Re: [Qemu-devel] [RFC] QMP: add query-hotpluggable-cpus

[Markus Armbruster]

Andreas Färber <address@hidden> writes:

> Am 16.02.2016 um 13:35 schrieb Markus Armbruster:
>> Igor Mammedov <address@hidden> writes:
>> 
>>> On Mon, 15 Feb 2016 20:43:41 +0100
>>> Markus Armbruster <address@hidden> wrote:
>>>
>>>> Igor Mammedov <address@hidden> writes:
>>>>
>>>>> it will allow mgmt to query present and possible to hotplug CPUs
>>>>> it is required from a target platform that wish to support
>>>>> command to set board specific MachineClass.possible_cpus() hook,
>>>>> which will return a list of possible CPUs with options
>>>>> that would be needed for hotplugging possible CPUs.
>>>>>
>>>>> For RFC there are:
>>>>>    'arch_id': 'int' - mandatory unique CPU number,
>>>>>                       for x86 it's APIC ID for ARM it's MPIDR
>>>>>    'type': 'str' - CPU object type for usage with device_add
>>>>>
>>>>> and a set of optional fields that would allows mgmt tools
>>>>> to know at what granularity and where a new CPU could be
>>>>> hotplugged;
>>>>> [node],[socket],[core],[thread]
>>>>> Hopefully that should cover needs for CPU hotplug porposes for
>>>>> magor targets and we can extend structure in future adding
>>>>> more fields if it will be needed.
>>>>>
>>>>> also for present CPUs there is a 'cpu_link' field which
>>>>> would allow mgmt inspect whatever object/abstraction
>>>>> the target platform considers as CPU object.
>>>>>
>>>>> For RFC purposes implements only for x86 target so far.  
>>>>
>>>> Adding ad hoc queries as we go won't scale.  Could this be solved by a
>>>> generic introspection interface?
>>> Do you mean generic QOM introspection?
>> 
>> Possibly, but I don't want to prematurely limit the conversation to QOM
>> introspection.
>> 
>>> Using QOM we could have '/cpus' container and create QOM links
>>> for exiting (populated links) and possible (empty links) CPUs.
>>> However in that case link's name will need have a special format
>>> that will convey an information necessary for mgmt to hotplug
>>> a CPU object, at least:
>>>   - where: [node],[socket],[core],[thread] options
>>>   - optionally what CPU object to use with device_add command
>> 
>> Encoding information in names feels wrong.
>> 
>>> Another approach to do QOM introspection would be to model hierarchy 
>>> of objects like node/socket/core..., That's what Andreas
>>> worked on. Only it still suffers the same issue as above
>>> wrt introspection and hotplug, One can pre-create empty
>>> [nodes][sockets[cores]] containers at startup but then
>>> leaf nodes that could be hotplugged would be a links anyway
>>> and then again we need to give them special formatted names
>>> (not well documented at that mgmt could make sense of).
>>> That hierarchy would need to become stable ABI once
>>> mgmt will start using it and QOM tree is quite unstable
>>> now for that. For some targets it involves creating dummy
>>> containers like node/socket/core for x86 where just modeling
>>> a thread is sufficient.
>> 
>> I acknowledge your concern regarding QOM tree stability.  We have QOM
>> introspection commands since 1.2.  They make the QOM tree part of the
>> external interface, but we've never spelled out which parts of it (if
>> any) are ABI.  Until we do, parts become de facto ABI by being used in
>> anger.  As a result, we don't know something's ABI until it breaks.
>> 
>> Andreas, do you have an opinion on proper use of QOM by external
>> software?
>
> This is absolutely untrue, there have been ABI rules in place and I held
> a presentation covering them in 2012...

I stand corrected!

Got a pointer to the current ABI rules?