Re: [PATCH v5 2/2] s390/kvm: diagnose 318 handling

[Christian Borntraeger <borntraeger@de.ibm.com>]

On 02.07.19 21:50, Collin Walling wrote:
> On 6/26/19 10:31 AM, David Hildenbrand wrote:
>> On 26.06.19 16:30, Collin Walling wrote:
>>> On 6/26/19 6:28 AM, Christian Borntraeger wrote:
>>>>
>>>>
>>>> On 26.06.19 11:45, David Hildenbrand wrote:
>>>>
>>>>>
>>>>> BTW. there is currently no mechanism to fake absence of diag318. Should
>>>>> we have one? (in contrast, for CMMA we have, which is also a CPU feature)
>>>>
>>>> Yes, we want to be able to disable diag318 via a CPU model feature. That actually
>>>> means that the kernel must not answer this if we disable it.
>>>>
>>> Correct. If the guest specifies diag318=off, then the instruction
>>> shouldn't be executed (it is fenced off in the kernel by checking the
>>> Read SCP Info bit).
>>
>> But the guest *could* execute it and not get an exception.
>>
> 
> IIUC, you're talking about the situation where QEMU supports diag318,
> but KVM does not. The worst case is the guest specifies diag318=on, and
> nothing will stop the guest from attempting to execute the instruction.
> 
> However, this is fenced in my QEMU patches. In (v5), I have this
> following snippet:
> 
> @@ -2323,6 +2345,13 @@ void kvm_s390_get_host_cpu_model(S390CPUModel *model, Error **errp)
>          KVM_S390_VM_CRYPTO_ENABLE_APIE)) {
>          set_bit(S390_FEAT_AP, model->features);
>      }
> +
> +    /* if KVM supports interception of diag318, then let's provide the bit */
> +    if (kvm_vm_check_attr(kvm_state, KVM_S390_VM_MISC,
> +        KVM_S390_VM_MISC_DIAG318)) {
> +        set_bit(S390_FEAT_DIAG318, model->features);
> +    }
> +
>      /* strip of features that are not part of the maximum model */
>      bitmap_and(model->features, model->features, model->def->full_feat,
>                 S390_FEAT_MAX);
> 
> If the guest specifies diag318=on, and KVM does *not* support emulation,
> then the following message will be observed:
> 
> qemu-system-s390x: Some features requested in the CPU model are not
> available in the configuration: diag318
> 
> and the guest will fail to start. Does this suffice?

But what happens if the guest ignores read scp info and executes diag318 anyway.
It should get a specification exception, but it does not instead diag318 is
executed.