RE: xen tsc problems?

[Dan Magenheimer <dan.magenheimer@oracle.com>]

> From: Keir Fraser [mailto:keir.fraser@eu.citrix.com]
> 
> On 13/07/2010 15:37, "Stefano Stabellini"
> <Stefano.Stabellini@eu.citrix.com>
> wrote:
> 
> > Does this mean that the host has some serious tsc issues?
> > Can this be a symptom of a bug in xen?
> > Suggestion are welcome.
> 
> The 's' and 't' debug key handlers will be useful to get an idea of how
> stable host TSCs are.
> 
>  -- Keir

Also you can try max_cstate=0 as a Xen boot parameter to rule
out power management screwing up the tsc.

> > Does this mean that the host has some serious tsc issues?

Probably.  But the default tsc_mode (0) is intended to hide all
such issues.  Could you check the 's' debug-key output to
ensure your guest is actually running with tsc_mode=0?

> > Can this be a symptom of a bug in xen?

Well, if the guest has problems with the default tsc_mode (0),
which does complete tsc emulation, I suppose it could be
a bug in Xen.  In particular, I wonder if the code that
recovers from deep C-states (and writes to the TSC) is broken.
IIRC, there was some changesets in that area recently.
If the problem goes away with max_cstate=0, that would be
a good place to start.

Dan