Kernel Memory Layout on ARM Linux

Russell King <rmk@arm.linux.org.uk>

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().