From mboxrd@z Thu Jan 1 00:00:00 1970 From: Vadim Rozenfeld Subject: Re: [RFC PATCH v2 2/2] add support for Hyper-V invariant TSC Date: Fri, 24 May 2013 05:57:00 -0400 (EDT) Message-ID: <1931417502.7037126.1369389420388.JavaMail.root@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> <519DB4CF.8050003@dlhnet.de> <20130523091316.GJ4725@redhat.com> <20130523133559.GA14454@amt.cnet> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Cc: Gleb Natapov , Peter Lieven , kvm@vger.kernel.org, pl@dlh.net To: Marcelo Tosatti Return-path: Received: from mx4-phx2.redhat.com ([209.132.183.25]:34504 "EHLO mx4-phx2.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751998Ab3EXJ5H (ORCPT ); Fri, 24 May 2013 05:57:07 -0400 In-Reply-To: <20130523133559.GA14454@amt.cnet> Sender: kvm-owner@vger.kernel.org List-ID: ----- Original Message ----- From: "Marcelo Tosatti" To: "Gleb Natapov" Cc: "Peter Lieven" , "Vadim Rozenfeld" , kvm@vger.kernel.org, pl@dlh.net Sent: Thursday, May 23, 2013 11:35:59 PM Subject: Re: [RFC PATCH v2 2/2] add support for Hyper-V invariant TSC On Thu, May 23, 2013 at 12:13:16PM +0300, Gleb Natapov wrote: > > > > > >"Reference TSC during Save and Restore and Migration > > > > > >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? > > > > > > > > > > what exactly do we heed the reference TSC for? the reference counter alone works great and it seems > > that there is a lot of trouble and crash possibilities involved with the referece tsc. > > > Reference TSC is even faster. There should be no crashed with proper > implementation. > > -- > Gleb. Lack of invariant TSC support in the host. if there is no iTSC in the host -> set sequence to 0 and go with reference counter. It is why they both scaled to 10 MHz, and it's why reference counters is a fall-back for iTSC.