From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jan Kiszka Subject: Re: [Qemu-devel] qemu-kvm upstreaming: Do we need -no-kvm-pit and -no-kvm-pit-reinjection semantics? Date: Fri, 20 Jan 2012 12:13:48 +0100 Message-ID: <4F194C6C.1070603@siemens.com> References: <4F17D56F.9090309@siemens.com> <20120119172532.GC11381@amt.cnet> <4F1854F9.6030400@siemens.com> <20120119175300.GE11381@amt.cnet> <4F185A88.5030904@siemens.com> <20120120101441.GA31499@amt.cnet> <4F194063.60307@siemens.com> <20120120102549.GB28798@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Cc: Marcelo Tosatti , qemu-devel , kvm To: "Daniel P. Berrange" Return-path: Received: from thoth.sbs.de ([192.35.17.2]:21423 "EHLO thoth.sbs.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751820Ab2ATLNz (ORCPT ); Fri, 20 Jan 2012 06:13:55 -0500 In-Reply-To: <20120120102549.GB28798@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: On 2012-01-20 11:25, Daniel P. Berrange wrote: > On Fri, Jan 20, 2012 at 11:22:27AM +0100, Jan Kiszka wrote: >> On 2012-01-20 11:14, Marcelo Tosatti wrote: >>> On Thu, Jan 19, 2012 at 07:01:44PM +0100, Jan Kiszka wrote: >>>> On 2012-01-19 18:53, Marcelo Tosatti wrote: >>>>>> What problems does it cause, and in which scenarios? Can't they be >>>>>> fixed? >>>>> >>>>> If the guest compensates for lost ticks, and KVM reinjects them, guest >>>>> time advances faster then it should, to the extent where NTP fails to >>>>> correct it. This is the case with RHEL4. >>>>> >>>>> But for example v2.4 kernel (or Windows with non-acpi HAL) do not >>>>> compensate. In that case you want KVM to reinject. >>>>> >>>>> I don't know of any other way to fix this. >>>> >>>> OK, i see. The old unsolved problem of guessing what is being executed. >>>> >>>> Then the next question is how and where to control this. Conceptually, >>>> there should rather be a global switch say "compensate for lost ticks of >>>> periodic timers: yes/no" - instead of a per-timer knob. Didn't we >>>> discussed something like this before? >>> >>> I don't see the advantage of a global control versus per device >>> control (in fact it lowers flexibility). >> >> Usability. Users should not have to care about individual tick-based >> clocks. They care about "my OS requires lost ticks compensation, yes or no". > > FYI, at the libvirt level we model policy against individual timers, for > example: > > > > > Are the various modes of tickpolicy fully specified somewhere? Jan -- Siemens AG, Corporate Technology, CT T DE IT 1 Corporate Competence Center Embedded Linux