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

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 21/09/16 11:09, Julien Grall wrote:
> 
> 
> 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:
>>>>> 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.
>>
>> 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?

I would suggest that internally Xen recognize an arbitrary number of
processor "classes", and order them according to more powerful -> less
powerful.  Then if at some point someone makes a platform with three
processors, you can say "class 0", "class 1" or "class 2".  "big" would
be an alias for "class 0" and "little" would be an alias for "class 1".

And in my suggestion, we allow a richer set of labels, so that the user
could also be more specific -- e.g., asking for "A15" specifically, for
example, and failing to build if there are no A15 cores present, while
allowing users to simply write "big" or "little" if they want simplicity
/ things which work across different platforms.

 -George

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