Re: Xen timecounter issues

[Manuel Bouyer <bouyer%antioche.eu.org@localhost>]

On Mon, Jun 24, 2024 at 04:42:19AM -0400, Brad Spencer wrote:
> Manuel Bouyer <bouyer%antioche.eu.org@localhost> writes:
> 
> > On Sun, Jun 23, 2024 at 01:58:36PM +0000, Taylor R Campbell wrote:
> >> It came to my attention today that there has been a lot of discussion
> >> recently about timecounters on Xen:
> >> 
> >> https://mail-index.netbsd.org/port-xen/2024/02/01/msg010525.html
> >> https://mail-index.netbsd.org/port-xen/2024/06/20/msg010573.html
> >> 
> >> These threads are long and I wasn't following because I'm not
> >> subscribed to port-xen, but since I wrote xen_clock.c and I remember
> >> roughly how it works, maybe my input might be helpful.  Is there a
> >> summary of the issues?
> >> 
> >> 1. Is there an array of the following variables?
> >> 
> >>    - dom0 kernel (netbsd-8, netbsd-9, netbsd-10, linux, freebsd, ...)
> >>    - domU kernel, if misbehaviour observed in domU (ditto)
> >>    - Xen kernel version
> >>    - virtualization type (pv/pvh/hvm/...)
> >>    - Xen TSC configuration
> >>    - physical CPU (and, whether the physical CPU has invariant TSC)
> >>    - misbehaviour summary
> >
> > AFAIK no. From what I understood the misbehavior is only seen in dom0.
> > All I can say is that I've run NetBSD Xen dom0 on various generation of
> > Intel CPUs (from P4 to quite recent Xeon) and I never had any issue with
> > timekeeping in dom0 (all my dom0 runs ntpd)
> 
> Another factor might be the number of vcpus allocated to Domain-0.  I
> use only 1 and have no trouble with time keeping on two Intel i7/i8
> systems and one very old AMD Athlon II.  One of the other reporters is
> using more than one vcpu with Domain-0 and is having trouble with time
> keeping and has found that cpu pining solves the problem.  I am also
> running Xen 4.15 and he is running 4.18 (I believe).

I'm switching from 4.15 to 4.18, and with netbsd-10 I'm running
dom0 with all available CPUs (and I have done so on my test machine
running -current for some time now)

I don't think the number of vCPUs is the factor here, as even with one vCPU
it's not pinned to a physical CPU.

-- 
Manuel Bouyer <bouyer%antioche.eu.org@localhost>
     NetBSD: 26 ans d'experience feront toujours la difference
--