Re: [patch] Re: proposal: stop using the xen_system_time timecounter in dom0

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

On Tue, Jun 25, 2024 at 01:53:54PM -0700, Greg A. Woods wrote:
> I think this patch is indeed an answer to my main concern about the dom0
> timecounter on older systems without TSC_INVARIANT.
> 
> I'm not yet able to test it yet either as my limited set of hardware is
> still doing other experiments.
> 
> BTW, as it turns out it currently would not work to override
> kern.timecounter.choice after boot in a dom0.
> 
> In the current implementation the xen_system_time timecounter is
> critical to the periodic resetting and reading of the TODR, which is
> actually how the Xen hypervisor's so-called "wall clock time" is kept in
> sync with dom0 as adjusted by ntpd.  I think that could be worked
> around, but....
> 
> Maybe there should be a very strong warning to Xen users not to override
> the kern.timecounter in the mean time, and perhaps eventually, if we
> continue to use xen_system_time in dom0s then other choices should not
> even be allowed?

Actually, as long as the xen_system_time timecounter is attached, the
TODR will be updated independantly from the selected timecounter.
See xen_timepush_init() and xen_timepush_intr().

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