TSC calibration in virtual machines

[Alan Somers]

On Wed, Jun 27, 2018 at 10:36 AM, Jung-uk Kim <jkim at freebsd.org> wrote:

> On 06/27/2018 03:14, 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
>
> We already do that for VMware hosts since r221214.
>
> https://svnweb.freebsd.org/changeset/base/221214
>
> We should do the same for each hypervisor.
>
> Jung-uk Kim
>
>
We probably should.  But why does calibration fail in the first place?  If
it can fail in a VM, then it can probably fail on bare metal too.  It would
be worth investigating.