Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1762083AbZJJSLg (ORCPT ); Sat, 10 Oct 2009 14:11:36 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1762074AbZJJSLg (ORCPT ); Sat, 10 Oct 2009 14:11:36 -0400 Received: from mx1.redhat.com ([209.132.183.28]:10050 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1762060AbZJJSLd (ORCPT ); Sat, 10 Oct 2009 14:11:33 -0400 Message-ID: <4AD0CDFB.9030704@redhat.com> Date: Sat, 10 Oct 2009 20:10:03 +0200 From: Avi Kivity User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.1) Gecko/20090814 Fedora/3.0-2.6.b3.fc11 Thunderbird/3.0b3 MIME-Version: 1.0 To: Jeremy Fitzhardinge 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> <4ACFD43E.6000506@goop.org> In-Reply-To: <4ACFD43E.6000506@goop.org> Content-Type: text/plain; charset=UTF-8; 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: 1571 Lines: 43 On 10/10/2009 02:24 AM, Jeremy Fitzhardinge wrote: > 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. > We only hit this if we have a double migration, otherwise we see p != p0. Most likely all existing implementations do have a write barrier on the guest entry path, so if we add a read barrier between the two compares, that ensures we're reading from the same cpu again. > 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. > I think we can update the ABI after verifying all implementations do have a write barrier. -- Do not meddle in the internals of kernels, for they are subtle and quick to panic. -- 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/