Re: [PATCH] x86, TSC: Add a software TSC offset

[Andy Lutomirski]

On Mon, Jul 21, 2014 at 3:08 PM, Borislav Petkov <bp@xxxxxxxxx> 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
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