1 ================================= 2 Kernel Memory Layout on ARM Linux 3 ================================= 4 5 Russell King <rmk@arm.linux.org.uk> 6 7 November 17, 2005 (2.6.15) 8 9 This document describes the virtual memory layout which the Linux 10 kernel uses for ARM processors. It indicates which regions are 11 free for platforms to use, and which are used by generic code. 12 13 The ARM CPU is capable of addressing a maximum of 4GB virtual memory 14 space, and this must be shared between user space processes, the 15 kernel, and hardware devices. 16 17 As the ARM architecture matures, it becomes necessary to reserve 18 certain regions of VM space for use for new facilities; therefore 19 this document may reserve more VM space over time. 20 21 =============== =============== =============================================== 22 Start End Use 23 =============== =============== =============================================== 24 ffff8000 ffffffff copy_user_page / clear_user_page use. 25 For SA11xx and Xscale, this is used to 26 setup a minicache mapping. 27 28 ffff4000 ffffffff cache aliasing on ARMv6 and later CPUs. 29 30 ffff1000 ffff7fff Reserved. 31 Platforms must not use this address range. 32 33 ffff0000 ffff0fff CPU vector page. 34 The CPU vectors are mapped here if the 35 CPU supports vector relocation (control 36 register V bit.) 37 38 fffe0000 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 42 fffe8000 fffeffff DTCM mapping area for platforms with 43 DTCM mounted inside the CPU. 44 45 fffe0000 fffe7fff ITCM mapping area for platforms with 46 ITCM mounted inside the CPU. 47 48 ffc80000 ffefffff Fixmap mapping region. Addresses provided 49 by fix_to_virt() will be located here. 50 51 ffc00000 ffc7ffff Guard region 52 53 ff800000 ffbfffff Permanent, fixed read-only mapping of the 54 firmware provided DT blob 55 56 fee00000 feffffff Mapping of PCI I/O space. This is a static 57 mapping within the vmalloc space. 58 59 VMALLOC_START VMALLOC_END-1 vmalloc() / ioremap() space. 60 Memory returned by vmalloc/ioremap will 61 be dynamically placed in this region. 62 Machine specific static mappings are also 63 located here through iotable_init(). 64 VMALLOC_START is based upon the value 65 of the high_memory variable, and VMALLOC_END 66 is equal to 0xff800000. 67 68 PAGE_OFFSET high_memory-1 Kernel direct-mapped RAM region. 69 This maps the platforms RAM, and typically 70 maps all platform RAM in a 1:1 relationship. 71 72 PKMAP_BASE PAGE_OFFSET-1 Permanent kernel mappings 73 One way of mapping HIGHMEM pages into kernel 74 space. 75 76 MODULES_VADDR MODULES_END-1 Kernel module space 77 Kernel modules inserted via insmod are 78 placed here using dynamic mappings. 79 80 TASK_SIZE MODULES_VADDR-1 KASAn shadow memory when KASan is in use. 81 The range from MODULES_VADDR to the top 82 of the memory is shadowed here with 1 bit 83 per byte of memory. 84 85 00001000 TASK_SIZE-1 User space mappings 86 Per-thread mappings are placed here via 87 the mmap() system call. 88 89 00000000 00000fff CPU vector page / null pointer trap 90 CPUs which do not support vector remapping 91 place their vector page here. NULL pointer 92 dereferences by both the kernel and user 93 space are also caught via this mapping. 94 =============== =============== =============================================== 95 96 Please note that mappings which collide with the above areas may result 97 in a non-bootable kernel, or may cause the kernel to (eventually) panic 98 at run time. 99 100 Since future CPUs may impact the kernel mapping layout, user programs 101 must not access any memory which is not mapped inside their 0x0001000 102 to TASK_SIZE address range. If they wish to access these areas, they 103 must set up their own mappings using open() and mmap().
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