Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754211AbXJ3E0g (ORCPT ); Tue, 30 Oct 2007 00:26:36 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752630AbXJ3E03 (ORCPT ); Tue, 30 Oct 2007 00:26:29 -0400 Received: from il.qumranet.com ([82.166.9.18]:35125 "EHLO il.qumranet.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750993AbXJ3E02 (ORCPT ); Tue, 30 Oct 2007 00:26:28 -0400 Message-ID: <4726B218.8010101@qumranet.com> Date: Tue, 30 Oct 2007 06:24:56 +0200 From: Avi Kivity User-Agent: Thunderbird 2.0.0.5 (X11/20070727) MIME-Version: 1.0 To: Dan Hecht CC: Ingo Molnar , Zachary Amsden , jeremy@goop.org, avi@quramnet.com, kvm-devel@lists.sourceforge.net, linux-kernel@vger.kernel.org, Glauber de Oliveira Costa , Glauber de Oliveira Costa , --cc@redhat.com, Garrett Smith Subject: Re: [kvm-devel] [PATCH] raise tsc clocksource rating References: <11936994092607-git-send-email-gcosta@redhat.com> <1193697734.9793.86.camel@bodhitayantram.eng.vmware.com> <20071029224852.GA27547@elte.hu> <1193698505.9793.90.camel@bodhitayantram.eng.vmware.com> <20071029230213.GA1982@elte.hu> <47266B90.8000008@vmware.com> In-Reply-To: <47266B90.8000008@vmware.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1277 Lines: 29 Dan Hecht wrote: > Not really. In the case hardware TSC is perfect, the paravirt time > counter can be implemented directly in terms of hardware TSC; there is > no loss in optimization. This is done transparently. And virtual TSC > can be implemented this way too. > > The real improvement that a paravirt clocksource offers over the TSC > clocksource is that the guest does not need to measure the TSC frequency > itself against some other constant frequency source (which is > problematic on a virtual machine). Instead, the paravirt clocksource > queries the hypervisor for the frequency of the counter. As you know, > with clocksource style kernels, it's important to get this frequency > correct, or else the guest will have long-term time drift. > > In addition, a paravirt clocksource can compensate for events like vcpu migration to another host cpu. So I agree: a paravirt clocksource is always better than or equal to the tsc. -- Any sufficiently difficult bug is indistinguishable from a feature. - 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/