RE: Problem correlating TSC read from domU with Xentrace's TSC

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

> I am trying to correlating performance issue in guest VM with the
> scheduling trace from Xentrace. User-mode application in guest VM
> periodically dump APIC ID and RDTSC into trace. I also start Xentrace
> in Dom0 during the same period.
> 
> However, I notice that range of TSC values report both trace is
> completely disjointed. TSC values from Xentrace is always greater than
> what  guest VM see, even if I start capturing Xen's trace 30s before
> capturing trace in guest VM.
> 
> I understand that Xen don't emulate rdtsc() for PV guest and Xentrace
> also read rdtsc() during __insert_record(). However, I don't
> understand what cause my issue.  Any suggestion?   I am using Xen
> 4.1.1 on Fedora 15

Hi Thawan --

Depending on the underlying hardware, Xen may emulate rdtsc
for PV guests.  To force this off, set tsc_mode=2 in your
guest vm.cfg file (and reboot).  For more info on tsc_mode
see tscmode.txt in the Xen source tree (or google for it).

Dan