host, bhyve vm and ntpd

[Ian Lepore]

On Sat, 2017-10-21 at 17:07 -0400, Michael Voorhis wrote:
> Ian Lepore writes:
> > 
> > Beyond that, I'm not sure what else to try.  It might be necessary to
> > get some bhyve developers involved (I know almost nothing about it).
> NTPD behaves more normally on uniprocessor VMs.
> 
> A FreeBSD bhyve-guest running on a freebsd host will select a
> different timecounter depending on whether it is a multiprocessor or a
> uniprocessor.  My uniprocessor bhyve-vm selected TSC-low as the best
> timecounter in a uniprocessor.  NTP functions there as expected.
> 
> kern.timecounter.choice: TSC-low(1000) ACPI-fast(900) HPET(950) i8254(0) dummy(-1000000)
> kern.timecounter.hardware: TSC-low
> 
> The very same VM, when given two total CPUs, selected HPET (if I
> recall) and the timekeeping with NTPD was unreliable, with many
> step-resets to the clock.
> 

Hmm, I just had glance at the code in sys/amd64/vmm/io/vhpet.c and it
looks right.  I wonder if this is just a simple roundoff error in
converting between 10.0MHz and SBT units?  If so, that could be wished
away easily by using a power-of-2 frequency for the virtual HPET.  I
wonder if the attached patch is all that's needed?

-- Ian
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