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

[Laszlo Ersek]

On 10/17/17 17:07, Daniel P. Berrange 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.

Oh. This is the critical bit I've been missing.

Sorry about the noise I've made!

Thanks!
Laszlo


> 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
>