Am Dienstag, 21. Juni 2016, 09:22:31 schrieb John Stultz: Hi John, > On Tue, Jun 21, 2016 at 1:32 AM, Arnd Bergmann <arnd@xxxxxxxx> wrote: > > On Tuesday, June 21, 2016 8:20:10 AM CEST Stephan Mueller wrote: > >> Am Freitag, 17. Juni 2016, 17:59:41 schrieb Arnd Bergmann: > > Compared to the previous __getnstimeofday(), the difference is > > > > - using "monotonic" timebase instead of "real", so the zero time > > > > is when the system booted rather than Jan 1 1970 > > I haven't looked at the details of the calling code, but I'd worry for > crypto uses, especially if its being used for entropy collection, > using the monotonic clock instead of the realtime clock might be > problematic. Funnily it does not seem like that. All tests that I have conducted show that monotonic clocks behave equally as realtime clocks, because the uncertainty lies in the execution time of a set of instructions. All we need to do is to measure it with a timer that has a resolution that allows detecting these variations. > > > - "raw" means we don't honor updates for the rate based on ntp, > > > > which is probably better as the ntp state might be observable > > over the net (it probably doesn't matter, but it can't hurt) > > So... this feels like a very vague explanation, and the lack of > frequency correction here probably need a really good comment. Keeping > multiple time domains is usually asking for trouble, but we added the > MONOTONIC_RAW clock to address a few cases where people really wanted > an abstract hardware counter, which was unaffected by frequency > corrections. I'd really make sure its clear why this is what you want > vs the standard system time domain so we don't run into problems > understanding it later. Perfect, that is what I would be interested in. > > > - "fast" means that in very rare cases, the time might appear > > > > to go backwards (it probably can't happen here because you are not > > called in an NMI). > > "fast" really means "safe-for-nmi wrt to locking". The tradeoff being > that when frequency adjustments occur, and if your code is delayed, > you might see time go backwards by a small amount. This allows My code would not see that as an issue. > tracing/sched code (or other code called from NMI) to not have to > duplicate the timekeeping infrastructure. > > I think without a much better explanation, using the "fast" method > isn't really warranted here. Thanks a lot. With that, I would think that the proposed ktime_get_raw_fast_ns is good for use, which is supported with testing on my system. > > thanks > -john Ciao Stephan -- To unsubscribe from this list: send the line "unsubscribe linux-crypto" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
