Bug#599161: [Xen-devel] #599161: Xen debug patch for the "clock shifts by 50 minutes" bug.
On 08/11/2012 09:39, "Jan Beulich" <JBeulich@suse.com> 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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