svn commit: r345359 - in head/sys/cddl/dev/dtrace: amd64 i386

[Mark Johnston]

On Wed, Mar 20, 2019 at 08:20:18PM -0700, Rodney W. Grimes wrote:
> > Author: markj
> > Date: Thu Mar 21 02:52:22 2019
> > New Revision: 345359
> > URL: https://svnweb.freebsd.org/changeset/base/345359
> > 
> > Log:
> >   Don't attempt to measure TSC skew when running as a VM guest.
> >   
> >   It simply doesn't work in general since VCPUs may migrate between
> >   physical cores.  The approach used to measure skew also doesn't
> >   make much sense in a VM.
> 
> "May" is the important aspect here, and to my knowledge both
> bhyve and Vmware provide a way to pin cpus there should be
> a way for us to turn this back on if it is desired.  Sticking
> it under the big knob vm_guest is probably not the most flexiable
> solution.
> 
> Could we please have someway to force this back on?

Even with pinning the skew computation is bogus in a VM.  On an idle
host running bhyve with pinned vCPUs it gives offsets that are several
orders of magnitude larger than on the host.  I would prefer to see a
specific reason for wanting the old behaviour before adding a knob.