TSC calibration in virtual machines

[John Baldwin]

On 6/27/18 12:14 AM, Andriy Gapon wrote:
> 
> It seems that TSC calibration in virtual machines sometimes can do more harm
> than good.  Should we default to trusting the information provided by a hypervisor?
> 
> Specifically, I am observing a problem on GCE instances where calibrated TSC
> frequency is about 10% lower than advertised frequency.  And apparently the
> advertised frequency is the right one.
> 
> I found this thread with similar reports and a variety of workarounds from
> administratively disabling the calibration to switching to a different timecounter:
> https://lists.freebsd.org/pipermail/freebsd-cloud/2017-January/000080.html

I suspect you are probably right that we should just "trust" TSC frequencies
provided by a hypervisor.  We could perhaps choose to whitelist hypervisors
known to provide accurate values if we wanted to be cautious.

-- 
John Baldwin