TSC calibration in virtual machines

[Jung-uk Kim]

On 06/27/2018 12:47, Alan Somers wrote:
> On Wed, Jun 27, 2018 at 10:36 AM, Jung-uk Kim <jkim at freebsd.org
> <mailto: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
>     <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
>     <https://svnweb.freebsd.org/changeset/base/221214>
> 
>     We should do the same for each hypervisor.
> 
> We probably should.  But why does calibration fail in the first place?
Because multiple guests are sharing same physical CPUs and guest OS has
no control, timing cannot be 100% accurate.

> If it can fail in a VM, then it can probably fail on bare metal too.  It
> would be worth investigating.
It does not "fail" in bare metal because we have almost complete control.

Jung-uk Kim

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