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

[Keir Fraser <keir@xxxxxxx>]

On 08/11/2012 09:39, "Jan Beulich" <JBeulich@xxxxxxxx> 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



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