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

[Julien Grall <julien.grall@xxxxxxx>]

Hi Stefano,

On 20/09/2016 20:09, Stefano Stabellini wrote:
> On Tue, 20 Sep 2016, Julien Grall wrote:
> > Hi,
> >
> > On 20/09/2016 12:27, George Dunlap wrote:
> > > On Tue, Sep 20, 2016 at 11:03 AM, Peng Fan <van.freenix@xxxxxxxxx> wrote:
> > > > On Tue, Sep 20, 2016 at 02:54:06AM +0200, Dario Faggioli wrote:
> > > > > On Mon, 2016-09-19 at 17:01 -0700, Stefano Stabellini wrote:
> > > > > > On Tue, 20 Sep 2016, Dario Faggioli wrote:
> > > > I'd like to add a computing capability in xen/arm, like this:
> > > >
> > > > struct compute_capatiliby
> > > > {
> > > >    char *core_name;
> > > >    uint32_t rank;
> > > >    uint32_t cpu_partnum;
> > > > };
> > > >
> > > > struct compute_capatiliby cc=
> > > > {
> > > >   {"A72", 4, 0xd08},
> > > >   {"A57", 3, 0xxxx},
> > > >   {"A53", 2, 0xd03},
> > > >   {"A35", 1, ...},
> > > > }
> > > >
> > > > Then when identify cpu, we decide which cpu is big and which cpu is little
> > > > according to the computing rank.
> > > >
> > > > Any comments?
> > > 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.
> >
> > 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.
> That's right, see Documentation/devicetree/bindings/arm/cpus.txt. It is
> trivial to identify the two different CPU classes and which cores belong
> to which class.t, as
The class of the CPU can be found from the MIDR, there is no need to use 
the device tree/acpi for that. Note that I don't think there is an easy 
way in ACPI (i.e not in AML) to find out the class.
> It is harder to figure out which one is supposed to be
> big and which one LITTLE. Regardless, we could default to using the
> first cluster (usually big), which is also the cluster of the boot cpu,
> and utilize the second cluster only when the user demands it.
Why do you think the boot CPU will usually be a big one? In the case of 
Juno platform it is configurable, and the boot CPU is a little core on 
r2 by default.
In any case, what we care about is differentiate between two set of 
CPUs. I don't think Xen should care about migrating a guest vCPU between 
big and LITTLE cpus. So I am not sure why we would want to know that.
The only thing we need an identifier for each set (I might be the MIDR 
or the compatible in the device tree).
Note that, as Peng mentioned, Linaro is working on an energy-aware 
scheduler. So there is a way (maybe not yet upstreamed) to find the CPU 
topology.
Regards,

--
Julien Grall

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