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[2003:cb:c706:2c00:5f6:3120:375a:3e4]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-427f9372a3fsm63111605e9.13.2024.07.25.00.39.41 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 25 Jul 2024 00:39:42 -0700 (PDT) Message-ID: Date: Thu, 25 Jul 2024 09:39:40 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3] target/s390x: filter deprecated properties based on model expansion type To: Markus Armbruster Cc: Collin Walling , qemu-s390x@nongnu.org, qemu-devel@nongnu.org, thuth@redhat.com, wangyanan55@huawei.com, philmd@linaro.org, marcel.apfelbaum@gmail.com, eduardo@habkost.net, Jiri Denemark References: <20240719181741.35146-1-walling@linux.ibm.com> <87h6cksk4h.fsf@pond.sub.org> <28ea8260-a411-4651-8e2a-1fcc009f5043@linux.ibm.com> <87bk2nrzou.fsf@pond.sub.org> <9f8023a4-3edd-476f-9243-677138be3921@linux.ibm.com> <87cyn2ugyi.fsf@pond.sub.org> <87v80tudo7.fsf@pond.sub.org> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=170.10.133.124; envelope-from=david@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -21 X-Spam_score: -2.2 X-Spam_bar: -- X-Spam_report: (-2.2 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.136, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On 25.07.24 09:35, Markus Armbruster wrote: > Markus Armbruster writes: > >> Collin Walling writes: >> >>> On 7/24/24 3:56 AM, Markus Armbruster wrote: >>>> Collin Walling 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. -- Cheers, David / dhildenb