From mboxrd@z Thu Jan 1 00:00:00 1970 From: Aritoki TAKADA Subject: Re: Question: Timekeeping between Host and Guest with NTP Date: Fri, 24 Aug 2012 20:43:49 +0900 Message-ID: <503768F5.1000109@hitachi.com> References: <50349C7C.2070409@hitachi.com> <20120823181312.GA15127@amt.cnet> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: kvm@vger.kernel.org, ltc-kernel@ml.yrl.intra.hitachi.co.jp To: Marcelo Tosatti Return-path: Received: from mailxx.hitachi.co.jp ([133.145.228.50]:59934 "EHLO mailxx.hitachi.co.jp" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750908Ab2HXLqE (ORCPT ); Fri, 24 Aug 2012 07:46:04 -0400 In-Reply-To: <20120823181312.GA15127@amt.cnet> Sender: kvm-owner@vger.kernel.org List-ID: Thank you for your comment, (2012/08/24 3:13), Marcelo Tosatti wrote: > NTP should be running in the guest so as to synchronize the guest > time-of-day clocks to UTC. > > kvmclock exposes the monotonic clock from the host. The frequency > of the host monotonic clock is corrected by host ntpd. This is > probably where the confusion comes from. > > That is no guarantee that time-of-day clocks in the guest are > synchronized to UTC or even that clock frequency visible to userspace > applications in the guest is equal to the monotonic clock frequency > of the host. I understood your comment except one point. We know that the frequency of host monotonic clock is kept accurate by host ntpd and that kvmclock shows it to the guest. Doesn't this mean that applications in the guest can see the accurate clock frequency provided by the host? Sincerely, -- Aritoki TAKADA aritoki.takada.jt@hitachi.com Hitachi, Ltd., Yokohama Research Laboratory