Philippe Mathieu-Daudé <phi...@redhat.com> writes:
> On 6/10/21 1:06 PM, Markus Armbruster wrote:
>> Philippe Mathieu-Daudé <phi...@redhat.com> writes:
>>
>>> On 6/10/21 11:33 AM, Markus Armbruster wrote:
>>>> Eric Blake <ebl...@redhat.com> writes:
>>>>
>>>>> On Wed, Jun 09, 2021 at 08:49:54PM +0200, Philippe Mathieu-Daudé wrote:
>>>>>> In case we need to use QAPI types but no QAPI command / QAPI event
>>>>>> actually use them, the generated qmp_marshal_output() function will
>>>>>> trigger the compiler 'unused-function' warnings.
>>>>>> To prevent that, emit these functions inlined: the compiler will
>>>>>> ignore such unused functions.
>>>>>>
>>>>>> Signed-off-by: Philippe Mathieu-Daudé <phi...@redhat.com>
>>>>>> ---
>>>>>> RFC: No clue about QAPI...
>>>>>> Tested with GCC. If the compiler is picky we could use the 'unused'
>>>>>> function attribute.
>>>>>
>>>>> And I have no clue if clang will warn about an unused inline function.
>>>>> Going with the compiler attribute seems safer and just as easy to do
>>>>> in the same two-line change (remember, the "unused" attribute merely
>>>>> means "suppress warnings if I don't use this", and not "warn me if I
>>>>> use it in spite of calling it unused").
>>>>>
>>>>>> ---
>>>>>> scripts/qapi/commands.py | 4 ++--
>>>>>> 1 file changed, 2 insertions(+), 2 deletions(-)
>>>>>>
>>>>>> diff --git a/scripts/qapi/commands.py b/scripts/qapi/commands.py
>>>>>> index 0e13d510547..bbed776a909 100644
>>>>>> --- a/scripts/qapi/commands.py
>>>>>> +++ b/scripts/qapi/commands.py
>>>>>> @@ -91,8 +91,8 @@ def gen_call(name: str,
>>>>>> def gen_marshal_output(ret_type: QAPISchemaType) -> str:
>>>>>> return mcgen('''
>>>>>>
>>>>>> -static void qmp_marshal_output_%(c_name)s(%(c_type)s ret_in,
>>>>>> - QObject **ret_out, Error **errp)
>>>>>> +static inline void qmp_marshal_output_%(c_name)s(%(c_type)s ret_in,
>>>>>> + QObject **ret_out, Error **errp)
>>>>>
>>>>> On the other hand, the qapi generator is smart enough to only output
>>>>> introspection data for qapi types that were actually used by a command
>>>>> or event, so how is that working, and why is it not also being used to
>>>>> elide the generation of unused qmp_marshal_output_FOO functions? This
>>>>> is where I'll have to defer to Markus.
>>>>
>>>> This is a QAPI generator restriction. Let me explain.
>>>>
>>>> The qmp_marshal_output_T() are shared by all commands returning T.
>>>>
>>>> The commands may be conditional. The user is responsible for making T's
>>>> 'if' the conjunction of the commands'. See the FIXME in commands.py.
>>>
>>> Yes, I noticed the FIXME:
>>>
>>> # FIXME: If T is a user-defined type, the user is responsible
>>> # for making this work, i.e. to make T's condition the
>>> # conjunction of the T-returning commands' conditions. If T
>>> # is a built-in type, this isn't possible: the
>>> # qmp_marshal_output_T() will be generated unconditionally.
>>>
>>> Using inline / unused attributes don't invalidate this :)
>>
>> Generating the unused attribute lets us keep types unconditional even
>> when the commands returning them are conditional (also takes care of the
>> built-in case, where we cannot make the type conditional).
>>
>> However, conditional commands returning an unconditional type is a bit
>> of a code smell. In this particular case, the smell seems to lead to a
>> (minor) issue: too much TPM code is compiled even when CONFIG_TPM is
>> off. With the attribute in place, we wouldn't have learned this.
>>
>> We may still find non-smelly instances of this pattern. Until then, I'm
>> a bit reluctant to generate the attribute.
>
> I agree with your nose :)
>
>>>> If I do this for tpm.json (appended), then tpm.h misses TpmModel when
>>>> CONFIG_TPM is off, and tpm_backend.h misses TpmType and TpmInfo. I
>>>> suspect more TPM code needs to be guarded by CONFIG_TPM.
>>>
>>> Yes, this is what I did first, use the code below and add #ifdef'ry,
>>> but the code becomes ugly and harder to maintain because the enums
>>> are used in middle of a QOM interface structure:
>>>
>>> include/sysemu/tpm.h-37-struct TPMIfClass {
>>> include/sysemu/tpm.h-38- InterfaceClass parent_class;
>>> include/sysemu/tpm.h-39-
>>> include/sysemu/tpm.h:40: enum TpmModel model;
>>> include/sysemu/tpm.h-41- void (*request_completed)(TPMIf *obj, int ret);
>>> include/sysemu/tpm.h-42- enum TPMVersion (*get_version)(TPMIf *obj);
>>> include/sysemu/tpm.h-43-};
>>> include/sysemu/tpm.h-44-
>>>
>>> If you think using inline / unused attributes is not an option for
>>> QAPI, then the #ifdef'ry isn't worth it and I'd prefer use v1 which
>>> doesn't use conditional QAPI suggested by Marc-André.
>>
>> Ignorant question: why do we want to define QOM type "tpm-if" when
>> CONFIG_TPM is off?
>
> Good question. I suppose for historical reasons? Copy/pasting of
> another older include/sysemu/ files? Recently I saw these headers
> received more love, such better #ifdef'ry to allow code elision.
>
> I'll defer that to Stefan.
Stefan, would you be willing to look into this?
