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

[Markus Armbruster]

David Gibson <address@hidden> writes:

> 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.

I'm concerned this might suffer combinatorial explosion.

qemu-system-x86_64 --cpu help shows more than two dozen CPUs.  They can
be configured in numerous arrangements of sockets, cores, threads.  Many
of these wouldn't be physically possible with older CPUs.  Guest
software might work even with physically impossible configurations, but
arranging virtual models of physical hardware in physically impossible
configurations invites trouble, and should best be avoided.

I'm afraid I'm still in the guess-what-you-mean stage because I lack
concrete examples to go with the abstract description.  Can you
enumerate the pre-plugged CPU packages for a board of your choice to
give us a better idea of how your proposal would look like in practice?
Then describe briefly what a management application would need to know
about them, and what it would do with the knowledge?

Perhaps a PC board would be the most useful, because PCs are probably
second to none in random complexity :)

>> 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.

Do these CPU packages come with "soldered-in" CPUs?  Or do they provide
slots where CPUs can be plugged in?  From what I've read, I guess it's
the latter, together with a "thou shalt not plug in different CPUs"
commandment.  Correct?

If yes, then the CPU the board comes with would determine what you can
plug into the slots.

Conversely, the CPU the board comes with helps determine the CPU
packages.

>> > 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.
>> >> 
>> >> [...]
>> >> 
>>