Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753999AbZJGVjZ (ORCPT ); Wed, 7 Oct 2009 17:39:25 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753192AbZJGVjY (ORCPT ); Wed, 7 Oct 2009 17:39:24 -0400 Received: from mx1.redhat.com ([209.132.183.28]:45469 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752983AbZJGVjX (ORCPT ); Wed, 7 Oct 2009 17:39:23 -0400 Message-ID: <4ACD0A2B.1080307@redhat.com> Date: Wed, 07 Oct 2009 23:37:47 +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> In-Reply-To: <4ACD05D8.5090903@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: 2580 Lines: 77 On 10/07/2009 11:19 PM, Jeremy Fitzhardinge wrote: > >> When do you copy? >> >> I'd rather have a single copy for guest and host. >> > When Xen updates the parameters normally. The interface never really > needed to share the memory between hypervisor and guest, and I think > avoiding it is a bit more robust. > > But for KVM, you already use the MSR to place the pvclock_vcpu_time_info > structure, so you could just place it in the page and use the same > memory for kernel and usermode. > Yes. >> If the hypervisor does a pvclock->version = somethingelse->version++ >> then the guest may get confused. But I understand you have a >> guest-private ->version? >> > The guest should never get confused by the version being changed by the > hypervisor. It's already part of the ABI. Or did you mean something else? > 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'm not sure what you mean by "guest-private version"; the versions are > always guest-private: te version is part of the pvclock structure, > which is per-vcpu, which is private to each guest. The guest nevern > maintains a separate long-term copy of the structure, only a transient > snapshot while computing time from the tsc (that's the current pvclock.c > code). > Same for kvm. I'm not worried about cross-guest corruption, just the guest and host working together to confuse the guest. >> 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 >> > I don't think that's necessary, but I can certainly live with it if it > makes you happier. > I think the version issue requires it. -- 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/