Markus Armbruster <arm...@redhat.com> writes: > 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'.
If we want just query-cpu-model-expansion return deprecated properties, we can instead move @deprecated-props from CpuModelInfo to CpuModelExpansionInfo. > Thoughts?