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

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Message-ID: <20140721223009.GF11555@pd.tnic>
Date:	Tue, 22 Jul 2014 00:30:09 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Andy Lutomirski <luto@...capital.net>
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 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.

> > 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.

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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