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Re: [Xen-devel] per-domain configuration and inappropriate use of globals

On Mon, Oct 22, 2018 at 02:29:40PM +0100, Andrew Cooper wrote:
> On 19/10/18 19:22, Juergen Gross wrote:
> > On 19/10/2018 18:57, Andrew Cooper wrote:
> >> In practice, having fine grain control of all the features like would be
> >> excellent for testing purposes, because it allows you to boot two
> >> otherwise-identical VMs with one configuration difference between them.
> >>
> >> In the spirit of the already in progress domaincreate work, options like
> >> these should be selectable at domain creation time, and immutable
> >> thereafter.
> >>
> >> That said, there is a plethora of tweakables, and I'm not sure how best
> >> to expose them.  While most (all?) of these options are inherently
> >> supported (as playing with them simulates what Xen would chose on
> >> different hardware), I expect there will be ample opportunity for people
> >> to break their systems if they tweak too much.
> >>
> >> Is there liable to be any provision in xl/libxl to have "unstable"
> >> configuration, which is easily identified as "may stop working / cease
> >> to exist / become invalid at any point in the future?"
> >>
> >> Alternatively, are there any other suggestions for alternative mechanisms?
> > Per-domain parameters like in my series? You could guard the "dangerous"
> > ones by a global parameter (boot-time or run-time settable).
> I was hoping to separate the discussion of what information should be
> configurable, from the mechanism we used to provide said information.

How do you plan to express this information in SUPPORT.md?

> Using a text-based mechanism suffers from the same stable/unstable
> issues as xl.cfg, so the same concern applies there.

But that is a get-out-of-jail-free card for xl / libxl because they
wouldn't need to care what is supplied. :p

If the way you want this to work is inherently unstable, I think it'd be
better to leave xl / libxl out of the picture from the beginning.


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