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

[David Gibson <david@gibson.dropbear.id.au>]

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On Tue, Feb 16, 2016 at 11:36:55AM +0100, Igor Mammedov wrote:
> On Mon, 15 Feb 2016 20:43:41 +0100
> Markus Armbruster <armbru@redhat.com> wrote:
> 
> > Igor Mammedov <imammedo@redhat.com> 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?
> 
> 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

Hmm.. is it not enough to follow the link and get the topology
information by examining the target?

In the design Eduardo and I have been discussing we're actually not
planning to allow device_add to construct CPU packages - at least, not
for the time being.  The idea is that the machine type will construct
enough packages for maxcpus, and management just toggles them on and
off.

We can eventually allow construction of new packages with device_add,
but for now that gets hidden inside the platform until we've worked
out more details.

> 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'd prefer to avoid exposing the node/socket/core heirarchy through
the QOM interfaces as much as possible.  Although all systems I know
of have a heirarchy something like that, exactly what the levels may
vary, so I think it's better not to bake that into our interface.

Properties giving core/socket/node id values isn't too bad, but
building a whole tree mirroring that heirarchy seems like asking for
trouble.

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

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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