Re: Xen timecounter issues

[Brad Spencer <brad%anduin.eldar.org@localhost>]

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).

>> [...]
>> 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

I think 'xl vcpu-list Domain-0' ...  run it a couple of times in a
row..  at least on mine the CPU column seems to show Domain-0 floating
around the pCPUs like the DOMUs do.

>>    (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



-- 
Brad Spencer - brad%anduin.eldar.org@localhost - KC8VKS - http://anduin.eldar.org