From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from [140.186.70.92] (port=42031 helo=eggs.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1PmSrS-0006fc-8p for qemu-devel@nongnu.org; Mon, 07 Feb 2011 10:18:43 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1PmSrQ-0003Ts-TP for qemu-devel@nongnu.org; Mon, 07 Feb 2011 10:18:42 -0500 Received: from mail-bw0-f45.google.com ([209.85.214.45]:56129) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1PmSrQ-0003TM-Nn for qemu-devel@nongnu.org; Mon, 07 Feb 2011 10:18:40 -0500 Received: by bwz16 with SMTP id 16so5126802bwz.4 for ; Mon, 07 Feb 2011 07:18:39 -0800 (PST) Message-ID: <4D500D4B.4070701@codemonkey.ws> Date: Mon, 07 Feb 2011 09:18:35 -0600 From: Anthony Liguori MIME-Version: 1.0 Subject: Re: [Qemu-devel] Re: [RFC: 0/2] patch for QEMU HPET periodic timer emulation to alleviate time drift 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> <4D5008B3.90505@redhat.com> In-Reply-To: <4D5008B3.90505@redhat.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Avi Kivity Cc: Jan Kiszka , Glauber Costa , Ulrich Obergfell , kvm , qemu-devel On 02/07/2011 08:58 AM, Avi Kivity wrote: > 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. As an optimization, if you can do something useful with the knowledge that we missed 20 ticks instead of waiting for 20 callbacks, it's useful to have. But adding a getter makes that possible and I agree that it clutters the interface for an edge use-case. Regards, Anthony Liguori