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

[Keir Fraser <keir@xxxxxxx>]

On 08/11/2012 13:53, "Jan Beulich" <JBeulich@xxxxxxxx> wrote:

>>> 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;
> 
> I concur

Well, no, you don't really. You're about to point out the flaw in my
reasoning...

> : Given that what old_stamp is here was new_stamp for
> the last update, we should simply have
> 
> stamp64 = s0 + (s1 - s0) + (s2 - s1) + ...
> 
> (of course with the mask applied on each addend), which (for the
> low bits) is the same as just new_stamp.

Very good point. Silly me. Then the observed value of plt_stamp64 makes
perfect sense.

 -- Keir



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