Re: [Xen-devel] [PATCH RFC] handle tsc_tolerance during migration between identical hosts

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 11.04.17 at 11:39, <olaf@xxxxxxxxx> wrote:
> Testing has shown that domUs with 'tsc_mode=default' can be migrated
> safely between identical hardware, even if the measured clock frequency
> differs by a few kHz. A change like shown below would allow to migrate
> between "2.nnGHz" hosts without enforcing emulation. If the domU is
> migrated to a host with "2.mmGHz" the frequency jump might have bad
> effects, and tsc emulation might be good even if it comes with a
> perfromance penalty.
> 
> The change below adds a new boot option to set a tolerance value.
> I think its up to the host admin to decide, therefore a global option
> should be enough.
> 
> Any opinions,

I'm afraid successful testing is not a sufficient criteria here. At
the very least the (so far missing) documentation needs to very
clearly point out the possible risks associated with using this
option. But with there not being any functional change when
the option is not being made use of, I don't think there's a
reason not to add this (for the adventurous).

What I'm not sure about is whether having this as a global
(instead of per-guest) setting is really all that useful: As
different guests may require different tolerance, having a
single unique setting may not allow people to get very far.

> also on the name of the cmdline option?

I'm trying to advocate for dashes instead of underscores, and
I think it should say vtsc instead of tsc, to make clear it has an
effect on guests only.

Jan


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