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Kernel Memory Layout on ARM Linux
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Russell King <rmk@arm.linux.org.uk>
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November 17, 2005 (2.6.15)
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This document describes the virtual memory layout which the Linux
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kernel uses for ARM processors. It indicates which regions are
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free for platforms to use, and which are used by generic code.
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The ARM CPU is capable of addressing a maximum of 4GB virtual memory
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space, and this must be shared between user space processes, the
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kernel, and hardware devices.
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As the ARM architecture matures, it becomes necessary to reserve
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certain regions of VM space for use for new facilities; therefore
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this document may reserve more VM space over time.
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=============== =============== ===============================================
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Start End Use
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=============== =============== ===============================================
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ffff8000 ffffffff copy_user_page / clear_user_page use.
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For SA11xx and Xscale, this is used to
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setup a minicache mapping.
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ffff4000 ffffffff cache aliasing on ARMv6 and later CPUs.
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ffff1000 ffff7fff Reserved.
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Platforms must not use this address range.
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ffff0000 ffff0fff CPU vector page.
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The CPU vectors are mapped here if the
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CPU supports vector relocation (control
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register V bit.)
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fffe0000 fffeffff XScale cache flush area. This is used
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in proc-xscale.S to flush the whole data
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cache. (XScale does not have TCM.)
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fffe8000 fffeffff DTCM mapping area for platforms with
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DTCM mounted inside the CPU.
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fffe0000 fffe7fff ITCM mapping area for platforms with
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ITCM mounted inside the CPU.
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ffc80000 ffefffff Fixmap mapping region. Addresses provided
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by fix_to_virt() will be located here.
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ffc00000 ffc7ffff Guard region
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ff800000 ffbfffff Permanent, fixed read-only mapping of the
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firmware provided DT blob
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fee00000 feffffff Mapping of PCI I/O space. This is a static
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mapping within the vmalloc space.
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VMALLOC_START VMALLOC_END-1 vmalloc() / ioremap() space.
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Memory returned by vmalloc/ioremap will
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be dynamically placed in this region.
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Machine specific static mappings are also
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located here through iotable_init().
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VMALLOC_START is based upon the value
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of the high_memory variable, and VMALLOC_END
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is equal to 0xff800000.
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PAGE_OFFSET high_memory-1 Kernel direct-mapped RAM region.
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This maps the platforms RAM, and typically
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maps all platform RAM in a 1:1 relationship.
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PKMAP_BASE PAGE_OFFSET-1 Permanent kernel mappings
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One way of mapping HIGHMEM pages into kernel
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space.
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MODULES_VADDR MODULES_END-1 Kernel module space
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Kernel modules inserted via insmod are
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placed here using dynamic mappings.
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00001000 TASK_SIZE-1 User space mappings
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Per-thread mappings are placed here via
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the mmap() system call.
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00000000 00000fff CPU vector page / null pointer trap
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CPUs which do not support vector remapping
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place their vector page here. NULL pointer
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dereferences by both the kernel and user
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space are also caught via this mapping.
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=============== =============== ===============================================
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Please note that mappings which collide with the above areas may result
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in a non-bootable kernel, or may cause the kernel to (eventually) panic
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at run time.
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Since future CPUs may impact the kernel mapping layout, user programs
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must not access any memory which is not mapped inside their 0x0001000
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to TASK_SIZE address range. If they wish to access these areas, they
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must set up their own mappings using open() and mmap().
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