From mboxrd@z Thu Jan 1 00:00:00 1970 From: Keir Fraser Subject: Re: #599161: Xen debug patch for the "clock shifts by 50 minutes" bug. Date: Thu, 08 Nov 2012 10:38:20 +0000 Message-ID: References: <509B8BF802000078000A72A3@nat28.tlf.novell.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <509B8BF802000078000A72A3@nat28.tlf.novell.com> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: Jan Beulich , Philippe.Simonet@swisscom.com Cc: 599161@bugs.debian.org, xen-devel@lists.xen.org, mrsanna1@gmail.com, Ian Campbell List-Id: xen-devel@lists.xenproject.org On 08/11/2012 09:39, "Jan Beulich" wrote: >>>>> (XEN) XXX plt_overflow: plt_now=5ece12d34128 plt_wrap=5ece12d09306 >>>>> now=5ece12d16292 old_stamp=35c7c new_stamp=800366a5 >>>>> plt_stamp64=15b800366a5 plt_mask=ffffffff tsc=e3839fd23854 >>>>> tsc_stamp=e3839fcb0273 >>>> >>>> (below is the complete xm dmesg output) >>>> >>>> did that help you ? do you need more info ? >>> >>> I'll leave this to Keir (who wrote the debugging patch) to answer but it >>> looks to me like it should be useful! >> >> I'm scratching my head. plt_wrap is earlier than plt_now, which should be >> impossible. plt_stamp64 oddly has low 32 bits identical to new_stamp. That >> seems very very improbable! > > Is it? My understanding was that plt_stamp64 is just a software > extension to the more narrow HW counter, and hence the low > plt_mask bits would always be expected to be identical. No, plt_stamp is simply the HW counter time at which plt_stamp64 was last brought up to date. Hence plt_stamp64 is updated as: plt_stamp64 += (new_stamp - old_stamp) & plt_mask; Hence why seeing plt_stamp64&plt_mask == new_stamp is very unexpected! -- Keir