Re: [PATCH v3] target/s390x: filter deprecated properties based on model expansion type

[Collin Walling <walling@linux.ibm.com>]

On 7/25/24 3:39 AM, David Hildenbrand wrote:
> On 25.07.24 09:35, Markus Armbruster wrote:
>> Markus Armbruster <armbru@redhat.com> writes:
>>
>>> Collin Walling <walling@linux.ibm.com> writes:
>>>
>>>> On 7/24/24 3:56 AM, Markus Armbruster wrote:
>>>>> Collin Walling <walling@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'.
>>
>> If we want just query-cpu-model-expansion return deprecated properties,
>> we can instead move @deprecated-props from CpuModelInfo to
>> CpuModelExpansionInfo.
> 
> That might a bit more sense, because deprecated-props does not make any 
> sense as input parameter, for example.
> 

Will do.  Thanks for the feedback.  v4 in the works.

-- 
Regards,
  Collin