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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: Fri, 26 Feb 2016 11:37:17 +0100

On Fri, 26 Feb 2016 15:12:26 +1100
David Gibson <address@hidden> wrote:

> On Thu, Feb 25, 2016 at 01:43:05PM +0100, Igor Mammedov wrote:
> > On Thu, 25 Feb 2016 12:25:43 +1100
> > David Gibson <address@hidden> wrote:
> >   
> > > On Wed, Feb 24, 2016 at 03:17:54PM +0100, Igor Mammedov wrote:  
> > > > On Wed, 24 Feb 2016 12:54:17 +1100
> > > > David Gibson <address@hidden> wrote:
> > > >     
> > > > > On Tue, Feb 23, 2016 at 10:46:45AM +0100, Igor Mammedov wrote:    
> > > > > > On Mon, 22 Feb 2016 13:54:32 +1100
> > > > > > David Gibson <address@hidden> wrote:
> > > > > >       
> > > > > > > On Fri, Feb 19, 2016 at 04:49:11PM +0100, Igor Mammedov wrote:    
> > > > > > >   
> > > > > > > > On Fri, 19 Feb 2016 15:38:48 +1100
> > > > > > > > David Gibson <address@hidden> wrote:
> > > > > > > > 
> > > > > > > > CCing thread a couple of libvirt guys.
> > > > > > > >         
> > > > > > > > > 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 sorry but I couldn't parse above 2 paragraphs. The way I see
> > > > > > > > whatever placement info QEMU will provide to mgmt, mgmt will 
> > > > > > > > have
> > > > > > > > to deal with it in one way or another.
> > > > > > > > Perhaps rephrasing and adding some examples might help to 
> > > > > > > > explain
> > > > > > > > suggestion a bit better?        
> > > > > > > 
> > > > > > > Ok, so what I'm saying is that I think describing a location for 
> > > > > > > the
> > > > > > > package itself could be problematic.  For some cases it will be 
> > > > > > > ok,
> > > > > > > but depending on exactly what the package represents on a 
> > > > > > > particular
> > > > > > > platform there could be a lot of options for how to represent it.
> > > > > > > 
> > > > > > > What I'm suggesting instead is that instead of giving a location 
> > > > > > > for
> > > > > > > itself, the package lists the locations of all the threads it will
> > > > > > > contain when it is enabled/present/whatever.  Those locations can 
> > > > > > > be
> > > > > > > given as node/socket/core/thread tuples - which are properties 
> > > > > > > that
> > > > > > > cpu threads already need to have, so we're not making the possible
> > > > > > > inadequacy of that information any worse than it already was.
> > > > > > > 
> > > > > > > Examples.. so I'm not really sure how to write QOM objects, but I 
> > > > > > > hope
> > > > > > > this is clear enough:
> > > > > > > 
> > > > > > > On x86
> > > > > > >   .../cpu-package[0]              (type 'acpi-thread')
> > > > > > >          present = true
> > > > > > >          location[0] = (node 0, socket 0, core 0, thread 0)
> > > > > > >          thread[0] = <link to cpu thread object>
> > > > > > >   .../cpu-package[1]              (type 'acpi-thread')
> > > > > > >          present = false
> > > > > > >          location[0] = (node 0, socket 0, core 0, thread 1)
> > > > > > > 
> > > > > > > On Power
> > > > > > >   .../cpu-package[0]              (type 'spapr-core')
> > > > > > >          present = true
> > > > > > >          location[0] = (node 0, socket 0, core 0, thread 0)
> > > > > > >          location[1] = (node 0, socket 0, core 0, thread 1)
> > > > > > >          ...
> > > > > > >          location[7] = (node 0, socket 0, core 0, thread 7)
> > > > > > >          thread[0] = <link...>
> > > > > > >          ...
> > > > > > >          thread[7] = >link...>
> > > > > > >   .../cpu-package[1]              (type 'spapr-core')
> > > > > > >          present = false
> > > > > > >          location[0] = (node 0, socket 0, core 0, thread 0)
> > > > > > >          location[1] = (node 0, socket 0, core 0, thread 1)
> > > > > > >          ...
> > > > > > >          location[7] = (node 0, socket 0, core 0, thread 7)
> > > > > > > 
> > > > > > > Does that make sense?
> > > > > > >       
> > > > > > > > > > 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?        
> > > > > > > > In this RFC it's MachineClass.possible_cpus method which is a 
> > > > > > > > bit more
> > > > > > > > flexible as it allows a board to describe possible CPU devices 
> > > > > > > > (whatever
> > > > > > > > they might be: sockets|cores|threads|some_chip_module) and 
> > > > > > > > their properties
> > > > > > > > without forcing board to precreate cpu_package objects which 
> > > > > > > > should convey
> > > > > > > > the same info one way or another.        
> > > > > > > 
> > > > > > > Hmm.. so my RFC so far (at least the revised version based on
> > > > > > > Eduardo's comments) is that the cpu_package objects are always
> > > > > > > precreated.  In future we might allow dynamic construction, but 
> > > > > > > that
> > > > > > > will require a bunch more thinking to designt the right 
> > > > > > > interfaces.
> > > > > > >       
> > > > > > > > > > Considering that we would need to create HMP command so 
> > > > > > > > > > user could
> > > > > > > > > > inspect possible CPUs from monitor, it would need to do the 
> > > > > > > > > > same as
> > > > > > > > > > QMP command regardless of whether it's cpu-package objects 
> > > > > > > > > > or
> > > > > > > > > > just board calculated info a runtime.
> > > > > > > > > >            
> > > > > > > > > > > In the design Eduardo and I have been discussing we're 
> > > > > > > > > > > actually not
> > > > > > > > > > > planning to allow device_add to construct CPU packages - 
> > > > > > > > > > > at least, not
> > > > > > > > > > > for the time being.  The idea is that the machine type 
> > > > > > > > > > > will construct
> > > > > > > > > > > enough packages for maxcpus, and management just toggles 
> > > > > > > > > > > them on and
> > > > > > > > > > > off.          
> > > > > > > > > > Another question is how it would work wrt migration?        
> > > > > > > > > >   
> > > > > > > > > 
> > > > > > > > > I'm assuming the "present" bits would be added to the 
> > > > > > > > > migration
> > > > > > > > > stream; seems straightforward enough to me.  Is there some
> > > > > > > > > consideration I'm missing?        
> > > > > > > > It's hard to estimate how cpu-package objects might complicate
> > > > > > > > migration. It should not break migration for old machine types
> > > > > > > > and if possible it should work for backwards migration to older
> > > > > > > > QEMU versions (to be downstream friendly).        
> > > > > > > 
> > > > > > > So, the simple way to achieve that is to only instantiate the
> > > > > > > cpu-package objects on newer machine types.  Older machine types 
> > > > > > > will
> > > > > > > instatiate the cpu threads directly from the machine type in the 
> > > > > > > old
> > > > > > > way, and (except for x86) won't allow cpu hotplug.
> > > > > > > 
> > > > > > > I think that's a reasonable first approach.  Later we can look at
> > > > > > > migrating a non-package setup to a package setup, if it looks like
> > > > > > > that will be useful.
> > > > > > >       
> > > > > > > > If we go typical '-device/device_add 
> > > > > > > > whatever_cpu_device,foo_options_list'
> > > > > > > > route then it would allow us to replicate older device models 
> > > > > > > > without
> > > > > > > > issues (I don't expect any in x86 case) as it's what CPUs are 
> > > > > > > > now under the hood.
> > > > > > > > This RFC doesn't force us to re-factor device models in order 
> > > > > > > > to use
> > > > > > > > hotplug (where CPU objects are already self-sufficient 
> > > > > > > > devices/hotplug capable).
> > > > > > > > 
> > > > > > > > It rather tries completely split interface aspect from how we 
> > > > > > > > are
> > > > > > > > internally model CPU hotplug, and tries to solve issue with
> > > > > > > > 
> > > > > > > >  -device/device_add for which we need to provide
> > > > > > > >    'what type to plug' and 'where to plug, which options to set 
> > > > > > > > to what'
> > > > > > > > 
> > > > > > > > It's 1st level per you proposal, later we can do 2nd level on 
> > > > > > > > top of it
> > > > > > > > using cpu-packages(flip present property) to simplify mgmt's job
> > > > > > > > if it still would really needed (i.e. mgmt won't be able to 
> > > > > > > > cope with
> > > > > > > > -device, which it already has support for).        
> > > > > > > 
> > > > > > > Yeah.. so the thing is, in the short term I'm really more 
> > > > > > > interested
> > > > > > > in the 2nd layer interface.  It's something we can actually use,
> > > > > > > whereas the 1st layer interfaace still has a lot of potential
> > > > > > > complications.      
> > > > > > What complications do you see from POWER point if view?      
> > > > > 
> > > > > I don't relaly see any complications specific to Power.  But the
> > > > > biggest issue, as far as I can tell is how do we advertise to the user
> > > > > / management layer what sorts of CPUs can be hotplugged - how many,
> > > > > what types are possible and so forth.  The constraints here could in
> > > > > theory be pretty complex.    
> > > > that's what query-hotpluggable-cpus does, but not for theoretical
> > > > set of platforms but rather a practical set that we a wanting
> > > > CPU hotplug for.
> > > >  i.e. board returns a fixed board layout describing what cpu types
> > > >  could be hotplugged and where at in terms of [socket/core/thread]
> > > >  tuples, which maps well to current targets which need CPU hotplug
> > > >  (power/s390/x86/ARM).
> > > > 
> > > > The rest of interface (i.e.) add/remove actions are handled by
> > > > reused -device/device_add - that mgmt has already support for and
> > > > works pretty well for migration as well
> > > > (no need to maintain machine version-ed compat glue is plus).
> > > > 
> > > > So any suggestions how to improve layout description returned
> > > > by query-hotpluggable-cpus command are welcome.
> > > > Even if we end up using QOM interface, suggestions will still
> > > > be useful as the other interface will need to convey the same info
> > > > just via other means.    
> > > 
> > > Yeah, as I mentioned elsewhere, I'm starting to come around to this
> > > basic approach, although I'm still a bit dubious about the specific
> > > format suggested.  I don't have specific suggestions to improve it
> > > yet, but I'm working on it :).
> > > 
> > >   
> > > > > > > This is why Eduardo suggested - and I agreed - that it's probably
> > > > > > > better to implement the "1st layer" as an internal 
> > > > > > > structure/interface
> > > > > > > only, and implement the 2nd layer on top of that.  When/if we 
> > > > > > > need to
> > > > > > > we can revisit a user-accessible interface to the 1st layer.      
> > > > > > We are going around QOM based CPU introspecting interface for
> > > > > > years now and that's exactly what 2nd layer is, just another
> > > > > > implementation. I've just lost hope in this approach.
> > > > > > 
> > > > > > What I'm suggesting in this RFC is to forget controversial
> > > > > > QOM approach for now and use -device/device_add + QMP introspection,
> > > > > > i.e. completely split interface from how boards internally implement
> > > > > > CPU hotplug.      
> > > > > 
> > > > > I can see the appeal of that approach at this juncture.  Hmm..    
> > > > A lot of work has been done to make CPUs device_add compatible.    
> > > 
> > > So... it's been much discussed, but I'm still pretty unclear on how
> > > the device_add interface is supposed to work; at least in the context
> > > of non thread-granularity hotplug.
> > > 
> > > Basically, is it acceptable for:
> > >   device_add vendor-model-cpu-core
> > > 
> > > to create, in addition to the core device, a bunch of additional
> > > devices (the individual threads), or is that the "object mutating its
> > > own topology" that Andreas objects to violently?  
> > I think it's acceptable to have vendor-model-cpu-core device
> > considering it's platform limitation or socket if device model calls for it.
> > I'm not sure that mutating applies to all objects but for Device
> > inherited classes there shouldn't be any.
> > i.e.
> >  1. create Device with instance_init - constructor that shouldn't fail ever
> >  2. set properties -
> >       done by -device/device_add and also by device_post_init() for globals
> >  3. set 'realize' property to ON - allowed to fail, completes device 
> > initialization
> >     realize() hook must validate set earlier properties if it hasn't been
> >     done earlier and complete all child objects initialization,  
> 
> Ok, does that include the initial construction of child objects?
for x86 we do so, i.e. construct lapic child  since it's not known
at instance_init() time if CPU has it and known only after properties
are set i.e. at realize time.

> 
> >     children are should be at 'realized' state when parent's realize()
> >     hook finishes without error. No further children are allowed to be
> >     created and not properties are allowed to be set after Device is 
> > realized.
> >  4. Once realize() hook is executed, Device core code calls
> >     plug hook if it supported hotplug_handler_plug() which usually
> >     does the job of wiring Device to board. For more details see
> >     device_set_realized().
> >
> > On top of that Andreas would like that children weren't dynamically
> > allocated but embedded into parent, included in parent's
> > instance_size if possible i.e. children count is known at
> > instance_init() time.  
> 
> Right, which is not possible if we have a nr_threads property, as we
> want for the cases we're looking at now.
the same applies to x86 lapic mentioned above, so we do object_new(lapic)
at realize time.

> 
> > > If that is acceptable, where exactly should it be done?  In the
> > > device's instance_init? in realize? somewhere else?  
> > Not sure what question is about, does above answer it?
> >    
> > > > The missing piece is letting mgmt to know what CPUs and with
> > > > which options could be plugged in.    
> > > 
> > > Well, that's *a* missing piece, certainly..
> > >   
> > > > And adding a query-hotpluggable-cpus QMP command looks like
> > > > a path of the least resistance that would work for power/s390/x86/ARM.
> > > >     
> > >   
> >   
> 




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