From mboxrd@z Thu Jan 1 00:00:00 1970 From: Gleb Natapov Subject: Re: [RFC PATCH v2 2/2] add support for Hyper-V invariant TSC Date: Thu, 23 May 2013 18:31:10 +0300 Message-ID: <20130523153110.GI26157@redhat.com> References: <1368947197-9033-1-git-send-email-vrozenfe@redhat.com> <1368947197-9033-3-git-send-email-vrozenfe@redhat.com> <20130522005046.GA7589@amt.cnet> <643480794.5761686.1369207375451.JavaMail.root@redhat.com> <20130522212330.GA29387@amt.cnet> <20130523091229.GI4725@redhat.com> <20130523135338.GA15652@amt.cnet> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Vadim Rozenfeld , kvm@vger.kernel.org, pl@dlh.net To: Marcelo Tosatti Return-path: Received: from mx1.redhat.com ([209.132.183.28]:17655 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754876Ab3EWPbO (ORCPT ); Thu, 23 May 2013 11:31:14 -0400 Content-Disposition: inline In-Reply-To: <20130523135338.GA15652@amt.cnet> Sender: kvm-owner@vger.kernel.org List-ID: On Thu, May 23, 2013 at 10:53:38AM -0300, Marcelo Tosatti wrote: > On Thu, May 23, 2013 at 12:12:29PM +0300, Gleb Natapov wrote: > > > To address migration scenarios to physical platforms that do not support > > > iTSC, the TscSequence field is used. In the event that a guest partition > > > is migrated from an iTSC capable host to a non-iTSC capable host, the > > > hypervisor sets TscSequence to the special value of 0xFFFFFFFF, which > > > directs the guest operating system to fall back to a different clock > > > source (for example, the virtual PM timer)." > > > > > > Why it would not/does not work after migration? > > > > > Please read the whole discussion, we talked about it already. We > > definitely do not want to fall back to PM timer either, we want to use > > reference counter instead. > > Case 1) On migration of TSC page enabled Windows guest, from invariant TSC host, > to non-invariant TSC host, Windows guests fallback to PMTimer > and not to reference timer via MSR. > > This is suboptimal because pmtimer emulation is excessively slow. > > Is there a better option? > If setting TscSequence to zero makes Windows fall back to the MSR this is a better option. > Case 2) > Reference timer (via MSR) support is interesting for the case of non invariant TSC > host. -- Gleb.