Re: [RFC PATCH v2 1/2] qapi: Inline qmp_marshal_output() functions

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

On 6/10/21 1:06 PM, Markus Armbruster wrote:
> Philippe Mathieu-Daudé <philmd@redhat.com> writes:
> 
>> On 6/10/21 11:33 AM, Markus Armbruster wrote:
>>> Eric Blake <eblake@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 <philmd@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.

Thanks for the review,

Phil.