Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759112AbYJVT0x (ORCPT ); Wed, 22 Oct 2008 15:26:53 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1756545AbYJVT0d (ORCPT ); Wed, 22 Oct 2008 15:26:33 -0400 Received: from mx2.mail.elte.hu ([157.181.151.9]:53734 "EHLO mx2.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756217AbYJVT0c (ORCPT ); Wed, 22 Oct 2008 15:26:32 -0400 Date: Wed, 22 Oct 2008 21:26:22 +0200 From: Ingo Molnar To: Alok Kataria Cc: Andi Kleen , "H. Peter Anvin" , LKML , the arch/x86 maintainers , Daniel Hecht Subject: Re: [PATCH] Skip tsc synchronization checks if CONSTANT_TSC bit is set. Message-ID: <20081022192622.GA30930@elte.hu> References: <1224552902.2640.88.camel@alok-dev1> <874p36fflp.fsf@basil.nowhere.org> <1224607284.6161.22.camel@alok-dev1> <20081021174008.GH12825@one.firstfloor.org> <1224612294.6161.43.camel@alok-dev1> <20081021181536.GI12825@one.firstfloor.org> <1224616236.6161.60.camel@alok-dev1> <20081021192746.GJ12825@one.firstfloor.org> <1224703427.13953.8.camel@alok-dev1> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1224703427.13953.8.camel@alok-dev1> User-Agent: Mutt/1.5.18 (2008-05-17) X-ELTE-VirusStatus: clean X-ELTE-SpamScore: -1.5 X-ELTE-SpamLevel: X-ELTE-SpamCheck: no X-ELTE-SpamVersion: ELTE 2.0 X-ELTE-SpamCheck-Details: score=-1.5 required=5.9 tests=BAYES_00,DNS_FROM_SECURITYSAGE autolearn=no SpamAssassin version=3.2.3 -1.5 BAYES_00 BODY: Bayesian spam probability is 0 to 1% [score: 0.0000] 0.0 DNS_FROM_SECURITYSAGE RBL: Envelope sender in blackholes.securitysage.com Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 924 Lines: 26 * Alok Kataria wrote: > Skip tsc synchronization checks if CONSTANT_TSC bit is set. > > From: Alok N Kataria > > 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/