On Mon, Jul 21, 2014 at 3:08 PM, Borislav Petkov <b...@alien8.de> wrote: > On Mon, Jul 21, 2014 at 02:56:49PM -0700, Andy Lutomirski wrote: >> I expect that users of __vdso_clock_gettime (e.g. glibc) will get the >> correct time :) They use vread_tsc, and they can't use >> preempt_disable, because they're in userspace. They also can't >> directly access per-cpu variables. >> >> Turning off vdso tsc support on these machines would be an option. > > Right, this is what I was going to propose to tglx on IRC. Or we can try > to come up with something working for the vdso too, say RDTSCP :-) > > But that still won't work as it needs the per-cpu variables. So I guess > vdso loses... > >> I actually own one of these systems. It's a Sandy Bridge Core-i7 >> Extreme or something like that. > > Ha, cool, so I've got my tester! :-)
Ha. Ha ha. Muahaha. Because IIRC this box is synced until the first time it suspends. > >> I wonder if that's a bug in get_cycles. >> >> The basic issue is that rdtsc is not ordered with respect to nearby >> loads, so it's fairly easy to see it behaving non-monotonically across >> CPUs. rdtscp is ordered, but it's a little slower. > > Yah, that I know. But I don't see get_cycles() having the barriers. So > it might be a bug. We can certainly try to "fix" it and see what happens > :-) > > diff --git a/arch/x86/include/asm/tsc.h b/arch/x86/include/asm/tsc.h > index 94605c0e9cee..ad7d5e449c0b 100644 > --- a/arch/x86/include/asm/tsc.h > +++ b/arch/x86/include/asm/tsc.h > @@ -27,7 +27,9 @@ static inline cycles_t get_cycles(void) > if (!cpu_has_tsc) > return 0; > #endif > + rdtsc_barrier(); > rdtscll(ret); > + rdtsc_barrier(); > Only the first of these is necessary. There was a long thread on this a couple of years ago, and the conclusion was that the code in vread_tsc in vclock_gettime.c is correct. --Andy > return ret; > } > > -- > Regards/Gruss, > Boris. > > Sent from a fat crate under my desk. Formatting is fine. > -- -- Andy Lutomirski AMA Capital Management, LLC -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
