RE: [Xen-devel] Xen 4 TSC problems

[Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>]

> From: Philippe.Simonet@xxxxxxxxxxxx [mailto:Philippe.Simonet@xxxxxxxxxxxx]
> Subject: RE: [Xen-devel] Xen 4 TSC problems
> 
> Hi Xen developpers
> 
> i need some good tips to go forward with my TSC problem :
> 
> Could you give me some tips that I could test or implement ?
>       - try other hypervisor version ?

Hi Phillipe --

It would definitely be worthwhile to see if you can reproduce
the problem on the latest xen-unstable bits.  (Please make sure
that the bug George reported below is fixed in your build.)
A lot has changed since 4.0.1.

Dan

P.S. I will be mostly offline for the next week or so...

> > -----Original Message-----
> > From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx [mailto:xen-devel-
> > bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of George Dunlap
> > Sent: Monday, September 19, 2011 12:40 PM
> > To: Dan Magenheimer
> > Cc: Keir Fraser; jeremy@xxxxxxxx; xen-devel@xxxxxxxxxxxxxxxxxxx; Philippe
> > Simonet; Konrad Wilk
> > Subject: Re: [Xen-devel] Xen 4 TSC problems
> >
> > On Thu, Sep 15, 2011 at 7:38 PM, Dan Magenheimer
> > <dan.magenheimer@xxxxxxxxxx> wrote:
> > >> 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?
> >
> > Well the comment in 20823:89907dab1aef seems to indicate that's where the
> > "assume it's reliable on AMD until proven otherwise" started; that would be
> > January 2010.
> >
> > I looked as far back as 20705:a74aca4b9386, and there the TSC reliability
> > checks were again in init_xen_time().  Figuring out where things were before
> > then is getting into archeology. :-)
> >
> > The comment at the top of init_xen_time() is correct now, but from the time
> > it was first written through 4.1 is was just plain wrong -- it said
> > init_xen_time() happened after all cpus were up, which has never been true.
> >
> >  -George
> >
> > _______________________________________________
> > Xen-devel mailing list
> > Xen-devel@xxxxxxxxxxxxxxxxxxx
> > http://lists.xensource.com/xen-devel

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