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

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

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 :)

> 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.

> diff --git a/qapi/tpm.json b/qapi/tpm.json
> index 09332e6f99..e74c881ea6 100644
> --- a/qapi/tpm.json
> +++ b/qapi/tpm.json
> @@ -17,7 +17,9 @@
>  #
>  # Since: 1.5
>  ##
> -{ 'enum': 'TpmModel', 'data': [ 'tpm-tis', 'tpm-crb', 'tpm-spapr' ] }
> +{ 'enum': 'TpmModel', 'data': [ 'tpm-tis', 'tpm-crb', 'tpm-spapr' ],
> +  'if': 'defined(CONFIG_TPM)' }
> +
>  ##
>  # @query-tpm-models:
>  #
> @@ -47,7 +49,8 @@
>  #
>  # Since: 1.5
>  ##
> -{ 'enum': 'TpmType', 'data': [ 'passthrough', 'emulator' ] }
> +{ 'enum': 'TpmType', 'data': [ 'passthrough', 'emulator' ],
> +  'if': 'defined(CONFIG_TPM)' }
>  
>  ##
>  # @query-tpm-types:
> @@ -124,7 +127,8 @@
>  { 'struct': 'TPMInfo',
>    'data': {'id': 'str',
>             'model': 'TpmModel',
> -           'options': 'TpmTypeOptions' } }
> +           'options': 'TpmTypeOptions' },
> +  'if': 'defined(CONFIG_TPM)' }
>  
>  ##
>  # @query-tpm:
>