On 29/11/2022 18.42, Pierre Morel wrote:
The KVM capability, KVM_CAP_S390_CPU_TOPOLOGY is used to activate the S390_FEAT_CONFIGURATION_TOPOLOGY feature and the topology facility for the guest in the case the topology is available in QEMU and in KVM. The feature is fenced for SE (secure execution).
Out of curiosity: Why does it not work yet?
To allow smooth migration with old QEMU the feature is disabled by default using the CPU flag -disable-topology.
I stared at this code for a while now, but I have to admit that I don't quite get it. Why do we need a new "disable" feature flag here? I think it is pretty much impossible to set "ctop=on" with an older version of QEMU, since it would require the QEMU to enable KVM_CAP_S390_CPU_TOPOLOGY in the kernel for this feature bit - and older versions of QEMU don't set this capability yet.
Which scenario would fail without this disable-topology feature bit? What do I miss?
Making the S390_FEAT_CONFIGURATION_TOPOLOGY belonging to the default features makes the -ctop CPU flag is no more necessary,
too many verbs in this sentence ;-)
turning the topology feature on is done with -disable-topology only.
... Thomas
