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Re: [Xen-devel] [RFC 0/5] xen/arm: support big.little SoC

On Tue, 2016-09-20 at 17:34 +0200, Julien Grall wrote:
> On 20/09/2016 12:27, George Dunlap wrote:
> > I think we definitely need to have Xen have some kind of idea the
> > order between processors, so that the user doesn't need to figure
> > out
> > which class / pool is big and which pool is LITTLE.  Whether this
> > sort
> > of enumeration is the best way to do that I'll let Julien and
> > Stefano
> > give their opinion.
> I don't think an hardcoded list of processor in Xen is the right 
> solution. There are many existing processors and combinations for 
> big.LITTLE so it will nearly be impossible to keep updated.
As far as either the scheduler or cpupools go, what's necessary would
 - in Xen, a function (or an array acting as a map, or whatever) to 
   call to know whether pcpu X is big or LITTLE;
 - at toolstack level, an hypercal (or a field, bit, whatever in a
   struct already returned by an existing hypercall) to know the same 
   thing, i.e., whether c pcpu is big or LITTLE.

Once we have this, we can do everything. We will probably want to
abstract things a bit, and make them as generic as practical, so that
the same interface can be used not only for ARM big.LITTLE, but for
whatever future heterogeneous cpu arch we'll support... but really, the
actual information that we need is "just" that.

I've absolutely no idea how such info could be achieved, and I have no
ARM big.LITTLE hardware to test on.

> I would expect the firmware table (device tree, ACPI) to provide 
> relevant data for each processor and differentiate big from LITTLE
> core.
> Note that I haven't looked at it for now. A good place to start is 
> looking at how Linux does.
Makes sense.

<<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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