From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from [140.186.70.92] (port=37221 helo=eggs.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1Pcyqf-0000qp-6J for qemu-devel@nongnu.org; Wed, 12 Jan 2011 06:26:43 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1PcyqS-0002ZD-TJ for qemu-devel@nongnu.org; Wed, 12 Jan 2011 06:26:30 -0500 Received: from david.siemens.de ([192.35.17.14]:24633) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1PcyqS-0002Yb-Ko for qemu-devel@nongnu.org; Wed, 12 Jan 2011 06:26:28 -0500 Message-ID: <4D2D8FE1.200@siemens.com> Date: Wed, 12 Jan 2011 12:26:25 +0100 From: Jan Kiszka MIME-Version: 1.0 Subject: Re: [Qemu-devel] [RFC][PATCH 0/3] Let RTC follow backward jumps of host clock immediately References: <4D13B4DB.8030801@redhat.com> In-Reply-To: <4D13B4DB.8030801@redhat.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Zachary Amsden Cc: Anthony Liguori , "qemu-devel@nongnu.org" Am 23.12.2010 21:45, Zachary Amsden wrote: > On 12/17/2010 04:58 AM, Jan Kiszka wrote: >> By default, we base the mc146818 RTC on the host clock (CLOCK_REALTIME). >> This works fine if only the frequency of the host clock is tuned (e.g. >> by NTP) or if it is set to a future time. However, if the host is tuned >> backward, e.g. because NTP obtained the correct time after the guest was >> already started or the admin decided to tune the local time, we see an >> unpleasant effect in the guest: The RTC will stall for the period the >> host clock is set back. >> >> This series tries to address the issue more gracefully. By detecting >> those warps and providing a callback mechanism to device models, the >> RTC is enabled to update its timers and register content immediately. >> Tested successfully with a hwclock readout loop in a Linux guest while >> fiddling with the host time. >> >> Note that if this kind of RTC adjustment is not wanted, the user is >> still free to decouple the RTC from the host clock and base it on the >> VM clock - just like before. >> > > Did you test this with a Windows guest? They rely heavily on RTC, this > is probably a better behavior for that case. I'd be curious if Windows > accepts the RTC register changing underneath it, but based on earlier > versions of Windows Time Service, would be surprised if it did not. I haven't tried with Windows yet. When does it read the RTC and how can I check the outcome? Jan -- Siemens AG, Corporate Technology, CT T DE IT 1 Corporate Competence Center Embedded Linux