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Diffstat (limited to 'Documentation/arm/swp_emulation')
-rw-r--r-- | Documentation/arm/swp_emulation | 27 |
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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 | |||