Re: What time is it kvm-clock?

[Marcelo Tosatti <mtosatti@redhat.com>]

On Tue, Feb 23, 2016 at 06:31:59PM -0800, Owen Hofmann wrote:
> Specifically, what underlying source of time should be exposed through
> kvm-clock and other paravirtual ABIs like the HyperV reference tsc
> page?  Recently a couple of threads on kvm-list, along with attempts
> to produce reliable behavior from kvm-clock on our systems have

What is there in place are testcases to measure particular kvmclock
issues, such as time backwards events and maximum offset/frequency 
against NTP. 

There is no "true" clock, you can only measure one clock against 
another (i think Radim raised that point as well). UTC is the
global standard, an average of atomic clocks.
http://tf.nist.gov/general/pdf/1498.pdf

What you'd like to do is measure kvmclock stability regarding
some parameter. So to improve that situation one could find what
parameters are important (such as whether clock-A should not stop 
counting for more than some time units of clock-B, thats the "opposite"
side effect of the bug uncovered by Hyper-V fixes, the other being
time backwards events).

(clock-A being kvmclock, clock-B being a GPS clock for example).

One useful activity would be to compare (only the minimum measurements
out of many measurements)
kvmclock in a guest with a GPS clock. I bought a Garmin GPS clock but never got 
around to enabling the RS-232 connection required to bypass the USB
latency.
It costs less than 100US$.

http://www.lammertbies.nl/comm/info/GPS-time.html