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

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

On 1/21/20 3:22 PM, Markus Armbruster wrote:
> Paolo Bonzini <pbonzini@redhat.com> writes:
> 
>> 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".

I am happy enough if this 'crude fix' works, I'll test it, thanks!

>>>>
>>>> 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?
> 
> Ommitting the kind of #if we've been discussing is relatively harmless:
> 
>      #if defined(TARGET_I386)
>      void qmp_rtc_reset_reinjection(Error **errp);
>      void qmp_marshal_rtc_reset_reinjection(QDict *args, QObject **ret, Error **errp);
>      #endif /* defined(TARGET_I386) */
> 
> But what about this one, in qapi-types-block-core.h:
> 
>      typedef enum BlockdevDriver {
>          BLOCKDEV_DRIVER_BLKDEBUG,
>          [...]
>      #if defined(CONFIG_REPLICATION)
>          BLOCKDEV_DRIVER_REPLICATION,
>      #endif /* defined(CONFIG_REPLICATION) */
>          [...]
>          BLOCKDEV_DRIVER__MAX,
>      } BlockdevDriver;
> 
> If I omit it in the header, I then have to omit it in
> qapi-types-block-core.c's
> 
>      const QEnumLookup BlockdevDriver_lookup = {
>          .array = (const char *const[]) {
>              [BLOCKDEV_DRIVER_BLKDEBUG] = "blkdebug",
>              [...]
>      #if defined(CONFIG_REPLICATION)
>              [BLOCKDEV_DRIVER_REPLICATION] = "replication",
>      #endif /* defined(CONFIG_REPLICATION) */
>              [...]
>          },
>          .size = BLOCKDEV_DRIVER__MAX
>      };
> 
> and God knows what else.  But I must not omit it in qapi-introspect.c's
> 
>          QLIT_QDICT(((QLitDictEntry[]) {
>              { "meta-type", QLIT_QSTR("enum"), },
>              { "name", QLIT_QSTR("245"), },
>              { "values", QLIT_QLIST(((QLitObject[]) {
>                  QLIT_QSTR("blkdebug"),
>                  [...]
>      #if defined(CONFIG_REPLICATION)
>                  QLIT_QSTR("replication"),
>      #endif /* defined(CONFIG_REPLICATION) */
>                  [...]
>                  {}
>              })), },
>              {}
>          })),
> 
> because that would defeat introspection.
> 
> I smell a swamp.
> 
> I'd rather not complicate the generator to support not including a
> header I feel we *should* include.  #ifdef CONFIG_FOO can occur not just
> in QAPI-generated code, and neglecting to include the relevant header
> can cause *nasty* problems not just in QAPI-generated code.  Like
> inconsistent struct definitions in separate compilation units.  Been
> there, debugged that, wasn't fun, do not want to go there again.
>