0001 =================================
0002 Kernel Memory Layout on ARM Linux
0003 =================================
0004
0005 Russell King <rmk@arm.linux.org.uk>
0006
0007 November 17, 2005 (2.6.15)
0008
0009 This document describes the virtual memory layout which the Linux
0010 kernel uses for ARM processors. It indicates which regions are
0011 free for platforms to use, and which are used by generic code.
0012
0013 The ARM CPU is capable of addressing a maximum of 4GB virtual memory
0014 space, and this must be shared between user space processes, the
0015 kernel, and hardware devices.
0016
0017 As the ARM architecture matures, it becomes necessary to reserve
0018 certain regions of VM space for use for new facilities; therefore
0019 this document may reserve more VM space over time.
0020
0021 =============== =============== ===============================================
0022 Start End Use
0023 =============== =============== ===============================================
0024 ffff8000 ffffffff copy_user_page / clear_user_page use.
0025 For SA11xx and Xscale, this is used to
0026 setup a minicache mapping.
0027
0028 ffff4000 ffffffff cache aliasing on ARMv6 and later CPUs.
0029
0030 ffff1000 ffff7fff Reserved.
0031 Platforms must not use this address range.
0032
0033 ffff0000 ffff0fff CPU vector page.
0034 The CPU vectors are mapped here if the
0035 CPU supports vector relocation (control
0036 register V bit.)
0037
0038 fffe0000 fffeffff XScale cache flush area. This is used
0039 in proc-xscale.S to flush the whole data
0040 cache. (XScale does not have TCM.)
0041
0042 fffe8000 fffeffff DTCM mapping area for platforms with
0043 DTCM mounted inside the CPU.
0044
0045 fffe0000 fffe7fff ITCM mapping area for platforms with
0046 ITCM mounted inside the CPU.
0047
0048 ffc80000 ffefffff Fixmap mapping region. Addresses provided
0049 by fix_to_virt() will be located here.
0050
0051 ffc00000 ffc7ffff Guard region
0052
0053 ff800000 ffbfffff Permanent, fixed read-only mapping of the
0054 firmware provided DT blob
0055
0056 fee00000 feffffff Mapping of PCI I/O space. This is a static
0057 mapping within the vmalloc space.
0058
0059 VMALLOC_START VMALLOC_END-1 vmalloc() / ioremap() space.
0060 Memory returned by vmalloc/ioremap will
0061 be dynamically placed in this region.
0062 Machine specific static mappings are also
0063 located here through iotable_init().
0064 VMALLOC_START is based upon the value
0065 of the high_memory variable, and VMALLOC_END
0066 is equal to 0xff800000.
0067
0068 PAGE_OFFSET high_memory-1 Kernel direct-mapped RAM region.
0069 This maps the platforms RAM, and typically
0070 maps all platform RAM in a 1:1 relationship.
0071
0072 PKMAP_BASE PAGE_OFFSET-1 Permanent kernel mappings
0073 One way of mapping HIGHMEM pages into kernel
0074 space.
0075
0076 MODULES_VADDR MODULES_END-1 Kernel module space
0077 Kernel modules inserted via insmod are
0078 placed here using dynamic mappings.
0079
0080 TASK_SIZE MODULES_VADDR-1 KASAn shadow memory when KASan is in use.
0081 The range from MODULES_VADDR to the top
0082 of the memory is shadowed here with 1 bit
0083 per byte of memory.
0084
0085 00001000 TASK_SIZE-1 User space mappings
0086 Per-thread mappings are placed here via
0087 the mmap() system call.
0088
0089 00000000 00000fff CPU vector page / null pointer trap
0090 CPUs which do not support vector remapping
0091 place their vector page here. NULL pointer
0092 dereferences by both the kernel and user
0093 space are also caught via this mapping.
0094 =============== =============== ===============================================
0095
0096 Please note that mappings which collide with the above areas may result
0097 in a non-bootable kernel, or may cause the kernel to (eventually) panic
0098 at run time.
0099
0100 Since future CPUs may impact the kernel mapping layout, user programs
0101 must not access any memory which is not mapped inside their 0x0001000
0102 to TASK_SIZE address range. If they wish to access these areas, they
0103 must set up their own mappings using open() and mmap().
