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xen-devel
RE: [Xen-devel] Problem correlating TSC read from domU with Xentrace's T
> 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
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