addresses provided | |
45 | by fix_to_virt() will be located here. | |
46 | ||
47 | ffc00000 ffefffff DMA memory mapping region. Memory returned | |
1da177e4 LT |
48 | by the dma_alloc_xxx functions will be |
49 | dynamically mapped here. | |
50 | ||
51 | ff000000 ffbfffff Reserved for future expansion of DMA | |
52 | mapping region. | |
53 | ||
c2794437 RH |
54 | fee00000 feffffff Mapping of PCI I/O space. This is a static |
55 | mapping within the vmalloc space. | |
56 | ||
1da177e4 LT |
57 | VMALLOC_START VMALLOC_END-1 vmalloc() / ioremap() space. |
58 | Memory returned by vmalloc/ioremap will | |
59 | be dynamically placed in this region. | |
0536bdf3 NP |
60 | Machine specific static mappings are also |
61 | located here through iotable_init(). | |
62 | VMALLOC_START is based upon the value | |
63 | of the high_memory variable, and VMALLOC_END | |
64 | is equal to 0xff000000. | |
1da177e4 LT |
65 | |
66 | PAGE_OFFSET high_memory-1 Kernel direct-mapped RAM region. | |
67 | This maps the platforms RAM, and typically | |
68 | maps all platform RAM in a 1:1 relationship. | |
69 | ||
18fe1cad FB |
70 | PKMAP_BASE PAGE_OFFSET-1 Permanent kernel mappings |
71 | One way of mapping HIGHMEM pages into kernel | |
72 | space. | |
73 | ||
74 | MODULES_VADDR MODULES_END-1 Kernel module space | |
1da177e4 LT |
75 | Kernel modules inserted via insmod are |
76 | placed here using dynamic mappings. | |
77 | ||
78 | 00001000 TASK_SIZE-1 User space mappings | |
79 | Per-thread mappings are placed here via | |
80 | the mmap() system call. | |
81 | ||
82 | 00000000 00000fff CPU vector page / null pointer trap | |
83 | CPUs which do not support vector remapping | |
84 | place their vector page here. NULL pointer | |
85 | dereferences by both the kernel and user | |
86 | space are also caught via this mapping. | |
87 | ||
88 | Please note that mappings which collide with the above areas may result | |
89 | in a non-bootable kernel, or may cause the kernel to (eventually) panic | |
90 | at run time. | |
91 | ||
92 | Since future CPUs may impact the kernel mapping layout, user programs | |
93 | must not access any memory which is not mapped inside their 0x0001000 | |
94 | to TASK_SIZE address range. If they wish to access these areas, they | |
95 | must set up their own mappings using open() and mmap(). |