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

Collin Walling
2019-07-02T19:50:22Z

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?