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

[Igor Mammedov]

On Tue, 16 Feb 2016 16:48:34 +1100
David Gibson <address@hidden> wrote:

> On Mon, Feb 15, 2016 at 08:43:41PM +0100, Markus Armbruster 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?  
> 
> That's my main concern as well.
> 
> Igor,  did you see my post with a proposal for how to organize
> hotpluggable packages of CPUs?  I believe that would also solve the
> problem at hand here, by having a standard QOM location with
> discoverable cpu objects.
> 
> The interface in your patch in particular would *not* solve the
> problem of advertising to management layers what the granularity of
> CPU hotplug is, which we absolutely need for Power.
I've had in mind Power as well, as topology items are optional
a query can respond with what granularity board would like
to use and what type of object it could be hotplugged:

-> { "execute": "query-hotpluggable-cpus" }
<- {"return": [
     {"core": 2, "socket": 2, "arch_id": 2, "type": "power-foo-core-cpu"},
     {"core": 1, "socket": 1, "arch_id": 1, "type": "power-foo-core-cpu"},
     {"core": 0, "socket": 0, "arch_id": 0, "type": "power-foo-core-cpu", 
"cpu_link": "/machine/unattached/device[3]"}
   ]}