From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: [Qemu-devel] Re: [RFC: 0/2] patch for QEMU HPET periodic timer emulation to alleviate time drift Date: Mon, 07 Feb 2011 16:58:59 +0200 Message-ID: <4D5008B3.90505@redhat.com> References: <480481933.225059.1296734409954.JavaMail.root@zmail07.collab.prod.int.phx2.redhat.com> <1375835067.226263.1296740625327.JavaMail.root@zmail07.collab.prod.int.phx2.redhat.com> <4D4AC99A.2070803@siemens.com> <4D4B0B07.2040904@codemonkey.ws> <4D4B1CF8.8040800@web.de> <4D4B5F23.7040801@codemonkey.ws> <4D4BBF55.9060000@web.de> <4D4FE6BF.5080502@redhat.com> <4D4FEF81.1040603@codemonkey.ws> <4D4FF02F.2030309@redhat.com> <4D4FF24A.7000004@codemonkey.ws> <4D4FFD3B.2030903@siemens.com> <4D5001A0.8020503@codemonkey.ws> <4D5004FC.80000@siemens.com> <4D5007B9.7060806@codemonkey.ws> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Jan Kiszka , Ulrich Obergfell , Glauber Costa , kvm , qemu-devel To: Anthony Liguori Return-path: Received: from mx1.redhat.com ([209.132.183.28]:18479 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753172Ab1BGO7G (ORCPT ); Mon, 7 Feb 2011 09:59:06 -0500 In-Reply-To: <4D5007B9.7060806@codemonkey.ws> Sender: kvm-owner@vger.kernel.org List-ID: On 02/07/2011 04:54 PM, Anthony Liguori wrote: > >> Why the accumulated_ticks argument? > > Then the missing ticks is stored in the PeriodicTimer instead of > storing it in the device state. That means we won't forget to save it > in vmstate. > > It's convenient because then if we lose ticks in the PeriodicTimer > layer, the devices have instance access to that info. When you do a > read() from timerfd, it returns the number of coalesced events. > That's the interface I had in my mind. > > We could just add a getter for PeriodicTimer and it would serve the > same purpose. If a drift compensation policy is in effect, you don't need the missed ticks, since you will get one callback for each (delayed) tick. If there is no drift compensation policy, presumably you aren't interested in lost ticks. So the ticks argument isn't very useful. On the other hand, we need a way to inject lost ticks into a PeriodicTimer. If interrupt injection detects that an interrupt was coalesced, we want the timer to schedule a new tick for us. -- error compiling committee.c: too many arguments to function