Re: [PATCH v3 2/4] s390x: kvm: topology: interception of PTF instruction

[Pierre Morel <pmorel@linux.ibm.com>]

On 10/14/21 10:09, Pierre Morel wrote:
> 
> 
> On 10/13/21 11:11, Thomas Huth wrote:
>> On 13/10/2021 09.55, Pierre Morel wrote:
>>>
>>>
>>> On 10/13/21 09:25, Thomas Huth wrote:
>>>> On 16/09/2021 15.50, Pierre Morel wrote:
>>>>> When the host supports the CPU topology facility, the PTF
>>>>> instruction with function code 2 is interpreted by the SIE,
>>>>> provided that the userland hypervizor activates the interpretation
>>>>> by using the KVM_CAP_S390_CPU_TOPOLOGY KVM extension.
>>>>>
>>>>> The PTF instructions with function code 0 and 1 are intercepted
>>>>> and must be emulated by the userland hypervizor.
>>>>>
>>>>> Signed-off-by: Pierre Morel <pmorel@linux.ibm.com>
>>>>> ---
>> ...
>>>>> diff --git a/target/s390x/kvm/kvm.c b/target/s390x/kvm/kvm.c
>>>>> index 5b1fdb55c4..dd036961fe 100644
>>>>> --- a/target/s390x/kvm/kvm.c
>>>>> +++ b/target/s390x/kvm/kvm.c
>>>>> @@ -97,6 +97,7 @@
>>>>>   #define PRIV_B9_EQBS                    0x9c
>>>>>   #define PRIV_B9_CLP                     0xa0
>>>>> +#define PRIV_B9_PTF                     0xa2
>>>>>   #define PRIV_B9_PCISTG                  0xd0
>>>>>   #define PRIV_B9_PCILG                   0xd2
>>>>>   #define PRIV_B9_RPCIT                   0xd3
>>>>> @@ -362,6 +363,7 @@ int kvm_arch_init(MachineState *ms, KVMState *s)
>>>>>       kvm_vm_enable_cap(s, KVM_CAP_S390_USER_SIGP, 0);
>>>>>       kvm_vm_enable_cap(s, KVM_CAP_S390_VECTOR_REGISTERS, 0);
>>>>>       kvm_vm_enable_cap(s, KVM_CAP_S390_USER_STSI, 0);
>>>>> +    kvm_vm_enable_cap(s, KVM_CAP_S390_CPU_TOPOLOGY, 0);
>>>>
>>>> Should this maybe rather be done in the last patch, to avoid a state 
>>>> where PTF is available, but STSI 15 is not implemented yet (when 
>>>> bisecting through these commits later)?
>>>>
>>>>   Thomas
>>>>
>>>
>>> Yes you are right, thanks.
>>
>> I'm also still a little bit surprised that there is really no 
>> migration code involved here yet. What if a guest gets started on a 
>> system with KVM_CAP_S390_CPU_TOPOLOGY support and later migrated to a 
>> system without KVM_CAP_S390_CPU_TOPOLOGY support? Is there already 
>> some magic in place that rejects such a migration? If not, the guest 
>> might first learn that it could use the PTF instruction, but suddenly 
>> it is then not available anymore? Does Linux cope right with PTF 
>> becoming unavailable during runtime? But even if it does, I think it's 
>> likely not in the sense of the architecture if certain instructions 
>> might disappear during runtime? Or do I miss something?
>>
>>   Thomas
>>
> 
> 
> I check on this and take the consequences.
> 
> Pierre
> 

I can use a solution using pre_save/postload migration entries to verify 
that both side of the migration use PTF and STSI_15 the same way.

Seems this direction OK ?

Regards,
Pierre

-- 
Pierre Morel
IBM Lab Boeblingen