[PATCH 5/5] qemu: Prefer -numa cpu over -numa node,cpus=

[Peter Krempa]

On Wed, Oct 20, 2021 at 13:07:59 +0200, Michal Prívozník wrote:
> On 10/6/21 3:32 PM, Igor Mammedov wrote:
> > On Thu, 30 Sep 2021 14:08:34 +0200
> > Peter Krempa <pkrempa at redhat.com> wrote:

[...]

> 2) In my experiments I try to mimic what libvirt does. Here's my cmd
> line:
> 
> qemu-system-x86_64 \
> -S \
> -preconfig \
> -cpu host \
> -smp 120,sockets=2,dies=3,cores=4,threads=5 \
> -object '{"qom-type":"memory-backend-memfd","id":"ram-node0","size":4294967296,"host-nodes":[0],"policy":"bind"}' \
> -numa node,nodeid=0,memdev=ram-node0 \
> -no-user-config \
> -nodefaults \
> -no-shutdown \
> -qmp stdio
> 
> and here is my QMP log:
> 
> {"QMP": {"version": {"qemu": {"micro": 50, "minor": 1, "major": 6}, "package": "v6.1.0-1552-g362534a643"}, "capabilities": ["oob"]}}
> 
> {"execute":"qmp_capabilities"}
> {"return": {}}
> 
> {"execute":"query-hotpluggable-cpus"}
> {"return": [{"props": {"core-id": 3, "thread-id": 4, "die-id": 2, "socket-id": 1}, "vcpus-count": 1, "type": "host-x86_64-cpu"}, {"props": {"core-id": 3, "thread-id": 3, "die-id": 2, "socket-id": 1}, "vcpus-count": 1, "type": "host-x86_64-cpu"}, {"props": {"core-id": 3, "thread-id": 2, "die-id": 2, "socket-id": 1}, "vcpus-count": 1, "type": "host-x86_64-cpu"}, {"props": {"core-id": 3, "thread-id": 1, "die-id": 2, "socket-id": 1}, "vcpus-count": 1, "type": "host-x86_64-cpu"}, {"props": {"core-id": 3, "thread-id": 0, "die-id": 2, "socket-id": 1}, "vcpus-count": 1, "type": "host-x86_64-cpu"}, {"props": {"core-id": 2, "thread-id": 4, "die-id": 2, "socket-id": 1}, "vcpus-count": 1, "type": "host-x86_64-cpu"},
> <snip/>
> {"props": {"core-id": 0, "thread-id": 0, "die-id": 0, "socket-id": 0}, "vcpus-count": 1, "type": "host-x86_64-cpu"}]}
> 
> 
> I can see that query-hotpluggable-cpus returns an array. Can I safely
> assume that vCPU ID == index in the array? I mean, if I did have -numa

No, this assumption would be incorrect on the aforementioned PPC
platform where one entry in the returned array can describe multiple
cores.

qemuDomainFilterHotplugVcpuEntities is the code that cross-references
the libvirt "index" with the data returned by query-hotpluggable cpus.

The important bit is the 'vcpus-count' property. The code which deals
with hotplug is already fetching everything that's needed.

> node,cpus=X  can I do  array[X] to obtain mapping onto Core/Thread/
> Die/Socket which would then be fed to 'set-numa-node' command. If not,
> what is the proper way to do it?