Re: #599161: Xen debug patch for the "clock shifts by 50 minutes" bug.

[Keir Fraser <keir@xen.org>]

On 08/11/2012 16:45, "Tim Deegan" <tim@xen.org> 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