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Revision 238d1d0f79f619d75c2cc741d6770fb0986aef24 authored by Linus Torvalds on 30 December 2016, 17:32:26 UTC, committed by Linus Torvalds on 30 December 2016, 17:32:26 UTC 
Merge tag 'docs-4.10-rc1-fix' of git://git.lwn.net/linux
 
Pull documentation fixes from Jonathan Corbet:
 "Two small fixes:

   - A merge error on my part broke the DocBook build. I've
     requisitioned one of tglx's frozen sharks for appropriate
     disciplinary action and resolved to be more careful about testing
     the DocBook stuff as long as it's still around.

   - Fix an error in unaligned-memory-access.txt"

* tag 'docs-4.10-rc1-fix' of git://git.lwn.net/linux:
  Documentation/unaligned-memory-access.txt: fix incorrect comparison operator
  docs: Fix build failure
2 parent s f3de082 + 36f671b
Raw File
legacy_instructions.txt 
The arm64 port of the Linux kernel provides infrastructure to support
emulation of instructions which have been deprecated, or obsoleted in
the architecture. The infrastructure code uses undefined instruction
hooks to support emulation. Where available it also allows turning on
the instruction execution in hardware.

The emulation mode can be controlled by writing to sysctl nodes
(/proc/sys/abi). The following explains the different execution
behaviours and the corresponding values of the sysctl nodes -

* Undef
  Value: 0
  Generates undefined instruction abort. Default for instructions that
  have been obsoleted in the architecture, e.g., SWP

* Emulate
  Value: 1
  Uses software emulation. To aid migration of software, in this mode
  usage of emulated instruction is traced as well as rate limited
  warnings are issued. This is the default for deprecated
  instructions, .e.g., CP15 barriers

* Hardware Execution
  Value: 2
  Although marked as deprecated, some implementations may support the
  enabling/disabling of hardware support for the execution of these
  instructions. Using hardware execution generally provides better
  performance, but at the loss of ability to gather runtime statistics
  about the use of the deprecated instructions.

The default mode depends on the status of the instruction in the
architecture. Deprecated instructions should default to emulation
while obsolete instructions must be undefined by default.

Note: Instruction emulation may not be possible in all cases. See
individual instruction notes for further information.

Supported legacy instructions
-----------------------------
* SWP{B}
Node: /proc/sys/abi/swp
Status: Obsolete
Default: Undef (0)

* CP15 Barriers
Node: /proc/sys/abi/cp15_barrier
Status: Deprecated
Default: Emulate (1)

* SETEND
Node: /proc/sys/abi/setend
Status: Deprecated
Default: Emulate (1)*
Note: All the cpus on the system must have mixed endian support at EL0
for this feature to be enabled. If a new CPU - which doesn't support mixed
endian - is hotplugged in after this feature has been enabled, there could
be unexpected results in the application.
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