Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756453AbYJVTbA (ORCPT ); Wed, 22 Oct 2008 15:31:00 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1759228AbYJVTan (ORCPT ); Wed, 22 Oct 2008 15:30:43 -0400 Received: from smtp-outbound-1.vmware.com ([65.115.85.69]:52597 "EHLO smtp-outbound-1.vmware.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757047AbYJVTal (ORCPT ); Wed, 22 Oct 2008 15:30:41 -0400 Subject: Re: [PATCH] Skip tsc synchronization checks if CONSTANT_TSC bit is set. From: Alok Kataria Reply-To: akataria@vmware.com To: Ingo Molnar Cc: Andi Kleen , "H. Peter Anvin" , LKML , the arch/x86 maintainers , Daniel Hecht In-Reply-To: <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> <20081022192622.GA30930@elte.hu> Content-Type: text/plain Organization: VMware INC. Date: Wed, 22 Oct 2008 12:30:40 -0700 Message-Id: <1224703840.13953.15.camel@alok-dev1> Mime-Version: 1.0 X-Mailer: Evolution 2.8.0 (2.8.0-40.el5_1.1) Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1150 Lines: 34 On Wed, 2008-10-22 at 12:26 -0700, Ingo Molnar wrote: > * 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. Then does adding a new flag to skip this check be acceptable ? Something like the patch that i had sent yesterday ? Thanks, Alok > > 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/