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

From: Philippe Mathieu-Daudé
Subject: Re: [RFC PATCH] qapi: Incorrect attempt to fix building with MC146818RTC=n
Date: Tue, 21 Jan 2020 15:48:15 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.2.2

On 1/21/20 3:22 PM, Markus Armbruster wrote:
Paolo Bonzini <address@hidden> writes:

On 21/01/20 06:49, Markus Armbruster wrote:
Paolo Bonzini <address@hidden> writes:

On 13/01/20 15:01, Markus Armbruster wrote:
Philippe Mathieu-Daudé <address@hidden> 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 
   qapi/qapi-commands-misc-target.c:46: undefined reference to 
   /usr/bin/ld: qapi/qapi-commands-misc-target.c:46: undefined reference to 
   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

   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 <address@hidden>
  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 
     #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 
     #endif /* defined(TARGET_I386) */

But what about this one, in qapi-types-block-core.h:

     typedef enum BlockdevDriver {
     #if defined(CONFIG_REPLICATION)
     #endif /* defined(CONFIG_REPLICATION) */
     } BlockdevDriver;

If I omit it in the header, I then have to omit it in

     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[]) {
     #if defined(CONFIG_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.

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