Re: [PATCH v12 6/7] s390x/cpu_topology: activating CPU topology

[Thomas Huth <thuth@redhat.com>]

On 01/12/2022 12.52, Pierre Morel wrote:
> 
> 
> On 12/1/22 11:15, Thomas Huth wrote:
>> 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?
> 
> The only scenario it provides is that ctop is then disabled by default on 
> newer QEMU allowing migration between old and new QEMU for older machine 
> without changing the CPU flags.
> 
> Otherwise, we would need -ctop=off on newer QEMU to disable the topology.

Ah, it's because you added S390_FEAT_CONFIGURATION_TOPOLOGY to the default 
feature set here:

  static uint16_t default_GEN10_GA1[] = {
      S390_FEAT_EDAT,
      S390_FEAT_GROUP_MSA_EXT_2,
+    S390_FEAT_DISABLE_CPU_TOPOLOGY,
+    S390_FEAT_CONFIGURATION_TOPOLOGY,
  };

?

But what sense does it make to enable it by default, just to disable it by 
default again with the S390_FEAT_DISABLE_CPU_TOPOLOGY feature? ... sorry, I 
still don't quite get it, but maybe it's because my sinuses are quite 
clogged due to a bad cold ... so if you could elaborate again, that would be 
very appreciated!

However, looking at this from a distance, I would not rather not add this to 
any default older CPU model at all (since it also depends on the kernel to 
have this feature enabled)? Enabling it in the host model is still ok, since 
the host model is not migration safe anyway.

  Thomas