From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jan Kiszka Subject: Re: qemu-kvm upstreaming: Do we need -no-kvm-pit and -no-kvm-pit-reinjection semantics? Date: Fri, 20 Jan 2012 11:22:27 +0100 Message-ID: <4F194063.60307@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> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: kvm , qemu-devel To: Marcelo Tosatti Return-path: Received: from goliath.siemens.de ([192.35.17.28]:29702 "EHLO goliath.siemens.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750736Ab2ATKWh (ORCPT ); Fri, 20 Jan 2012 05:22:37 -0500 In-Reply-To: <20120120101441.GA31499@amt.cnet> Sender: kvm-owner@vger.kernel.org List-ID: 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". Jan -- Siemens AG, Corporate Technology, CT T DE IT 1 Corporate Competence Center Embedded Linux