xref: /kernel/linux/linux-5.10/Documentation/arm/memory.rst

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Kernel Memory Layout on ARM Linux
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		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().