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

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Message-ID: <20081022192622.GA30930@elte.hu>
Date:	Wed, 22 Oct 2008 21:26:22 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Alok Kataria <akataria@...are.com>
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.


* 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.

	Ingo
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