Kernel Memory Layout on ARM Linux

Russell King <>

November 17, 2005 (2.6.15)

This document describes the virtual memory layout which the Linux kernel uses for ARM processors. It indicates which regions are free for platforms to use, and which are used by generic code.

The ARM CPU is capable of addressing a maximum of 4GB virtual memory space, and this must be shared between user space processes, the kernel, and hardware devices.

As the ARM architecture matures, it becomes necessary to reserve certain regions of VM space for use for new facilities; therefore this document may reserve more VM space over time.

Start End Use
ffff8000 ffffffff copy_user_page / clear_user_page use. For SA11xx and Xscale, this is used to setup a minicache mapping.
ffff4000 ffffffff cache aliasing on ARMv6 and later CPUs.
ffff1000 ffff7fff Reserved. Platforms must not use this address range.
ffff0000 ffff0fff CPU vector page. The CPU vectors are mapped here if the CPU supports vector relocation (control register V bit.)
fffe0000 fffeffff XScale cache flush area. This is used in proc-xscale.S to flush the whole data cache. (XScale does not have TCM.)
fffe8000 fffeffff DTCM mapping area for platforms with DTCM mounted inside the CPU.
fffe0000 fffe7fff ITCM mapping area for platforms with ITCM mounted inside the CPU.
ffc80000 ffefffff Fixmap mapping region. Addresses provided by fix_to_virt() will be located here.
ffc00000 ffc7ffff Guard region
ff800000 ffbfffff Permanent, fixed read-only mapping of the firmware provided DT blob
fee00000 feffffff Mapping of PCI I/O space. This is a static mapping within the vmalloc space.
VMALLOC_START VMALLOC_END-1 vmalloc() / ioremap() space. Memory returned by vmalloc/ioremap will be dynamically placed in this region. Machine specific static mappings are also located here through iotable_init(). VMALLOC_START is based upon the value of the high_memory variable, and VMALLOC_END is equal to 0xff800000.
PAGE_OFFSET high_memory-1 Kernel direct-mapped RAM region. This maps the platforms RAM, and typically maps all platform RAM in a 1:1 relationship.
PKMAP_BASE PAGE_OFFSET-1 Permanent kernel mappings One way of mapping HIGHMEM pages into kernel space.
MODULES_VADDR MODULES_END-1 Kernel module space Kernel modules inserted via insmod are placed here using dynamic mappings.
TASK_SIZE MODULES_VADDR-1 KASAn shadow memory when KASan is in use. The range from MODULES_VADDR to the top of the memory is shadowed here with 1 bit per byte of memory.
00001000 TASK_SIZE-1 User space mappings Per-thread mappings are placed here via the mmap() system call.
00000000 00000fff CPU vector page / null pointer trap CPUs which do not support vector remapping place their vector page here. NULL pointer dereferences by both the kernel and user space are also caught via this mapping.

Please note that mappings which collide with the above areas may result in a non-bootable kernel, or may cause the kernel to (eventually) panic at run time.

Since future CPUs may impact the kernel mapping layout, user programs must not access any memory which is not mapped inside their 0x0001000 to TASK_SIZE address range. If they wish to access these areas, they must set up their own mappings using open() and mmap().