[Bug 208238] [Hyper-V] TSC frequency is not correctly detected: "calcru: runtime went backwards"

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=208238

--- Comment #2 from Dexuan Cui <decui at microsoft.com> ---
The tsc-calibration fix was temporarily taken back last Friday because we found
an interesting thing: with some kind of config, Xen can pretend to be Hyper-V
by mimicking the hypervisor CPUID signature (this is usually to make Windows VM
run better on Xen), but Xen doesn’t support Hyper-V timecounter, so the fix
causes an unexpected panic when FreeBSD VM runs on Xen in this case.

We're making another patch to better detect if the underlying hypervisor
supports Hyper-V timecounter or not:
https://lists.freebsd.org/pipermail/freebsd-current/2016-April/060519.html

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