Re: [Qemu-devel] [PATCH RFC 00/21] Modularize generated QAPI code

[Daniel P . Berrangé]

On Fri, Feb 02, 2018 at 02:03:15PM +0100, Markus Armbruster wrote:
> Our qapi-schema.json is composed of modules connected by include
> directives, but the generated code is monolithic all the same: one
> qapi-types.h with all the types, one qapi-visit.h with all the
> visitors, and so forth.  These monolithic headers get included all
> over the place.  In my "build everyhing" tree, adding a QAPI type
> recompiles about 4500 out of 4800 objects.
> 
> Nobody would write such monolithic headers by hand.  It stands to
> reason that one shouldn't generate them, either.
> 
> This series' basic idea is to split up generated headers to mirror the
> schema's modular structure: one header per module.  That way, you can
> include just what you need.
> 
> The series is RFC for a number of reasons:
> 
> * The split is implemented only for qapi-types.h.  That one should
>   provide the biggest benefits, though.
> 
> * There's a bit of code duplication.
> 
> * I haven't re-read my patches, yet.
> 
> Even in this incomplete state, the compile-time improvements can be
> massive.  Before this series, any QAPI schema change recompiles some
> 4500 out of 4800 objects in my "build everything" tree.  Afterwards,
> adding a type to qapi/migration.json recompiles less than 400, adding
> a QMP event recompiles less than 200, and a documentation change no
> longer recompiles anything.

Having gone through the same exercise for trace.h, I very much welcome
this effort for QAPI too !

If I consider the crypto stuff I maintain, the QAPI definitions are
in qapi/crypto.json.

>From my POV, I feel it would be natural to have them move to instead
be at  crypto/qapi.json, and have the generated headers/source files be
crypto/qapi-types.h, crypto/qapi-visit.h, etc.

This would mirror what we did with tracing, crypto/trace-events, and
generating crypto/trace*.{c,h}

Regards,
Daniel
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