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

[Daniel P. Berrange]

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

(perhaps -device instead of -object above, but that's a minor detail)

Regards,
Daniel
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