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


From: Markus Armbruster
Subject: Re: [Qemu-devel] [RFC] QMP: add query-hotpluggable-cpus
Date: Tue, 16 Feb 2016 13:35:42 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.5 (gnu/linux)

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?

> The similar but a bit more abstract approach was suggested
> by David https://lists.gnu.org/archive/html/qemu-ppc/2016-02/msg00000.html

Cc'ing him.  If I understand the high-level idea correctly, David
proposes to have an abstract type cpu-package with generic properties.
Its concrete subtypes are composed of whatever components make up the
hot-pluggable unit.

Management software can then use the generic properties to deal with hot
plug without having to know about the concrete subtypes, at least to
some useful degree.

Similarly, the generic properties suffice for implementing generic
high-level interfaces like -smp.

David, is that a fair summary?

Naturally, we need a way to introspect available subtypes of cpu-package
to answer questions like what concrete types can actually be plugged
into this board.

This could be an instance of the generic QOM introspection question
"what can plug into this socket"?  Unfortunately, I don't know enough
QOM to put that into more concrete terms.  Andreas, Paolo, can you help
out?

> Benefit of dedicated CPU hotplug focused QMP command is that
> it can be quite abstract to suite most targets and not depend
> on how a target models CPUs internally and still provide
> information needed for hotplugging a CPU object.
> That way we can split efforts on how we model/refactor CPUs
> internally and how mgmt would work with them using
> -device/device_add.

CPUs might be special enough to warrant special commands.  Nevertheless,
non-special solutions should be at least explored.  That's what we're
doing here.



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