Re: [Xen-devel] [DOC RFC] Heterogeneous Multi Processing Support in Xen

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 08.12.16 at 22:54, <dario.faggioli@xxxxxxxxxx> wrote:
> On Thu, 2016-12-08 at 03:14 -0700, Jan Beulich wrote:
>> > > > On 07.12.16 at 19:29, <dario.faggioli@xxxxxxxxxx> wrote:
>> > The list of classes is kept ordered from the more powerful to the
>> > less
>> > powerful.
>> > **TODO:** this has been [proposed by 
>> > George](https://lists.xenproject.org/archives/html/xen-devel/2016-0 
>> > 9/msg02212.html).
>> > I like the idea, what do others think? If we agree on that, note
>> > that there
>> > has been no discussion on defining what "more powerful" means,
>> > neither on
>> > x86 (although, not really that interesting, for now, I'd say), nor
>> > on ARM.
>> 
>> Indeed I think there should be no assumption about the ability to
>> order things here: Even if for some initial set of hardware it may
>> be possible to clearly tell which one's more powerful and which
>> one's more weak, already the moment you extend this from
>> compute power to different ISA extensions you'll immediately end
>> up with the possibility of two CPUs have a distinct extra feature
>> compared to one another (say one a crypto extension and the
>> other a wider vector compute engine).
>> 
> Yeah, that was what was puzzling me too. Keeping them ordered has the
> nice property that if a user says the following in a config file:
> 
>  vcpuclass=["0-3:class0", "4-7:class1"]
> 
> (assuming that class0 and class1 are the always available Xen names) it

This, btw, is another aspect I think has a basic problem: class0 and
class1 say nothing about the properties of a class, and hence are
tied to one particular host. I think class names need to be descriptive
and uniform across hosts. That would allow migration of such VMs as
well as prevent starting them on a host not having suitable hardware.

Jan


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