Re: [Xen-devel] [PATCH v2 2/3] xen: add hypercall option to temporarily pin a vcpu

[Dario Faggioli <dario.faggioli@xxxxxxxxxx>]

On Wed, 2016-03-02 at 16:34 +0100, Juergen Gross wrote:
> On 02/03/16 10:27, Dario Faggioli wrote:
> > 
> > However, an xl flag is easier to add, easier to document and easier
> > and
> > more natural to find, from the point of view of an user that really
> > needs it. And perhaps it could turn out useful for other situations
> > in
> > future. So, I guess I'd say:
> >  - yes, let's add that
> >  - let's do it as a "force flag" of `xl vcpu-pin'.
> Which raises the question: how to do that on the libxl level?
> 
Ah, right.

> a) expand libxl_set_vcpuaffinity() with another parameter (is this
> even
>    possible? I could do some ifdeffery, but the API would change...)
> 
> b) add a libxl_set_vcpuaffinity_force() variant
> 
> c) imply the force flag by specifying both hard and soft maps as NULL
>    (it _is_ basically just that: keep both affinity sets), implying
> that
>    it makes no sense to specify any affinities with the -f flag
> (which
>    renders the "force" meaning rather strange, would be more a
> "restore"
>    now).
> 
Eheh, tools' maintainers' call. My preference would be b).

I don't like a), mostly because that would mean everyone will need to
specify a parameter that it is really only necessary in special cases.

I could live with c), but it indeed makes the semantic too convoluted
for my taste.

I guess, however, that even if going for b), we need to decide whether
to require a cpumask or not, and what to do if one passes NULL. Maybe
we can have a cpumask parameter and,
 - if it is not NULL, force affinity to that,
 - if it is NULL, just 'restore';
what do you think?

Actually, at Xen level, the override only acts on hard affinity...
should libxl take only one cpumask (for hard affinity only), or both
hard and soft?
I'd say just one for hard is enough, unless we want to make space for a
potential future situation where we will want to break and restore soft
affinity as well...

Dario
-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

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