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

On 20/09/16 21:17, Stefano Stabellini wrote:
On Tue, 20 Sep 2016, Julien Grall wrote:
Hi Stefano,

On 20/09/2016 20:09, Stefano Stabellini wrote:
On Tue, 20 Sep 2016, Julien Grall wrote:

On 20/09/2016 12:27, George Dunlap wrote:
On Tue, Sep 20, 2016 at 11:03 AM, Peng Fan <van.freenix@xxxxxxxxx>
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
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
nearly be impossible to keep updated.

I would expect the firmware table (device tree, ACPI) to provide relevant
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
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

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.

No, it is not about migrating (at least yet). It is about giving useful
information to the user. It would be nice if the user had to choose
between "big" and "LITTLE" rather than "class 0x1" and "class 0x100", or
even "A7" or "A15".

I don't think it is wise to assume that we may have only 2 kind of CPUs on the platform. We may have more in the future, if so how would you name them?

IHMO, asking the user to specify the type of CPUs he wants would be the easiest way (though a bit difficult for the user) and avoid us to rely on non-upstreamed bindings.


Julien Grall

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