Re: [RFC PATCH] qapi: Incorrect attempt to fix building with MC146818RTC=n

[Paolo Bonzini <pbonzini@redhat.com>]

On 21/01/20 06:49, Markus Armbruster wrote:
> Paolo Bonzini <pbonzini@redhat.com> writes:
> 
>> On 13/01/20 15:01, Markus Armbruster wrote:
>>> Philippe Mathieu-Daudé <philmd@redhat.com> writes:
>>>
>>>> When configured with --without-default-devices and setting
>>>> MC146818RTC=n, the build fails:
>>>>
>>>>     LINK    x86_64-softmmu/qemu-system-x86_64
>>>>   /usr/bin/ld: qapi/qapi-commands-misc-target.o: in function `qmp_marshal_rtc_reset_reinjection':
>>>>   qapi/qapi-commands-misc-target.c:46: undefined reference to `qmp_rtc_reset_reinjection'
>>>>   /usr/bin/ld: qapi/qapi-commands-misc-target.c:46: undefined reference to `qmp_rtc_reset_reinjection'
>>>>   collect2: error: ld returned 1 exit status
>>>>   make[1]: *** [Makefile:206: qemu-system-x86_64] Error 1
>>>>   make: *** [Makefile:483: x86_64-softmmu/all] Error 2
>>>>
>>>> This patch tries to fix this, but this is incorrect because QAPI
>>>> scripts only provide TARGET definitions, so with MC146818RTC=y we
>>>> get:
>>>>
>>>>   hw/rtc/mc146818rtc.c:113:6: error: no previous prototype for ‘qmp_rtc_reset_reinjection’ [-Werror=missing-prototypes]
>>>>     113 | void qmp_rtc_reset_reinjection(Error **errp)
>>>>         |      ^~~~~~~~~~~~~~~~~~~~~~~~~
>>>>   cc1: all warnings being treated as errors
>>>>   make[1]: *** [rules.mak:69: hw/rtc/mc146818rtc.o] Error 1
>>>>
>>>> Any idea? :)
>>>>
>>>> Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
>>>> ---
>>>>  qapi/misc-target.json | 2 +-
>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/qapi/misc-target.json b/qapi/misc-target.json
>>>> index a00fd821eb..8e49c113d1 100644
>>>> --- a/qapi/misc-target.json
>>>> +++ b/qapi/misc-target.json
>>>> @@ -41,7 +41,7 @@
>>>>  #
>>>>  ##
>>>>  { 'command': 'rtc-reset-reinjection',
>>>> -  'if': 'defined(TARGET_I386)' }
>>>> +  'if': 'defined(TARGET_I386) && defined(CONFIG_MC146818RTC)' }
>>>>  
>>>>  
>>>>  ##
>>>
>>> The generated qapi-commands-misc-target.h duly has
>>>
>>>     #if defined(TARGET_I386) && defined(CONFIG_MC146818RTC)
>>>     void qmp_rtc_reset_reinjection(Error **errp);
>>>     void qmp_marshal_rtc_reset_reinjection(QDict *args, QObject **ret, Error **errp);
>>>     #endif /* defined(TARGET_I386) && defined(CONFIG_MC146818RTC) */
>>>
>>> mc146818rtc.c includes it.  But since it doesn't include
>>> config-devices.h, CONFIG_MC146818RTC remains undefined, and the
>>> prototype gets suppressed.
>>>
>>> Crude fix: make mc146818rtc.c #include "config-devices.h".
>>
>> Can we modify the code generator to leave out the #if from the header,
>> and only include it in the .c file?  An extra prototype is harmless.
> 
> Is *everything* we generate into headers just as harmless?

It should be, since it's just the C version of some JSON.  The only
problematic thing could be different definitions of the same command for
multiple targets, and I think we want to avoid that anyway.

To see it a different way, these are the "C bindings" to QMP, just that
the implementation is an in-process call rather than RPC.  If the QAPI
code generator was also able to generate Python bindings and the like,
they would have to be the same for all QEMU binaries, wouldn't they?

Paolo