Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754496AbZJGVz1 (ORCPT ); Wed, 7 Oct 2009 17:55:27 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753466AbZJGVz0 (ORCPT ); Wed, 7 Oct 2009 17:55:26 -0400 Received: from mx1.redhat.com ([209.132.183.28]:29912 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751933AbZJGVz0 (ORCPT ); Wed, 7 Oct 2009 17:55:26 -0400 Message-ID: <4ACD0DEE.80404@redhat.com> Date: Wed, 07 Oct 2009 23:53:50 +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> <4ACCF565.30804@redhat.com> <4ACD05D8.5090903@goop.org> <4ACD0A2B.1080307@redhat.com> <4ACD0D78.2090006@goop.org> In-Reply-To: <4ACD0D78.2090006@goop.org> 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: 1284 Lines: 35 On 10/07/2009 11:51 PM, Jeremy Fitzhardinge wrote: > On 10/07/09 14:37, Avi Kivity wrote: > >> If the guest does a RMW on the version, but the host does not (copying >> it from somewhere else), then the guest RMW can be lost. >> >> Looking at the code, that's what kvm does: >> >> vcpu->hv_clock.version += 2; >> >> shared_kaddr = kmap_atomic(vcpu->time_page, KM_USER0); >> >> memcpy(shared_kaddr + vcpu->time_offset,&vcpu->hv_clock, >> sizeof(vcpu->hv_clock)); >> >> so a guest-side ++version can be lost. >> > I see, yes. The Xen code explicitly reads back the guest version and > increments that (I realize now that's what you meant by guest-private > version). If you were to have a second version number it would have to > be separated as well to avoid being overwritten by the hypervisor. > Yes. We have the space since a cacheline is 64 bytes (minimum) vs 32 bytes of pvclock data. -- I have a truly marvellous patch that fixes the bug which this signature is too narrow to contain. -- 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/