Re: [Qemu-devel] [RFC 0/6] enable numa configuration before machine_init() from HMP/QMP

[Igor Mammedov]

----- Original Message -----
> From: "Daniel P. Berrange" <address@hidden>
> To: "Igor Mammedov" <address@hidden>
> Cc: "peter maydell" <address@hidden>, address@hidden, address@hidden, 
> address@hidden,
> address@hidden, address@hidden, address@hidden, address@hidden
> Sent: Wednesday, October 18, 2017 5:30:10 PM
> Subject: Re: [Qemu-devel] [RFC 0/6] enable numa configuration before 
> machine_init() from HMP/QMP
> 
> On Tue, Oct 17, 2017 at 06:06:35PM +0200, Igor Mammedov wrote:
> > On Tue, 17 Oct 2017 16:07:59 +0100
> > "Daniel P. Berrange" <address@hidden> wrote:
> > 
> > > On Tue, Oct 17, 2017 at 09:27:02AM +0200, Igor Mammedov wrote:
> > > > On Mon, 16 Oct 2017 17:36:36 +0100
> > > > "Daniel P. Berrange" <address@hidden> wrote:
> > > >   
> > > > > On Mon, Oct 16, 2017 at 06:22:50PM +0200, Igor Mammedov wrote:
> > > > > > Series allows to configure NUMA mapping at runtime using QMP/HMP
> > > > > > interface. For that to happen it introduces a new '-paused' CLI
> > > > > > option
> > > > > > which allows to pause QEMU before machine_init() is run and
> > > > > > adds new set-numa-node HMP/QMP commands which in conjuction with
> > > > > > info hotpluggable-cpus/query-hotpluggable-cpus allow to configure
> > > > > > NUMA mapping for cpus.
> > > > > 
> > > > > What's the problem we're seeking solve here compared to what we
> > > > > currently
> > > > > do for NUMA configuration ?
> > > > From RHBZ1382425
> > > > "
> > > > Current -numa CLI interface is quite limited in terms that allow map
> > > > CPUs to NUMA nodes as it requires to provide cpu_index values which
> > > > are non obvious and depend on machine/arch. As result libvirt has to
> > > > assume/re-implement cpu_index allocation logic to provide valid
> > > > values for -numa cpus=... QEMU CLI option.
> > > 
> > > In broad terms, this problem applies to every device / object libvirt
> > > asks QEMU to create. For everything else libvirt is able to assign a
> > > "id" string, which is can then use to identify the thing later. The
> > > CPU stuff is different because libvirt isn't able to provide 'id'
> > > strings for each CPU - QEMU generates a psuedo-id internally which
> > > libvirt has to infer. The latter is the same problem we had with
> > > devices before '-device' was introduced allowing 'id' naming.
> > > 
> > > IMHO we should take the same approach with CPUs and start modelling
> > > the individual CPUs as something we can explicitly create with -object
> > > or -device. That way libvirt can assign names and does not have to
> > > care about CPU index values, and it all works just the same way as
> > > any other devices / object we create
> > > 
> > > ie instead of:
> > > 
> > >   -smp 8,sockets=4,cores=2,threads=1
> > >   -numa node,nodeid=0,cpus=0-3
> > >   -numa node,nodeid=1,cpus=4-7
> > > 
> > > we could do:
> > > 
> > >   -object numa-node,id=numa0
> > >   -object numa-node,id=numa1
> > >   -object cpu,id=cpu0,node=numa0,socket=0,core=0,thread=0
> > >   -object cpu,id=cpu1,node=numa0,socket=0,core=1,thread=0
> > >   -object cpu,id=cpu2,node=numa0,socket=1,core=0,thread=0
> > >   -object cpu,id=cpu3,node=numa0,socket=1,core=1,thread=0
> > >   -object cpu,id=cpu4,node=numa1,socket=2,core=0,thread=0
> > >   -object cpu,id=cpu5,node=numa1,socket=2,core=1,thread=0
> > >   -object cpu,id=cpu6,node=numa1,socket=3,core=0,thread=0
> > >   -object cpu,id=cpu7,node=numa1,socket=3,core=1,thread=0
> > the follow up question would be where do "socket=3,core=1,thread=0"
> > come from, currently these options are the function of
> > (-M foo -smp ...) and can be queried vi query-hotpluggble-cpus at
> > runtime after qemu parses -M and -smp options.
> 
> NB, I realize my example was open to mis-interpretation. The values I'm
> illustrating here for socket=3,core=1,thread=0 and *not* ID values, they
> are a plain enumeration of values. ie this is saying the 4th socket, the
> 2nd core and the 1st thread.  Internally QEMU might have the 2nd core
> with a core-id of 8, or 7038 or whatever architecture specific numbering
> scheme makes sense, but that's not what the mgmt app gives at the CLI
> level
Even though fixed properties/values simplicity is tempting and it might even
work for what we have implemented in qemu currently (well, SPAPR will need
refactoring (if possible) to meet requirements + compat stuff for current
machines with sparse IDs).
But I have to disagree here and try to oppose it.

QEMU models concrete platforms/hw with certain non abstract properties
and it's libvirt's domain to translate platform specific devices into
'spherical' devices with abstract properties.

Now back to cpus and suggestion to fix the set of 'address' properties
and their values into continuous enumeration range [0..N). That would
  1. put a burden of hiding platform/device details on QEMU
      (which is already bad as QEMU's job is to emulate it)
  2. with abstract 'address' properties and values, user won't have
     a clue as to where device is being attached (as qemu would magically
     remap that to fit specific machine needs)
  2.1. if abstract 'address' properties and values we can do away with
     socket/core/thread/whatnot since they won't mean the same when considered
     from platform point of view, so we can just drop all these nonsense
     and go back to cpu-index that has all the properties you've suggested
     /abstract, [0..N]/.
  3. we currently stopped with socket|core|thread-id properties as they are
     applicable to machines that support -device cpu, but it's up to machine
     to pick witch of these to use (x86: uses all, spar: uses core-id only),
     but current property set is open for extension if need arises without
     need to redefine interface. So fixed list of properties [even ignoring
     values impact] doesn't scale.

We even have cpu-add command which takes cpu-index as argument and
-numa node,cpus=0..X CLI option, good luck with figuring out which cpu goes
where and if it makes any sense from platform point of view.

That's why when designing hot plug for 'device_add cpu' interface, we ended up
with new query-hotpluggble-cpus QMP command, which is currently used by libvirt
for hot-plug:

Approach allows 
   1: machine to publish properties/values that make sense from emulated
      platform point of view but still understandable by user of given hw.
   2: user may use them as opaque mandatory properties to create cpu device if
      he/she doesn't care about where it's plugged.
   3: if user cares about which cpu goes where, properties defined by machine
      provide that info from emulated hw point of view including platform 
specific
      details.
   4: it's easy to extend set of properties/values if need arises without
      breaking users (provided user will put them all in -device/device_add
      options as it's supposed to)

But current approach has drawback, to call query-hotpluggble-cpus, machine has 
to
be started first, which is fine for hot plug but not for specifying CLI options.

Currently that could be solved by starting qemu twice when 'defining domain',
where on the first run mgmt queries board layout and caches it for all the next
times the defined machine is started (change in machine/version/-smp/-cpu will
invalidate, cache).

This series allows to avoid this 1st time restart, when creating domain for
the first time, mgmt can query layout and then specify numa mapping without
restarting, it can cache defined mapping as commands exactly match corresponding
CLI options and reuse cached options on the next domain starts.

This approach could be extended further with "device_add cpu" command
so it would be possible to start qemu with -smp 0,... and allow mgmt to
create cpus with explicit IDs controlled by mgmt, and again mgmt may cache
these commands and reuse them on CLI next time machine is started.

I think Eduardo's work on query-slots is superset of query-hotpluggble-cpus,
but working to the same goal to allow mgmt discover which hw is provided by
specific machine and where/which hw could be plugged (like which slot supports
which kind of device and which 'address' should be used to attach device
(socket|core... - for cpus, bus/function - for pic, ...)
 
> Regards,
> Daniel
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