Collin Walling <wall...@linux.ibm.com> writes:
> On 7/24/24 3:56 AM, Markus Armbruster wrote:
>> Collin Walling <wall...@linux.ibm.com> writes:
> Let me try to explain the purpose of @deprecated-props and see if it
> helps bring us closer to some semblance of a mutual understanding so we
> can work together on a concise documentation for this field.
>
> s390 has been announcing features as deprecated for some time now, which
> was fine as a way to let users know that they should tune their guests
> to no longer user these features. Now that we are approaching the
> release of generations that will drop these deprecated features
> outright, we encounter an issue: if users have not been mindful with
> disabling these announced-deprecated-features, then their guests running
> on older models will not be able to migrate to machines running on newer
> hardware.
>
> To alleviate this, I've added the @deprecated-props array to the
> CpuModelInfo struct, and this field is populated by a
> query-cpu-model-expansion* return. It is up the the user/management app
> to make use of this data.
>
> On the libvirt side (currently in development), I am able to easily
> retrieve the host-model with a full expansion, parse the
> @deprecated-props, and then cache them for later use (e.g. when
> reporting the host-model with these features disabled, or enabling a
> user to define their domain with deprecated-features disabled via a
> convenient XML attribute).
>
> tl;dr @deprecated-props is only reported via a
> query-cpu-model-expansion, and it is up to the user/management app to
> figure out what to do with them.
Got it.
Permit me a digression. In QAPI/QMP, we do something similar: we expose
deprecation in introspection (query-qmp-schema), and what to do with the
information is up to the management application. We provide one more
tool to it: policy for handling deprecated interfaces, set with -compat.
It permits "testing the future". See qapi/compat.json for details.
Whether such a thing would be usful in your case I can't say.
>> On closer examination, more questions on CpuModelInfo emerge. Uses:
>>
>
> I will attempt to expand on each input @model (CpuModelInfo) as if they
> were documented in the file.
>
>> * query-cpu-model-comparison both arguments
>>
>> Documentation doesn't say how exactly the command uses the members of
>> CpuModelInfo, i.e. @name, @props, @deprecated-props. Can you tell me?
>>
>
> Note: Compares ModelA and ModelB.
>
> Both @models must include @name. @props is optional. @deprecated-props
> is ignored.
>
> @name: the name of the CPU model definition to look up. The definition
> will be compared against the generation, GA level, and a static set of
> properties of the opposing model.
>
> @props: a set of additional properties to include in the model's set of
> properties to be compared.
>
> @deprecated-props: ignored. The user should consider these properties
> beforehand and decide if these properties should be disabled/omitted on
> the respective model.
>
>> * query-cpu-model-expansion argument @model and return value member
>> @model.
>>
>> The other argument is the expansion type, on which the value of return
>> value model.deprecated-props depends, I believe. Fine.
>>
>> Documentation doesn't say how exactly the command uses the members of
>> CpuModelInfo arguments, i.e. @name, @props, @deprecated-props. Can
>> you tell me?
>>
>
> The @model must include @name. @props is optional. @deprecated-props
> is ignored.
>
> @name: the name of the CPU model definition to look up. The definition
> is associated with a set of properties that will populate the return data.
>
> @props: a set of additional properties to include in the model's set of
> expanded properties.
>
> @deprecated-props: ignored. The user should consider these properties
> beforehand and decide if these properties should be disabled/omitted on
> the model.
Return value member @model will have @name, may have @props and
@deprecated-props.
Absent @props is the same as {}. Only x86 uses {}.
Absent @deprecated-props is the same as {}. No target uses {}. Can be
present only on S390.
Aside: returning the same thing in two different ways, like absent and
{}, is slightly more complex than necessary. But let's ignore that
here.
>> * query-cpu-model-baseline both arguments and return value member
>> @model.
>>
>> Same, except we don't have an expansion type here. So same question,
>> plus another one: how does return value model.deprecated-props behave?
>>
>
> Note: Creates a baseline model based on ModelA and ModelB.
>
> The @models must include @name. @props is optional. @deprecated-props
> is ignored.
>
> @name: the name of the CPU model definition to look up. The definition,
> GA level, and a static set of properties will be used to determine the
> maximum model between ModelA and ModelB.
>
> @props: a set of additional properties to include in the model's set of
> properties to be baselined.
>
> @deprecated-props: ignored. The user should consider these properties
> beforehand and decide if these properties should be disabled/omitted on
> the respective model.
Return value member @model is just like in query-cpu-model-expansion.
Unlike query-cpu-model-expansion, we don't have an expansion type. The
value of @deprecated-props depends on the expansion type. Do we assume
a type? Which one?
>> If you can't answer my questions, we need to find someone who can.
>>
>
> Hopefully this provides clarity on how CpuModelInfo and its respective
> fields are used in each command. @David should be able to fill in any
> missing areas / expand / offer corrections.
>
>> [...]
This helps, thanks!
Arguments that are silently ignored is bad interface design.
Observe: when CpuModelInfo is an argument, @deprecated-props is always
ignored. When it's a return value, absent means {}, and it can be
present only for certain targets (currently S390).
The reason we end up with an argument we ignore is laziness: we use the
same type for both roles. We can fix that easily:
{ 'struct': 'CpuModel',
'data': { 'name': 'str',
'*props': 'any' } }
{ 'struct': 'CpuModelInfo',
'base': 'CpuModel',
'data': { '*deprecated-props': ['str'] } }
Use CpuModel for arguments, CpuModelInfo for return values.
Since @deprecated-props is used only by some targets, I'd make it
conditional, i.e. 'if': 'TARGET_S390X'.
Thoughts?
