Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934682AbZJJAvz (ORCPT ); Fri, 9 Oct 2009 20:51:55 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754199AbZJJAvy (ORCPT ); Fri, 9 Oct 2009 20:51:54 -0400 Received: from claw.goop.org ([74.207.240.146]:54707 "EHLO claw.goop.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753943AbZJJAvx (ORCPT ); Fri, 9 Oct 2009 20:51:53 -0400 Message-ID: <4ACFD43E.6000506@goop.org> Date: Fri, 09 Oct 2009 17:24:30 -0700 From: Jeremy Fitzhardinge User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.4pre) Gecko/20090922 Fedora/3.0-2.7.b4.fc11 Lightning/1.0pre Thunderbird/3.0b4 MIME-Version: 1.0 To: Avi Kivity CC: Jeremy Fitzhardinge , Dan Magenheimer , Xen-devel , kurt.hackel@oracle.com, the arch/x86 maintainers , Linux Kernel Mailing List , Glauber de Oliveira Costa , Keir Fraser , Zach Brown , Chris Mason Subject: Re: [Xen-devel] Re: [PATCH 3/5] x86/pvclock: add vsyscall implementation References: <1254790211-15416-1-git-send-email-jeremy.fitzhardinge@citrix.com> <1254790211-15416-4-git-send-email-jeremy.fitzhardinge@citrix.com> <4ACB0833.2050203@redhat.com> <4ACB9074.1000804@goop.org> <4ACC6C9C.7080707@redhat.com> In-Reply-To: <4ACC6C9C.7080707@redhat.com> X-Enigmail-Version: 0.97a Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1009 Lines: 28 On 10/07/09 03:25, Avi Kivity wrote: > def try_pvclock_vtime(): > tsc, p0 = rdtscp() > v0 = pvclock[p0].version > tsc, p = rdtscp() > t = pvclock_time(pvclock[p], tsc) > if p != p0 or pvclock[p].version != v0: > raise Exception("Processor or timebased change under our feet") > return t This doesn't quite work. If we end up migrating some time after the first rdtscp, then the accesses to pvclock[] will be cross-cpu. Since we don't made any strong SMP memory ordering guarantees on updating the structure, the snapshot isn't guaranteed to be consistent even if we re-check the version at the end. So to use rdtscp we need to either redefine the update of pvclock_vcpu_time_info to be SMP-safe, or keep the additional migration check. J -- 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/