Re: [PATCH v2] target/s390x: filter deprecated properties based on model expansion type

[Markus Armbruster]

Thomas Huth <thuth@redhat.com> writes:

> On 18/07/2024 20.22, Collin Walling wrote:
>> On 7/18/24 9:39 AM, Markus Armbruster wrote:
>>> Collin Walling <walling@linux.ibm.com> writes:
>>>
>>>> As s390 CPU models progress and deprecated properties are dropped
>>>> outright, it will be cumbersome for management apps to query the host
>>>> for a comprehensive list of deprecated properties that will need to be
>>>> disabled on older models. To remedy this, the query-cpu-model-expansion
>>>> output now behaves by filtering deprecated properties based on the
>>>> expansion type instead of filtering based off of the model's full set
>>>> of features:
>>>>
>>>> When reporting a static CPU model, only show deprecated properties that
>>>> are a subset of the model's enabled features.
>>>>
>>>> When reporting a full CPU model, show the entire list of deprecated
>>>> properties regardless if they are supported on the model.
>>>>
>>>> Suggested-by: Jiri Denemark <jdenemar@redhat.com>
>>>> Signed-off-by: Collin Walling <walling@linux.ibm.com>

[...]

>>>> diff --git a/qapi/machine-target.json b/qapi/machine-target.json
>>>> index a8d9ec87f5..d151504f25 100644
>>>> --- a/qapi/machine-target.json
>>>> +++ b/qapi/machine-target.json
>>>> @@ -21,8 +21,12 @@
>>>>   # @props: a dictionary of QOM properties to be applied
>>>>   #
>>>>   # @deprecated-props: a list of properties that are flagged as deprecated
>>>> -#     by the CPU vendor.  These props are a subset of the full model's
>>>> -#     definition list of properties. (since 9.1)
>>>> +#     by the CPU vendor.  (since 9.1).
>>>> +#
>>>> +# .. note:: Since 9.1, the list of deprecated props were always a subset
>>>> +#    of the model's full-definition list of properites. Now, this list is
>>>> +#    populated with the model's enabled property set when delta changes
>>>> +#    are applied. All deprecated properties are reported otherwise.
>>>
>>> I'm confused.
>>>
>>> "Since 9.1, the list of deprecated props were ..." and "Now, this list
>>> is" sounds like you're explaining behavior before and after a change.
>>> What change?  Since only released behavior matters, and
>>> @deprecated-props is new, there is no old behavior to document, isn't
>>> it?
>>
>> I admittedly had some difficulty articulating the change introduced by
>> this patch.  The @deprecated-props array, as well as a way for s390x to
>> populate it, was introduced in release 9.1.  Prior to this patch, the
>> deprecated-props list was filtered by the CPU model's full feature set.
>> I attempted to explain this with:
>> "Since 9.1, the list of deprecated props were always a subset of the
>> model's full-definition list of properties."
>
> Version 9.1 has not been released yet (see 
> https://wiki.qemu.org/Planning/9.1), so I agree with Markus, this sounds 
> confusing/wrong to me, too.

User-visible changes between releases need to be documented in release
notes and the manual.  Don't for changes within a release, instead
update documentation to reflect the new state of things.

Regardless, do explain the change in the commit message.

Specifics are more helpful than such generalities, so let me try despite
my relative ignorance of the subject matter.

1. Update the description of @deprecated-props to match the new code.
Ask yourself what people need to know to use this interface.

2. Explain in the commit message how semantics of @deprecated-props
change in this patch.