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 16:53:29 +0000 Message-ID: References: <20121108164559.GK76638@ocelot.phlegethon.org> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20121108164559.GK76638@ocelot.phlegethon.org> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: Tim Deegan , Jan Beulich Cc: Ian Campbell , Philippe.Simonet@swisscom.com, 599161@bugs.debian.org, mrsanna1@gmail.com, xen-devel@lists.xen.org List-Id: xen-devel@lists.xenproject.org On 08/11/2012 16:45, "Tim Deegan" wrote: >>> I wonder whether the overflow handling should just be removed, or made >>> conditional on a command-line parameter, or on the 32-bit platform counter >>> being at least somewhat likely to overflow before a softirq occurs -- it >>> seems lots of systems are using 14MHz HPET, and that gives us a couple of >>> minutes for the plt_overflow softirq to do its work before overflow occurs. >>> I think we would notice that outage in other ways. :) >> >> Iirc we added this for a good reason - to cover the, however >> unlikely, event of Xen running for very long without preemption. >> Presumably most of the cases got fixed meanwhile, and indeed >> a wraparound time on the order of minutes should make this >> superfluous, but as the case here shows that code did spot a >> severe anomaly (whatever that may turn out to be). > > ISTR when this code went in we were dealing with a timer that had a > period of about 4 seconds (ACPI PMTIMER?). It might well be OTT for the > HPET, but if there's something weird going on I'd like to track it down > while we have some sort of a handle on it. It must have been the PMTIMER. It's the only counter narrower than 32 bits (legacy PIT we simulate as a 32-bit counter behind the scenes). -- Keir