Re: [Qemu-devel] [PATCH v7 01/16] hw/cpu: introduce CPU clusters

[Luc Michel]

On 12/4/18 8:45 PM, Eduardo Habkost wrote:
> On Tue, Dec 04, 2018 at 07:16:39PM +0000, Peter Maydell wrote:
>> On Tue, 4 Dec 2018 at 19:05, Eduardo Habkost <address@hidden> wrote:
>>> On Tue, Dec 04, 2018 at 06:24:19PM +0000, Peter Maydell wrote:
>>>> A cluster is a group of CPUs which are all identical and have
>>>> the same view of the rest of the system.
>>
>>> With that definition in mind, why can't QEMU cluster CPUs
>>> automatically by looking at CPU models and address space objects?
>>
>> That sounds like it is in theory feasible and in practice
>> quite tricky. You would have to look not just at the CPU
>> model name but also introspect all its properties for
>> ones which change features of the CPU and are set differently
>> on different CPUs (and I don't think there's any way to
>> automatically tell which properties are ones which make
>> the CPU different for which-cluster purposes and which aren't).
>> And if we automatically checked whether address space objects
>> were the same it would rule out implementing devices with
>> per-cpu banked memory mapped registers by mapping different
>> things into the AS for each CPU (though that's a hypothetical
>> at the moment -- I've thought about implementing stuff that
>> way but we tend to implement that sort of thing by looking
>> at current_cpu->cpu_index at the moment).
> 
> I see.
> 
> Can't we at least do something to make sure the cluster objects
> make sense?  e.g. by ensuring at least QOM CPU type is the same,
> and that cpu->address_space somehow points to
> cpu->cluster->address_space?
Where such a check should be placed? Cluster realize function is not
good since children can still be added after device realization. A good
place would be when object_property_add_child() is called, but I'm not
aware of a way of hooking into that with the QOM API...

-- 
Luc

> 
> CCing Paolo, so he can correct me if this doesn't make sense.
>