Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759989AbZJGUK5 (ORCPT ); Wed, 7 Oct 2009 16:10:57 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1759930AbZJGUK4 (ORCPT ); Wed, 7 Oct 2009 16:10:56 -0400 Received: from mx1.redhat.com ([209.132.183.28]:26795 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759921AbZJGUK4 (ORCPT ); Wed, 7 Oct 2009 16:10:56 -0400 Message-ID: <4ACCF565.30804@redhat.com> Date: Wed, 07 Oct 2009 22:09:09 +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> <4ACCEC18.90401@goop.org> In-Reply-To: <4ACCEC18.90401@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: 2211 Lines: 65 On 10/07/2009 09:29 PM, Jeremy Fitzhardinge wrote: > >> I'm a bit worried about the kernel playing with the hypervisor's >> version field. >> > For Xen I explicitly made it not a problem by adding the notion of a > secondary pvclock_vcpu_time_info structure which is updated by copying, > aside from the version number which is updated as-is. > When do you copy? I'd rather have a single copy for guest and host. > As far as I can tell it isn't a problem for KVM either. The guest > version number is atomic with respect to preemption by the hypervisor so > there's no scope for racing. (The ABI already guarantees that the > pvclock structures are never updated cross-cpu.) > > It ultimately doesn't matter what the version number is so long as it > changes when the parameters are updated, and version numbers can't be > reused within a window where things get confused. > If the hypervisor does a pvclock->version = somethingelse->version++ then the guest may get confused. But I understand you have a guest-private ->version? >> It's better to introduce yet a new version for the kernel, and check >> both. >> > Two version numbers are awkward to read atomically at least on 32-bit. > And I don't think its necessary. > No need to read them atomically. cpu1 = vgetcpu() hver1 = pvclock[cpu1].hver kver1 = pvclock[cpu1].kver tsc = rdtsc /* multipication magic with pvclock[cpu1]*/ cpu2 = vgetcpu() hver2 = pvclock[cpu2].hver kver2 = pvclock[cpu2].kver valid = cpu1 == cpu2 && hver1 == hver2 && kver1 == kver2 >> It's sufficient to increment a version counter on thread migration, no >> need to do it on context switch. >> >> > That's true; switch_out is a pessimistic approximation of that. But is > there a convenient hook to test for migration? > I'd guess not but it's probably easy to add one in the migration thread. -- 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/