Re: [Xen-devel] [PATCH v11] tolerate jitter in cpu_khz calculation to avoid TSC emulation

[Olaf Hering <olaf@xxxxxxxxx>]

Am Thu, 13 Dec 2018 03:41:44 -0700
schrieb "Jan Beulich" <JBeulich@xxxxxxxx>:

> In a second step, let's consider what impact errors in calibration have.
> Between two systems with exactly the same hardware crystal
> frequency there of course is going to be some drift. The problem
> though is - between the two calibrated clock values from two systems
> you can't easily tell what part of the difference is a result of the
> calibration being imprecise, and what part of it is because of the
> crystals not providing the exact same frequency. Without knowing
> the possible range of both errors, the argumentation of _one of
> them_ being tolerable within a certain range to consider _both_
> systems sufficiently equal is at least questionable.

We already have a knob for that, if a reference clock can not be provided:
 tsc_mode=always_emulate
This would exactly cover the case there the assumed frequency that is
available during the start of domU will be used after migration.

Olaf

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