From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: [PATCH 0/6] KVM support for TSC scaling Date: Tue, 22 Feb 2011 12:41:53 +0200 Message-ID: <4D6392F1.1030601@redhat.com> References: <4D57F677.3090004@redhat.com> <20110221172807.GD16508@amd.com> <4D638BDE.70602@redhat.com> <20110222103513.GF16508@amd.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Marcelo Tosatti , "kvm@vger.kernel.org" , "linux-kernel@vger.kernel.org" , Zachary Amsden To: "Roedel, Joerg" Return-path: In-Reply-To: <20110222103513.GF16508@amd.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On 02/22/2011 12:35 PM, Roedel, Joerg wrote: > > This doesn't really work, since we don't know on what host the TSC > > calibration loop ran: > > > > - start guest on host H1 > > - migrate it around, now it's on host H2 > > - guest reboots, reruns calibration loop > > - migrate it around some more, now it's on host H3 > > - migrate to host with tsc multiplier Hnew > > > > So, what should we set the multiplier to? H1, H2, or H3's tsc rate? > > This scenario doesn't matter. If the guest already detected its TSC to > be unstable there is nothing we can do and it doesn't really matter what > we set the tsc frequency to. Therefore software will always set the > guest tsc frequency to the same value it had on the last host. Ok, so your scenario is - boot on host H1 - no intervening migrations - migrate to host Hnew - all succeeding migrations are only to new hosts or back to H1 This is somewhat artificial, and not very different from an all-new cluster. [the whole thing is kind of sad; we went through a huge effort to make clocks work on virtual machines in spite of the tsc issues; then we have a hardware solution, but can't use it because of old hardware. Same thing happens with the effort put into shadow in the pre-npt days] -- error compiling committee.c: too many arguments to function