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. (XScale does not have TCM.) 37 38fffe8000 fffeffff DTCM mapping area for platforms with 39 DTCM mounted inside the CPU. 40 41fffe0000 fffe7fff ITCM mapping area for platforms with 42 ITCM mounted inside the CPU. 43 44fff00000 fffdffff Fixmap mapping region. Addresses provided 45 by fix_to_virt() will be located here. 46 47ffc00000 ffefffff DMA memory mapping region. Memory returned 48 by the dma_alloc_xxx functions will be 49 dynamically mapped here. 50 51ff000000 ffbfffff Reserved for future expansion of DMA 52 mapping region. 53 54VMALLOC_END feffffff Free for platform use, recommended. 55 VMALLOC_END must be aligned to a 2MB 56 boundary. 57 58VMALLOC_START VMALLOC_END-1 vmalloc() / ioremap() space. 59 Memory returned by vmalloc/ioremap will 60 be dynamically placed in this region. 61 VMALLOC_START may be based upon the value 62 of the high_memory variable. 63 64PAGE_OFFSET high_memory-1 Kernel direct-mapped RAM region. 65 This maps the platforms RAM, and typically 66 maps all platform RAM in a 1:1 relationship. 67 68PKMAP_BASE PAGE_OFFSET-1 Permanent kernel mappings 69 One way of mapping HIGHMEM pages into kernel 70 space. 71 72MODULES_VADDR MODULES_END-1 Kernel module space 73 Kernel modules inserted via insmod are 74 placed here using dynamic mappings. 75 7600001000 TASK_SIZE-1 User space mappings 77 Per-thread mappings are placed here via 78 the mmap() system call. 79 8000000000 00000fff CPU vector page / null pointer trap 81 CPUs which do not support vector remapping 82 place their vector page here. NULL pointer 83 dereferences by both the kernel and user 84 space are also caught via this mapping. 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