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

[Igor Mammedov]

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.

It's either mgtg asks qemu for values or it duplicates each board
logic (including compat hacks per machine version) to be able
generate values/properties on it's own.


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