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

[Markus Armbruster]

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.

> Thoughts?