Re: Default CPU models on s390x and ppc64

[David Hildenbrand]

> > I don't see how the default cpu model could not be "host" if qemu was
> > started with "--enable-kvm"
>
> I guess the problem could be that QEMU capabilities probing in libvirt
> is done with -machine none. We first probe with KVM enabled and then for
> a few specific commands reprobe with TCG only. We could do it with
> query-machines too to get different default CPU models depending on the
> accelerator, but it wouldn't actually help.
>
> The full command line used for capabilities probing is
>
>      qemu-system-s390x \
>          -S \
>          -no-user-config \
>          -nodefaults \
>          -nographic \
>          -machine none,accel=kvm:tcg \
>          -qmp unix:/tmp/qmp-ivG4UN/qmp.monitor,server,nowait \
>          -pidfile /tmp/qmp-ivG4UN/qmp.pid \
>          -daemonize
>
> One of the command we send is
>
>      {"execute":"query-kvm","id":"libvirt-5"}
>
> and the reply is
>
>      {"return": {"enabled": true, "present": true}, "id": "libvirt-5"}

Right.

> Which means KVM is usable and enabled, but still query-machines returns
>
>      {
>        "return": [
>          {
>            "hotpluggable-cpus": true,
>            "name": "s390-ccw-virtio-4.0",
>            "numa-mem-supported": false,
>            "default-cpu-type": "qemu-s390x-cpu",
>            "cpu-max": 248,
>            "deprecated": false
>          },
>          {
>            "hotpluggable-cpus": true,
>            "name": "s390-ccw-virtio-3.1",
>            "numa-mem-supported": false,
>            "default-cpu-type": "qemu-s390x-cpu",
>            "cpu-max": 248,
>            "deprecated": false
>          },
>          ...
>        ]
>      }
>
> So it seems the code is run during the machine initialization and thus
> it doesn't affect what query-machines returns with -machine none.

I can see what's going on. kvm_arch_init() will only modify the single, 
current machine, not all machines. That means, we would actually have to 
overwrite the default model in all machines, not just the one we are 
starting with (here:none).

Hm....

--

Thanks,

David / dhildenb