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

[Keir Fraser <keir@xxxxxxx>]

On 07/11/2012 17:40, "Keir Fraser" <keir@xxxxxxx> wrote:

> On 07/11/2012 13:22, "Ian Campbell" <ijc@xxxxxxxxxxxxxx> 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!

Jan has pointed out that the value of plt_stamp64 makes perfect sense, and
will in fact always have low 32 bits identical to new_stamp. At least that
is explained.

So, the question is then why plt_now (== __read_platform_stime(15b800366a5))
is greater than plt_wrap (== __read_platform_stime(15c800366a5)). Perhaps
the scale_delta() logic is failing for some reason, but we do use it a lot
elsewhere!

 -- Keir

> 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. :)
> 
>  -- Keir
> 
> 



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