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Message-ID: <CALCETrU1BWJ5h=GDRFKg_CXh4pQvB6MyCoRP=H3z8XGirhQDsQ@mail.gmail.com>
Date: Mon, 21 Jul 2014 15:43:36 -0700
From: Andy Lutomirski <luto@...capital.net>
To: Borislav Petkov <bp@...en8.de>
Cc: Peter Zijlstra <peterz@...radead.org>,
Thomas Gleixner <tglx@...utronix.de>, x86-ml <x86@...nel.org>,
lkml <linux-kernel@...r.kernel.org>,
Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH] x86, TSC: Add a software TSC offset
On Mon, Jul 21, 2014 at 3:30 PM, Borislav Petkov <bp@...en8.de> wrote:
> On Mon, Jul 21, 2014 at 03:13:33PM -0700, Andy Lutomirski wrote:
>> Ha. Ha ha. Muahaha. Because IIRC this box is synced until the first
>> time it suspends.
>
> Sweet, TSCs get fumbled in some S-state or maybe SMI... Who TF knows.
>
> Well, I'm thinking upon resume, we run through smpboot which should do
> the tsc sync check again. Will have to test to see.
I have some reason to believe that this is almost an intentional bug
on the part of the BIOS vendor.
>
>> > 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.
>
> * The various CPU manuals are unclear
> * as to whether rdtsc can be reordered with later loads,
> * but no one has ever seen it happen.
>
> until some future uarch proves you wrong. :-)
>
> I guess we can try with one pre-fence only first although if we're doing
> one already, I can't imagine the post-one to be all that expensive since
> we've retired the whole inflight crap already anyway.
>
IIRC it was actually quite expensive, at least on Sandy Bridge. Maybe
AMD is different.
Anyway, if some future uarch breaks this, I could resurrect my old
hack: do a TSC-dependent load prior to returning. Loads are ordered,
and the hackish load can't be reordered wrt RDTSC due to
data-dependency, so we're in business :)
--Andy
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