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From: Jung-uk Kim <jkim@FreeBSD.org>
To: John Baldwin <jhb@freebsd.org>
Date: Tue, 21 Jun 2011 12:11:58 -0400
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Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org,
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Subject: Re: svn commit: r222866 - head/sys/x86/x86
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On Tuesday 21 June 2011 11:56 am, John Baldwin wrote:
> On Tuesday, June 21, 2011 11:48:55 am Jung-uk Kim wrote:
> > On Tuesday 21 June 2011 09:10 am, John Baldwin wrote:
> > > On Monday, June 20, 2011 7:41:00 pm Jung-uk Kim wrote:
> > > > My questions to you:
> > > >
> > > > a) Why do we care TSC timecounter when it is not invariant
> > > > where we *know* it is unusable and set to negative quality?
> > >
> > > What if the user knows they will not enable CPU throttling so
> > > for them the TSC is safe?  In that case, TSC is a more
> > > efficient timecounter and if the user constrains the system to
> > > make the TSC safe we should let them use it.
> >
> > In that case, it must be a UP system, the quality is still 800,
> > and TSC value won't be shifted.
> >
> > My question was specific to SMP cases.  Sorry, I didn't make that
> > clear.
>
> What if the user has an SMP system where the TSCs are in sync but
> it's older so it doesn't set the TSC invariant bit set in cpuid. 
> Are we now forbidding that user from using the TSC?

We do not forbid it but we cannot increase the quality because we 
don't compensate drift.

Jung-uk Kim