Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756686Ab0GMXcv (ORCPT ); Tue, 13 Jul 2010 19:32:51 -0400 Received: from mx1.redhat.com ([209.132.183.28]:57494 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756185Ab0GMXcu (ORCPT ); Tue, 13 Jul 2010 19:32:50 -0400 Message-ID: <4C3CF79E.6040601@redhat.com> Date: Tue, 13 Jul 2010 13:32:46 -1000 From: Zachary Amsden User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.10) Gecko/20100621 Fedora/3.0.5-1.fc13 Thunderbird/3.0.5 MIME-Version: 1.0 To: "David S. Ahern" CC: Marcelo Tosatti , KVM , Avi Kivity , Glauber Costa , Linux-kernel Subject: Re: [PATCH 09/18] Robust TSC compensation References: <1278987938-23873-1-git-send-email-zamsden@redhat.com> <1278987938-23873-10-git-send-email-zamsden@redhat.com> <20100713203418.GA903@amt.cnet> <4C3CD77B.9050108@redhat.com> <4C3CDDD4.6010009@cisco.com> In-Reply-To: <4C3CDDD4.6010009@cisco.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1195 Lines: 29 On 07/13/2010 11:42 AM, David S. Ahern wrote: > > On 07/13/10 15:15, Zachary Amsden wrote: > > >>> What prevents a vcpu from seeing its TSC go backwards, in case the first >>> write in the 5 second window is smaller than the victim vcpu's last >>> visible TSC value ? >>> >>> >> Nothing, unfortunately. However, the TSC would already have to be out >> of sync in order for the problem to occur. It can never happen in >> normal circumstances on a stable hardware TSC except in one case; >> migration. During the CPU state transfer phase of migration, however, >> > What about across processor sockets? Aren't CPUs brought up at different > points such that their TSCs start at different times? > It depends on the platform. But it doesn't matter. The definition we use is different start TSCs == out of sync. Some systems have synchronized TSCs, some do not. See patch 18/18 - "Timekeeping documentation" for details. -- 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/