Re: [dpdk-dev] [PATCH v2 2/2] build: support building ABI versioned files twice

[Bruce Richardson <bruce.richardson@intel.com>]

On Tue, Oct 01, 2019 at 03:23:47PM +0200, Andrzej Ostruszka wrote:
> Thanks Bruce for the patch.  I like the idea of splitting versioning out
> of rte_compat.h, but I have some comments.
> 
> On 9/27/19 10:59 PM, Bruce Richardson wrote:
> [...]
> > --- a/config/common_base
> > +++ b/config/common_base
> > @@ -111,6 +111,7 @@ CONFIG_RTE_MAX_VFIO_CONTAINERS=64
> >  CONFIG_RTE_MALLOC_DEBUG=n
> >  CONFIG_RTE_EAL_NUMA_AWARE_HUGEPAGES=n
> >  CONFIG_RTE_USE_LIBBSD=n
> > +CONFIG_RTE_USE_FUNCTION_VERSIONING=y
> 
> I'm not fond of this config option - it is not really an option to be
> changed by the user.  I would prefer to just add flag to CFLAGS in
> mk/target/generic/rte.vars.mk.
> 

Ok, that sounds reasonable enough.

> >  #
> >  # Recognize/ignore the AVX/AVX512 CPU flags for performance/power testing.
> > diff --git a/config/rte_config.h b/config/rte_config.h
> > index 0bbbe274f..b63a2fdea 100644
> > --- a/config/rte_config.h
> > +++ b/config/rte_config.h
> > @@ -31,9 +31,6 @@
> >  
> >  /****** library defines ********/
> >  
> > -/* compat defines */
> > -#define RTE_BUILD_SHARED_LIB
> > -
> 
> So now everything builds "as static lib" (but with "-fPIC") apart from
> those libraries that use symbol versioning.  I'm OK with that however
> I'd like to note that code might be using RTE_BUILD_SHARED_LIB and do
> different things e.g. app/test-bbdev/test_bbdev_perf.c.  I know that was
> already the case - just wanted to say that aloud to make sure we are all
> aware of this :).

Thanks for pointing this out, I wasn't aware of it! Doing a git grep this
seems to be the only place in a C file (other than rte_config.h and
rte_compat.h) where the SHARED_LIB flag is being checked. I'll need to
follow up on that to see what the logic is there, because it seems strange
to require such a check.

> 
> > diff --git a/doc/guides/contributing/coding_style.rst b/doc/guides/contributing/coding_style.rst
> > index 449b33494..e95a1a2be 100644
> > --- a/doc/guides/contributing/coding_style.rst
> > +++ b/doc/guides/contributing/coding_style.rst
> > @@ -948,6 +948,13 @@ reason
> >  	built. For missing dependencies this should be of the form
> >  	``'missing dependency, "libname"'``.
> >  
> > +use_function_versioning
> > +	**Default Value = false**.
> > +	Specifies if the library in question has ABI versioned functions. If it
> > +	has, this value should be set to ensure that the C files are compiled
> > +	twice with suitable parameters for each of shared or static library
> > +	builds.
> > +
> 
> Maybe a different name for this option?  In general an "ideal
> theoretical" solution would be for build system to figure out on its own
> that separate build is necessary automatically - but that might incur
> some performance penalty (additional grep'ing of sources or so). 

I was thinking about that, and how we can do it automatically. The trouble
is that for correctness we would need to recheck every file after it had
changed, and since the result of the check means that we have different
build steps it would basically mean doing a full reconfiguration for every
file change. That's not really practical, hence this proposed solution.

> So I'm
> fine with this option however I'd like to rename it to actually indicate
> what it's effect is.  Like 'separate_build' or 'split_build' or
> 'rebuild_objects' or ...
> 
> The intention of using of versioned symbols is already indicated by
> inclusion of the relevant header.

I actually feel the opposite. I'd rather have the name tied in to the fact
that it's related to using function versioning - subject to what is found
on investigating the #ifdef in the bbdev perf test. However, if you feel
strongly about the name something else, I can probably compromise on it :-)

> 
> > diff --git a/lib/librte_eal/common/include/rte_function_versioning.h b/lib/librte_eal/common/include/rte_function_versioning.h
> > index ce963d4b1..55e88ffae 100644
> > --- a/lib/librte_eal/common/include/rte_function_versioning.h
> > +++ b/lib/librte_eal/common/include/rte_function_versioning.h
> > @@ -7,6 +7,10 @@
> >  #define _RTE_FUNCTION_VERSIONING_H_
> >  #include <rte_common.h>
> >  
> > +#ifndef RTE_USE_FUNCTION_VERSIONING
> > +#error Use of function versioning disabled, is "use_function_versioning=true" in meson.build?
> > +#endif
> 
> If you accept above suggestion then change this message to something
> like: "Function versioning requires 'separate_build=true' in meson.build"
> 

Agreed. This obviously needs to be kept in sync with whatever the build
flag is.

> BTW it turned out that this need for separate build for versioned
> symbols is a result of long standing gcc bug:
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=48200
> 
> I'll test this with clang and if this will work then maybe we could
> guard this #if with another check for 'gcc'.
> 
> Best regards
> Andrzej
> 
> Tested-by: Andrzej Ostruszka <amo@semihalf.com>

Let me know what your testing throws up, thanks.

/Bruce