RE: [Xen-devel] Xen 4 TSC problems

[Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>]

> From: George Dunlap [mailto:George.Dunlap@xxxxxxxxxxxxx]
> Sent: Thursday, September 15, 2011 4:36 AM
> To: Philippe Simonet
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-devel] Xen 4 TSC problems
> 
> On Tue, Sep 13, 2011 at 8:16 AM, Philippe Simonet
> <philippe.simonet@xxxxxxxxxx> wrote:
> > Hi Xen developers
> >
> > i just would like to inform you that I have exactly the same problem with
> > Debian squeeze and xen, with
> > 50 seconds time jump on my dom0 and domu. NTP is running on all dom0/domuU,
> > clocksource is 'xen'
> > everywhere.
> >
> > some messages :
> > syslog :
> > Sep 11 13:56:50 dnsit22 kernel: [571603.359863] Clocksource tsc unstable
> > (delta = -2999662111513 ns)
> >
> > xm dmesg :
> > ...
> > (XEN) Platform timer is 14.318MHz HPET
> > ...
> > (XEN) Platform timer appears to have unexpectedly wrapped 10 or more times.
> > (XEN) TSC marked as reliable, warp = 0 (count=2)
> > ...
> 
> I haven't been following this conversation, so I don't know if this is
> relevant, but I've just discovered this morning that the TSC warp
> check in Xen is done at the wrong time (before any secondary cpus are
> brought up), and thus always returns warp=0.  I've submitted a patch
> to do the check after secondary CPUs are brought up; that should cause
> Xen to do periodic synchronization of TSCs when there is drift.

Wow, nice catch, George!  I wonder if this is the underlying bug
for many of the mysterious time problems that have been reported
for a year or two now... at least on certain AMD boxes.
Any idea when this was introduced?  Or has it always been wrong?

Dan

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