1================================= 2Kernel Memory Layout on ARM Linux 3================================= 4 5 Russell King <rmk@arm.linux.org.uk> 6 7 November 17, 2005 (2.6.15) 8 9This document describes the virtual memory layout which the Linux 10kernel uses for ARM processors. It indicates which regions are 11free for platforms to use, and which are used by generic code. 12 13The ARM CPU is capable of addressing a maximum of 4GB virtual memory 14space, and this must be shared between user space processes, the 15kernel, and hardware devices. 16 17As the ARM architecture matures, it becomes necessary to reserve 18certain regions of VM space for use for new facilities; therefore 19this document may reserve more VM space over time. 20 21=============== =============== =============================================== 22Start End Use 23=============== =============== =============================================== 24ffff8000 ffffffff copy_user_page / clear_user_page use. 25 For SA11xx and Xscale, this is used to 26 setup a minicache mapping. 27 28ffff4000 ffffffff cache aliasing on ARMv6 and later CPUs. 29 30ffff1000 ffff7fff Reserved. 31 Platforms must not use this address range. 32 33ffff0000 ffff0fff CPU vector page. 34 The CPU vectors are mapped here if the 35 CPU supports vector relocation (control 36 register V bit.) 37 38fffe0000 fffeffff XScale cache flush area. This is used 39 in proc-xscale.S to flush the whole data 40 cache. (XScale does not have TCM.) 41 42fffe8000 fffeffff DTCM mapping area for platforms with 43 DTCM mounted inside the CPU. 44 45fffe0000 fffe7fff ITCM mapping area for platforms with 46 ITCM mounted inside the CPU. 47 48ffc00000 ffefffff Fixmap mapping region. Addresses provided 49 by fix_to_virt() will be located here. 50 51fee00000 feffffff Mapping of PCI I/O space. This is a static 52 mapping within the vmalloc space. 53 54VMALLOC_START VMALLOC_END-1 vmalloc() / ioremap() space. 55 Memory returned by vmalloc/ioremap will 56 be dynamically placed in this region. 57 Machine specific static mappings are also 58 located here through iotable_init(). 59 VMALLOC_START is based upon the value 60 of the high_memory variable, and VMALLOC_END 61 is equal to 0xff800000. 62 63PAGE_OFFSET high_memory-1 Kernel direct-mapped RAM region. 64 This maps the platforms RAM, and typically 65 maps all platform RAM in a 1:1 relationship. 66 67PKMAP_BASE PAGE_OFFSET-1 Permanent kernel mappings 68 One way of mapping HIGHMEM pages into kernel 69 space. 70 71MODULES_VADDR MODULES_END-1 Kernel module space 72 Kernel modules inserted via insmod are 73 placed here using dynamic mappings. 74 7500001000 TASK_SIZE-1 User space mappings 76 Per-thread mappings are placed here via 77 the mmap() system call. 78 7900000000 00000fff CPU vector page / null pointer trap 80 CPUs which do not support vector remapping 81 place their vector page here. NULL pointer 82 dereferences by both the kernel and user 83 space are also caught via this mapping. 84=============== =============== =============================================== 85 86Please note that mappings which collide with the above areas may result 87in a non-bootable kernel, or may cause the kernel to (eventually) panic 88at run time. 89 90Since future CPUs may impact the kernel mapping layout, user programs 91must not access any memory which is not mapped inside their 0x0001000 92to TASK_SIZE address range. If they wish to access these areas, they 93must set up their own mappings using open() and mmap(). 94