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

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

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On Thu, Feb 18, 2016 at 11:55:16AM +0100, Igor Mammedov wrote:
> On Thu, 18 Feb 2016 15:05:10 +1100
> David Gibson <david@gibson.dropbear.id.au> wrote:
> 
> > On Tue, Feb 16, 2016 at 11:52:42AM +0100, Igor Mammedov wrote:
> > > On Tue, 16 Feb 2016 16:48:34 +1100
> > > David Gibson <david@gibson.dropbear.id.au> wrote:
> > >   
> > > > On Mon, Feb 15, 2016 at 08:43:41PM +0100, Markus Armbruster 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?    
> > > > 
> > > > 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]"}
> > >    ]}  
> > 
> > Hrm.. except your arch_id is supplied by a CPUClass hook, making it a
> > per-thread property, whereas here it needs to be per-core.
> That's only for demo purposes, it could be something else that is fixed
> and stable. For example it could be QOM link path associated with it.
> Like: { 'path': '/cpu[0]', ... }, or just something else to enumerate
> a set of possible CPUs.

Hm, ok.

> > Other than that I guess this covers what we need for Power, however I
> > dislike the idea of typing the hotplug granularity to be at any fixed
> > level of the socket/core/thread heirarchy.  As noted elsewhere, while
> > all machines are likely to have some sort of similar heirarchy, giving
> > it fixed levels of "socket", "core" and "thread" may be limiting.
> That's an optional granularity, if target doesn't care, it could skip
> that parameters or even extend command to provide a target specific
> parameters to create a CPU object, socket/core/thread are provided here
> as they would fit majority usecases. These optional parameters are
> basically a set of mandatory CPU object properties with values
> that mgmt should supply at -device/device_add time to create a CPU with
> expected properties.

It seems really weird to me to tell management a bunch of parameters
which it then needs to echo back to device_add.  If we're adding an
interface, why not just add a "add/remove cpu unit" interface.

-- 
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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