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 48ffc80000 ffefffff Fixmap mapping region. Addresses provided 49 by fix_to_virt() will be located here. 50 51ffc00000 ffc7ffff Guard region 52 53ff800000 ffbfffff Permanent, fixed read-only mapping of the 54 firmware provided DT blob 55 56fee00000 feffffff Mapping of PCI I/O space. This is a static 57 mapping within the vmalloc space. 58 59VMALLOC_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 68PAGE_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 72PKMAP_BASE PAGE_OFFSET-1 Permanent kernel mappings 73 One way of mapping HIGHMEM pages into kernel 74 space. 75 76MODULES_VADDR MODULES_END-1 Kernel module space 77 Kernel modules inserted via insmod are 78 placed here using dynamic mappings. 79 80TASK_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 8500001000 TASK_SIZE-1 User space mappings 86 Per-thread mappings are placed here via 87 the mmap() system call. 88 8900000000 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 96Please note that mappings which collide with the above areas may result 97in a non-bootable kernel, or may cause the kernel to (eventually) panic 98at run time. 99 100Since future CPUs may impact the kernel mapping layout, user programs 101must not access any memory which is not mapped inside their 0x0001000 102to TASK_SIZE address range. If they wish to access these areas, they 103must set up their own mappings using open() and mmap(). 104