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

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Date:	Wed, 22 Oct 2008 12:30:40 -0700
From:	Alok Kataria <akataria@...are.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Andi Kleen <andi@...stfloor.org>, "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 12:26 -0700, Ingo Molnar wrote:
> * Alok Kataria <akataria@...are.com> wrote:
> 
> > Skip tsc synchronization checks if CONSTANT_TSC bit is set.
> >
> > From: Alok N Kataria <akataria@...are.com>
> >
> > TSC synchronization checks between CPU's bail out even if we see a
> > distortion of a single cycle. This makes the TSC mostly unsuable in a
> > virtualized environment.
> >
> > The CONSTANT_TSC bit tells us if the hardware exports a constant TSC,
> > we can use this bit to trust the hardware and skip the TSC sync checks
> > at bootup.
> 
> the sync check is there to check the _offset_ between CPUs. CONSTANT_TSC
> is not a guarantee that the TSC will be coherent across all CPUs.
> 
> so this patch is fundamentally wrong.

Then does adding a new flag to skip this check be acceptable ? 
Something like the patch that i had sent yesterday ?

Thanks,
Alok

> 
>         Ingo

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