[linux.git] / Documentation / arm / swp_emulation

Software emulation of deprecated SWP instruction (CONFIG_SWP_EMULATE)
---------------------------------------------------------------------

ARMv6 architecture deprecates use of the SWP/SWPB instructions, and recommeds
moving to the load-locked/store-conditional instructions LDREX and STREX.

ARMv7 multiprocessing extensions introduce the ability to disable these
instructions, triggering an undefined instruction exception when executed.
Trapped instructions are emulated using an LDREX/STREX or LDREXB/STREXB
sequence. If a memory access fault (an abort) occurs, a segmentation fault is
signalled to the triggering process.

/proc/cpu/swp_emulation holds some statistics/information, including the PID of
the last process to trigger the emulation to be invocated. For example:
---
Emulated SWP:		12
Emulated SWPB:		0
Aborted SWP{B}:		1
Last process:		314
---

NOTE: when accessing uncached shared regions, LDREX/STREX rely on an external
transaction monitoring block called a global monitor to maintain update
atomicity. If your system does not implement a global monitor, this option can
cause programs that perform SWP operations to uncached memory to deadlock, as
the STREX operation will always fail.
Commit	Line	Data
64d2dc38 LL	1	Software emulation of deprecated SWP instruction (CONFIG_SWP_EMULATE)
	2	---------------------------------------------------------------------
	3
	4	ARMv6 architecture deprecates use of the SWP/SWPB instructions, and recommeds
	5	moving to the load-locked/store-conditional instructions LDREX and STREX.
	6
	7	ARMv7 multiprocessing extensions introduce the ability to disable these
	8	instructions, triggering an undefined instruction exception when executed.
	9	Trapped instructions are emulated using an LDREX/STREX or LDREXB/STREXB
	10	sequence. If a memory access fault (an abort) occurs, a segmentation fault is
	11	signalled to the triggering process.
	12
	13	/proc/cpu/swp_emulation holds some statistics/information, including the PID of
	14	the last process to trigger the emulation to be invocated. For example:
	15	---
	16	Emulated SWP: 12
	17	Emulated SWPB: 0
	18	Aborted SWP{B}: 1
	19	Last process: 314
	20	---
	21
	22	NOTE: when accessing uncached shared regions, LDREX/STREX rely on an external
	23	transaction monitoring block called a global monitor to maintain update
	24	atomicity. If your system does not implement a global monitor, this option can
	25	cause programs that perform SWP operations to uncached memory to deadlock, as
	26	the STREX operation will always fail.
	27