Re: Xen timecounter issues

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

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)

> [...]
> 4. (a) Can a dom0 vCPU be interrupted and switched from one pCPU to
>        another pCPU on the same host?

Yes, I guess this can happen, as I've not seen any special case for
dom0 in the Xen scheduler. I don't know how to verify this though


>    (b) Can a dom0 be migrated from one host to another host?

No dom0 can't be migrated

>    (c) Can a domain be migrated from a host with one Xen TSC
>        configuration to a host with a different Xen TSC configuration?

I've not checked migration constrains for quite some time, but I don't
remember seeing restriction in this area for migration

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