From mboxrd@z Thu Jan 1 00:00:00 1970 From: Marcelo Tosatti Subject: Re: [RFC PATCH v2 2/2] add support for Hyper-V invariant TSC Date: Thu, 23 May 2013 10:53:38 -0300 Message-ID: <20130523135338.GA15652@amt.cnet> 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> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Vadim Rozenfeld , kvm@vger.kernel.org, pl@dlh.net To: Gleb Natapov Return-path: Received: from mx1.redhat.com ([209.132.183.28]:23966 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758523Ab3EWNyA (ORCPT ); Thu, 23 May 2013 09:54:00 -0400 Content-Disposition: inline In-Reply-To: <20130523091229.GI4725@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: 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? Case 2) Reference timer (via MSR) support is interesting for the case of non invariant TSC host.