linux-kernel - Re: [PATCH] Skip tsc synchronization checks if CONSTANT

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Date:	Wed, 22 Oct 2008 15:11:58 -0700
From:	Alok Kataria <akataria@...are.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	LKML <linux-kernel@...r.kernel.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Daniel Hecht <dhecht@...are.com>
Subject: Re: [PATCH] Skip tsc synchronization checks if CONSTANT_TSC bit is
	set.

On Wed, 2008-10-22 at 15:13 -0700, Andi Kleen wrote:
> > > Adding it with vmware detection code is not useful.
> >
> > The VMware detection code is something that's needed anyways for other
> > purposes(getting tsc_freq right now). So the code which force sets this
> > TSC_CONSTANT bit is trivial.
> 
> It would be better to eliminate that too. Why do you need it anyways?
> 
> > Apart from that, as i said yesterday, this should be viewed as a special
> > case for VMware products which don't already set this bit. Changing the
> > behavior for already existing products is not feasible.
> 
> On old hypervisors Linux already runs fine without TSC, doesn't it?

Not really, there are problems with the pm timer too, the one about
missing the counter wrap and time dropping in chunks of 4 seconds.
Tried to explain it over here, http://lkml.org/lkml/2008/10/22/525

So TSC is the ideal clocksource from performance and correctness point
of view for VMware. 

Thanks,
Alok

> 
> -Andi

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