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.
