Re: [PATCH 09/13] qapi: transform target specific 'if' in runtime checks

[Pierrick Bouvier <pierrick.bouvier@linaro.org>]

On 5/13/25 12:08 AM, Markus Armbruster wrote:
> Pierrick Bouvier <pierrick.bouvier@linaro.org> writes:
> 
>> On 5/9/25 11:57 PM, Markus Armbruster wrote:
>>>> The build system would need generate an input document for the
>>>> QAPI visitor that defines whether each constant is set to true
>>>> or false, based on suitable CONFIG/TARGET conditions from meson.
>>>
>>> I think the conditions that are evaluated at build time in handwritten C
>>> code (with #if) should also be evaluated at build time in generated C
>>> code.
>>>
>>> Certain conditions are evaluated at build time in target-specific code,
>>> and at runtime in target-independent code.  Again, I think handwritten
>>> and generated code should work the same way.
>>>
>>> Thus, to eliminate target-specific QAPI-generated code, we either
>>> evaluate them at runtime, or simply eliminate them.  Elsewhere, we've
>>> come to the conclusion (I think) that the latter should do at least for
>>> now, likely forever, so we should try that first.
>>>
>>
>> I'm not sure if you mean you'd prefer to eradicate #if completely.
> 
> I do not!
> 
>> We have to keep in mind that some config host #if have to stay there, or
>> they expose things that the rest of QEMU code is not supposed to see
>> (hidden under those same CONFIG_ ifdef also).
> 
> Letting people configure their QEMU build is useful and must stay.
> 
> We provide this via conditional compilation, of complete source files
> (done in meson), as well as within source files (#if in C and 'if' in
> QAPI).
> 
>> So we would need both if and runtime_if.
> 
> I don't understand the need for runtime_if.  Can you give an example?
>

That is the point of this whole series, which explores introducing a 
'runtime' if in the schema, to keep it as it is today, while removing 
target specific compile time defines.
It is another approach that one Daniel followed on his series.

I invite you to give a quick read to this series, especially the related 
commit introducing 'runtime_if' 
(20250507231442.879619-2-pierrick.bouvier@linaro.org).

As well, I would appreciate if you could state clearly where we are 
going with all this (or at least, where you *don't* want this to go), so 
we can avoid spending time in the wrong direction.

I am ok to pursue Daniel's approach, or continue the approach in the 
current series, no strong opinion in my side, I am just trying to move 
QAPI generated code out of the way for the single binary goal.

Thanks,
Pierrick