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

[Ian Campbell <ijc@xxxxxxxxxxxxxx>]

On Wed, 2012-11-07 at 17:40 +0000, Keir Fraser 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.

impossible due to guarantees made by the h/w or by construction in Xen.
There appears to be a certain amount of hardware-specificness to the
issue -- so I'm wondering if maybe there are some platforms whose tsc is
not as monotonically increasing as it needs to be...

>  plt_stamp64 oddly has low 32 bits identical to new_stamp. That
> seems very very improbable!

Does this code run on all cpus or just one? Is it always the same one?

> 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
> 
> 
> 
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@xxxxxxxxxxxxx
> http://lists.xen.org/xen-devel



_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel