Re: [Qemu-devel] [RFC PATCH 2/2] numa: Add node_id data in query-hotpluggable-cpus

[Peter Krempa <pkrempa@redhat.com>]

On Fri, Jul 08, 2016 at 12:23:08 +1000, David Gibson wrote:
> On Thu,  7 Jul 2016 17:17:14 +0200
> Peter Krempa <pkrempa@redhat.com> wrote:
> 
> > Add a helper that looks up the NUMA node for a given CPU and use it to
> > fill the node_id in the PPC and X86 impls of query-hotpluggable-cpus.
> 
> 
> IIUC how the query thing works this means that the node id issued by
> query-hotpluggable-cpus will be echoed back to device add by libvirt.

It will be echoed back, but the problem is that it's configured
separately ...

> I'm not sure we actually process that information in the core at
> present, so I don't know that that's right.
> 
> We need to be clear on which direction information is flowing here.
> Does query-hotpluggable-cpus *define* the NUMA node allocation which is
> then passed to the core device which implements it.  Or is the NUMA
> allocation defined elsewhere, and query-hotpluggable-cpus just reports
> it.

Currently (in the pre-hotplug era) the NUMA topology is defined by
specifying a CPU numbers (see previous patch) on the commandline:

-numa node=1,cpus=1-5,cpus=8,cpus=11...

This is then reported to the guest.

For a machine started with:

-smp 5,maxcpus=8,sockets=2,cores=2,threads=2
-numa node,nodeid=0,cpus=0,cpus=2,cpus=4,cpus=6,mem=500
-numa node,nodeid=1,cpus=1,cpus=3,cpus=5,cpus=7,mem=500

you get the following topology that is not really possible with
hardware:

# lscpu
Architecture:          x86_64
CPU op-mode(s):        32-bit, 64-bit
Byte Order:            Little Endian
CPU(s):                5
On-line CPU(s) list:   0-4
Thread(s) per core:    1
Core(s) per socket:    2
Socket(s):             2
NUMA node(s):          2
Vendor ID:             GenuineIntel
CPU family:            6
Model:                 6
Model name:            QEMU Virtual CPU version 2.5+
Stepping:              3
CPU MHz:               3504.318
BogoMIPS:              7008.63
Hypervisor vendor:     KVM
Virtualization type:   full
L1d cache:             32K
L1i cache:             32K
L2 cache:              4096K
NUMA node0 CPU(s):     0,2,4
NUMA node1 CPU(s):     1,3

Note that the count of cpus per numa node does not need to be identical.

As of the above 'query-hotpluggable-cpus' will need to report the data
that was configured above even if it doesn't make much sense in a real
world.

I did not try the above on a PPC host and thus I'm not sure whether the
config above is allowed or not.

While for the hotplug cpus it would be possible to plug in the correct
one according to the requested use the queried data but with the current
approach it's impossible to set the initial vcpus differently.

Peter

Note: For libvirt it's a no-go to start a throwaway qemu process just to
query the information and thus it's desired to have a way to configure
all this without the need to query with a specific machine
type/topology.