swp_emulation.rst

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