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linux-next/Documentation/arm/memory.txt
Nicolas Pitre 6ff0966052 ARM: 8432/1: move VMALLOC_END from 0xff000000 to 0xff800000
There is a 12MB unused region in our memory map between the vmalloc and
fixmap areas. This became unused with commit e9da6e9905, confirmed
with commit 64d3b6a3f4.

We also have a 8MB guard area before the vmalloc area.  With the default
240MB vmalloc area size and the current VMALLOC_END definition, that
means the end of low memory ends up at 0xef800000 which is unfortunate
for 768MB machines where 8MB of RAM is lost to himem.

Let's move VMALLOC_END to 0xff800000 so the guard area won't chop the
top of the 768MB low memory area while keeping the default vmalloc area
size unchanged and still preserving a gap between the vmalloc and fixmap
areas.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
2015-09-22 08:13:57 +01:00

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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.
ffc00000 ffefffff Fixmap mapping region. Addresses provided
by fix_to_virt() will be located here.
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.
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().