From mboxrd@z Thu Jan 1 00:00:00 1970 From: Marcelo Tosatti Subject: Re: KVM timekeeping and TSC virtualization Date: Wed, 25 Aug 2010 13:55:54 -0300 Message-ID: <20100825165554.GA27263@amt.cnet> References: <1282291669-25709-1-git-send-email-zamsden@redhat.com> <4C6E8284.6020200@cisco.com> <4C6F0EB8.7050906@redhat.com> <4C707E23.80209@cisco.com> <4C73240F.5010902@redhat.com> <4C7336C9.8000006@cisco.com> <4C735D09.6080506@redhat.com> <4C73C9D0.7040007@cisco.com> <4C744F52.8070207@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: "David S. Ahern" , kvm@vger.kernel.org To: Zachary Amsden Return-path: Received: from mx1.redhat.com ([209.132.183.28]:23872 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751329Ab0HYR1z (ORCPT ); Wed, 25 Aug 2010 13:27:55 -0400 Content-Disposition: inline In-Reply-To: <4C744F52.8070207@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: On Tue, Aug 24, 2010 at 01:01:38PM -1000, Zachary Amsden wrote: > >>With this patchset, KVM now has a much stronger guarantee: If you have > >>old guest software running on broken hardware, using SMP virtual > >>machines, you do not get hardware performance and error-free time > >>virtualization. However, if you have new guest software, non-broken > >>hardware, or can simply run UP guests instead of SMP, you can have > >>hardware performance, and it is now error free. Alternatively, you can > >>sacrifice some accuracy and have hardware performance, even for SMP > >>guests, if you can tolerate some minor cross-CPU TSC variation. No > >>other vendor I know of can make that guarantee. > >> > >>Zach > >If the processor has a stable TSC why trap it? I realize you are trying > >to cover a gauntlet of hardware and guests, so maybe a nerd knob is > >needed to disable. > > Exactly. If you have a stable TSC, we don't trap it. If you don't > have a stable TSC, we do. That's the point of these patches. Wait, don't you trap if host TSC is faster than guest TSC?