Re: proposal: stop using the xen_system_time timecounter in dom0

[Martin Husemann <martin%duskware.de@localhost>]

On Fri, Jun 21, 2024 at 08:59:01AM +0200, Manuel Bouyer wrote:
> On Thu, Jun 20, 2024 at 05:16:38PM -0400, Greg Troxel wrote:
> > Manuel Bouyer <bouyer%antioche.eu.org@localhost> writes:
> > 
> > > On Thu, Jun 20, 2024 at 01:53:34PM -0700, Greg A. Woods wrote:
> > >> I would like to propose that we stop using the xen_system_time
> > >> timecounter in NetBSD dom0, i.e. in XEN3_DOM0.
> > >
> > > Why can't you just set kern.timecounter.hardware to something else
> > > if xen_system_time isn't working for you ?
> > 
> > I suppose he can, but I read that as an argument that xen_system_time is
> > not reliable, and thus should be withdrawn.  People should not have to
> > add workarounds.
> 
> it is perfectly reliable for me. In my case I'm not sure (really)
> that other sources would be better.

This perfectly matches the real world on many amd64 machines where not
all timecounter sources work as expected (due to firmware bugs or whatever).

However, in this case an easy workaround would be to lower the quality
estimate of the timecounter if the cpu in use does not have an
invariant TSC - I think we do something like that (with a bad list and
some cpu ident magic) on some other timecounters, maybe that logic could
be reused here?

Martin