Re: [Qemu-devel] [PATCH v1] s390x/cpu_models: Add "-cpu max" support

[David Hildenbrand]

On 25.07.2018 13:36, Cornelia Huck wrote:
> On Wed, 25 Jul 2018 11:12:33 +0200
> David Hildenbrand <address@hidden> wrote:
> 
>> The "max" CPU model behaves like "-cpu host" when KVM is enabled, and like
>> a CPU with the maximum possible feature set when TCG is enabled.
>>
>> While the "host" model can not be used under TCG ("kvm_required"), the
>> "max" model can and "Enables all features supported by the accelerator in
>> the current host".
>>
>> So we can treat "host" just as a special case of "max" (like x86 does).
>> It differs to the "qemu" CPU model under TCG such that compatibility
>> handling will not be performed and that some experimental CPU features
>> not yet part of the "qemu" model might be indicated.
>>
>> These are right now under TCG (see "qemu_MAX"):
>> - stfle53
> 
> That's a z13 feature, so I think it's fine as we don't care about
> machine generations for the max mode anyway, correct?

Yes the max model really just is "give me anything you got and that you
can expand to a static model (i.e. fully specify on the command line)".

> 
>> - msa5-base
> 
> That's just the warning, but as things are continuing to work, it's
> fine as well.

Yes, as these are z13 models we don't but them yet into our "stable"
qemu model which is based on a z12.

> 
>> - zpci
> 
> That one theoretically has a dependency on CONFIG_PCI, but as that is
> always set, I think it's fine as well.

That is even already handled correctly, see
        target/s390x/cpu_models.c:register_types()

1314 #ifndef CONFIG_USER_ONLY
1315     if (!pci_available) {
1316         clear_bit(S390_FEAT_ZPCI, qemu_max_cpu_feat);
1317     }
1318 #endif

[...]

> Maybe add a comment that for kvm we try the host model, and only that
> can fail (i.e., for tcg this will always work)?

"we expect only errors under KVM, when we actually query the kernel"

> 
>> +        error_report_err(local_err);
>> +        /* fallback to unsupported CPU models */
>> +        return;
>> +    }
>> +
>> +    cpu->model = g_new(S390CPUModel, 1);
>> +    /* copy the CPU model so we can modify it */
>> +    memcpy(cpu->model, max_model, sizeof(*cpu->model));
>> +}
>> +
>>  static void s390_cpu_model_finalize(Object *obj)
>>  {
>>      S390CPU *cpu = S390_CPU(obj);
> 
> Looks sane to me.
> 

Thanks!

-- 

Thanks,

David / dhildenb