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

[David Gibson]

On Mon, Feb 22, 2016 at 10:05:54AM +0100, Markus Armbruster wrote:
> David Gibson <address@hidden> writes:
> 
> > On Fri, Feb 19, 2016 at 10:51:11AM +0100, Markus Armbruster wrote:
> >> David Gibson <address@hidden> writes:
> >> 
> >> > On Thu, Feb 18, 2016 at 11:37:39AM +0100, Igor Mammedov wrote:
> >> >> On Thu, 18 Feb 2016 14:39:52 +1100
> >> >> David Gibson <address@hidden> wrote:
> >> >> 
> >> >> > On Tue, Feb 16, 2016 at 11:36:55AM +0100, Igor Mammedov wrote:
> >> >> > > 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?
> >> >> > > 
> >> >> > > 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?
> >> >> One can't follow a link if it's an empty one, hence
> >> >> CPU placement information should be provided somehow,
> >> >> either:
> >> >
> >> > Ah, right, so the issue is determining the socket/core/thread
> >> > addresses that cpus which aren't yet present will have.
> >> >
> >> >>  * by precreating cpu-package objects with properties that
> >> >>    would describe it /could be inspected via OQM/
> >> >
> >> > So, we could do this, but I think the natural way would be to have the
> >> > information for each potential thread in the package.  Just putting
> >> > say "core number" in the package itself assumes more than I'd like
> >> > about how packages sit in the heirarchy.  Plus, it means that
> >> > management has a bunch of cases to deal with: package has all the
> >> > information, package has just a core id, package has just a socket id,
> >> > and so forth.
> >> >
> >> > It is a but clunky that when the package is plugged, this information
> >> > will have to sit parallel to the array of actual thread links.
> >> >
> >> > Markus or Andreas is there a natural way to present a list of (node,
> >> > socket, core, thread) tuples in the package object?  Preferably
> >> > without having to create a whole bunch of "potential thread" objects
> >> > just for the purpose.
> >> 
> >> I'm just a dabbler when it comes to QOM, but I can try.
> >> 
> >> I view a concrete cpu-package device (subtype of the abstract
> >> cpu-package device) as a composite device containing stuff like actual
> >> cores.
> >
> > So.. the idea is it's a bit more abstract than that.  My intention is
> > that the package lists - in some manner - each of the threads
> > (i.e. vcpus) it contains / can contain.  Depending on the platform it
> > *might* also have internal structure such as cores / sockets, but it
> > doesn't have to.  Either way, the contained threads will be listed in
> > a common way, as a flat array.
> >
> >> To create a composite device, you start with the outer shell, then plug
> >> in components one by one.  Components can be nested arbitrarily deep.
> >> 
> >> Perhaps you can define the concrete cpu-package shell in a way that lets
> >> you query what you need to know from a mere shell (no components
> >> plugged).
> >
> > Right.. that's exactly what I'm suggesting, but I don't know enough
> > about the presentation of basic data in QOM to know quite how to
> > accomplish it.
> >
> >> >> or
> >> >>  * via QMP/HMP command that would provide the same information
> >> >>    only without need to precreate anything. The only difference
> >> >>    is that it allows to use -device/device_add for new CPUs.
> >> >
> >> > I'd be ok with that option as well.  I'd be thinking it would be
> >> > implemented via a class method on the package object which returns the
> >> > addresses that its contained threads will have, whether or not they're
> >> > present right now.  Does that make sense?
> >> 
> >> If you model CPU packages as composite cpu-package devices, then you
> >> should be able to plug and unplug these with device_add, unless plugging
> >> them requires complex wiring that can't be done in qdev / device_add,
> >> yet.
> >
> > There's a whole bunch of issues raised by allowing device_add of
> > cpus.  Although they're certainly interesting and probably useful, I'd
> > really like to punt on them for the time being, so we can get some
> > sort of cpu hotplug working on Power (and s390 and others).
> 
> If you make it a device, you can still set
> cannot_instantiate_with_device_add_yet to disable -device / device_add
> for now, and unset it later, when you're ready for it.

Yes, that was the plan.

> > The idea of the cpu packages is that - at least for now - the user
> > can't control their contents apart from the single "present" bit.
> > They already know what they can contain.
> 
> Composite devices commonly do.  They're not general containers.
> 
> The "present" bit sounds like you propose to "pre-plug" all the possible
> CPU packages, and thus reduce CPU hot plug/unplug to enabling/disabling
> pre-plugged CPU packages.

Yes.

> What if a board can take different kinds of CPU packages?  Do we
> pre-plug all combinations?  Then some combinations are non-sensical.
> How would we reject them?

I'm not trying to solve all cases with the present bit handling - just
the currently common case of a machine with fixed maximum number of
slots which are expected to contain identical processor units.

> For instance, PC machines support a wide range of CPUs in various
> arrangements, but you generally need to use a single kind of CPU, and
> the kind of CPU restricts the possible arrangements.  How would you
> model that?

The idea is that the available slots are determined by the machine,
possibly using machine or global options.  So for PC, -cpu and -smp
would determine the number of slots and what can go into them.

> > There are a bunch of potential use cases this doesn't address, but I
> > think it *does* address a useful subset of currently interesting
> > cases, without precluding more flexible extensions in future.
> >
> >> If that's the case, a general solution for "device needs complex wiring"
> >> would be more useful than a one-off for CPU packages.
> >> 
> >> [...]
> >> 
> 

-- 
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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