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


From: Igor Mammedov
Subject: Re: [Qemu-devel] [RFC] QMP: add query-hotpluggable-cpus
Date: Thu, 18 Feb 2016 11:55:16 +0100

On Thu, 18 Feb 2016 15:05:10 +1100
David Gibson <address@hidden> 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 <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]"}
> >    ]}  
> 
> 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.
 
> 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.



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