Re: [PATCH v13 0/7] s390x: CPU Topology

[Thomas Huth]

On 08/12/2022 10.44, Pierre Morel wrote:
> Hi,
>
> Implementation discussions
> ==========================
>
> CPU models
> ----------
>
> Since the S390_FEAT_CONFIGURATION_TOPOLOGY is already in the CPU model
> for old QEMU we could not activate it as usual from KVM but needed
> a KVM capability: KVM_CAP_S390_CPU_TOPOLOGY.
> Checking and enabling this capability enables
> S390_FEAT_CONFIGURATION_TOPOLOGY.
>
> Migration
> ---------
>
> Once the S390_FEAT_CONFIGURATION_TOPOLOGY is enabled in the source
> host the STFL(11) is provided to the guest.
> Since the feature is already in the CPU model of older QEMU,
> a migration from a new QEMU enabling the topology to an old QEMU
> will keep STFL(11) enabled making the guest get an exception for
> illegal operation as soon as it uses the PTF instruction.

I now thought that it is not possible to enable "ctop" on older QEMUs since 
the don't enable the KVM capability? ... or is it still somehow possible? 
What did I miss?

 Thomas


> A VMState keeping track of the S390_FEAT_CONFIGURATION_TOPOLOGY
> allows to forbid the migration in such a case.
>
> Note that the VMState will be used to hold information on the
> topology once we implement topology change for a running guest.