From: Markus Armbruster <armbru@redhat.com>
To: Stefan Berger <stefanb@linux.vnet.ibm.com>
Cc: "Michael Roth" <michael.roth@amd.com>,
qemu-devel@nongnu.org, "Eric Blake" <eblake@redhat.com>,
"Dr. David Alan Gilbert" <dgilbert@redhat.com>,
"Paolo Bonzini" <pbonzini@redhat.com>,
"Philippe Mathieu-Daudé" <philmd@redhat.com>
Subject: Re: [RFC PATCH v2 1/2] qapi: Inline qmp_marshal_output() functions
Date: Fri, 11 Jun 2021 10:02:49 +0200 [thread overview]
Message-ID: <87im2kvnfa.fsf@dusky.pond.sub.org> (raw)
In-Reply-To: <76d76c8e-5c0a-d676-f3d7-2d256d033294@redhat.com> ("Philippe Mathieu-Daudé"'s message of "Thu, 10 Jun 2021 17:59:55 +0200")
Philippe Mathieu-Daudé <philmd@redhat.com> writes:
> 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.
Stefan, would you be willing to look into this?
next prev parent reply other threads:[~2021-06-11 8:03 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-09 18:49 [PATCH v2 0/2] tpm: Return QMP error when TPM is disabled in build Philippe Mathieu-Daudé
2021-06-09 18:49 ` [RFC PATCH v2 1/2] qapi: Inline qmp_marshal_output() functions Philippe Mathieu-Daudé
2021-06-09 20:29 ` Eric Blake
2021-06-10 9:33 ` Markus Armbruster
2021-06-10 10:06 ` Philippe Mathieu-Daudé
2021-06-10 11:06 ` Markus Armbruster
2021-06-10 15:59 ` Philippe Mathieu-Daudé
2021-06-11 8:02 ` Markus Armbruster [this message]
2021-06-11 12:32 ` Stefan Berger
2021-06-09 18:49 ` [PATCH v2 2/2] tpm: Return QMP error when TPM is disabled in build Philippe Mathieu-Daudé
2021-06-09 20:33 ` Eric Blake
2021-06-10 9:35 ` Markus Armbruster
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=87im2kvnfa.fsf@dusky.pond.sub.org \
--to=armbru@redhat.com \
--cc=dgilbert@redhat.com \
--cc=eblake@redhat.com \
--cc=michael.roth@amd.com \
--cc=pbonzini@redhat.com \
--cc=philmd@redhat.com \
--cc=qemu-devel@nongnu.org \
--cc=stefanb@linux.vnet.ibm.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).