Re: [dpdk-dev] [PATCH] app: fix plugin load on static builds

[Olivier Matz <olivier.matz@6wind.com>]

Hi Bruce,

Thanks for the feedback.

On Thu, Nov 26, 2020 at 03:28:46PM +0000, Bruce Richardson wrote:
> On Thu, Nov 26, 2020 at 03:20:42PM +0100, Olivier Matz wrote:
> > When dpdk is compiled as static libraries, it is not possible
> > to load a plugin from an application. We get the following error:
> > 
> >   EAL: librte_pmd_xxxx.so: undefined symbol: per_lcore__rte_errno
> > 
> > This happens because the dpdk symbols are not exported. Add them to the
> > dynamic symbol table by using '-Wl,--export-dynamic'. This option was
> > previously present when compiled with Makefiles, it was introduced in
> > commit f9a08f650211 ("eal: add support for shared object drivers")
> > 
> > Fixes: 16ade738fd0d ("app/testpmd: build with meson")
> > Fixes: 89f0711f9ddf ("examples: build some samples with meson")
> > Cc: stable@dpdk.org
> > 
> > Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
> > ---
> > 
> > Hi,
> > 
> > Maybe the patch can be improved: I suppose that --export-dynamic
> > should only be added in case we are linking in static mode. Any
> > help about how to do that is welcome.
> 
> get_option('default_library') == 'static'
> 
> However, if the flag is harmless in shared linking mode, I don't think we
> need bother with this.

ok

> > 
> > There is probably a better place to define the default ldflags for
> > all applications (to factorize between app and example).
> > 
> > Also, should this flag be advertised in pkg-config?
> > 
> Perhaps. However, I'm not sure how common it would be for people to static
> link their own apps with DPDK and then want to plug in extra drivers after
> the fact? Are there any possible negative impacts to making that change?

I don't know if it is common, but this is something we do :)

> If we weren't right before the release deadline I'd definitely suggest
> adding it to the pkg-config files. At this late stage in release, I'm more
> cautious.

Yes, it is too late for 20.11. Maybe even for this patch without
updating pkg-config.

> 
> > Thanks,
> > Olivier
> > 
> > 
> >  app/meson.build      | 3 +++
> >  examples/meson.build | 4 +++-
> >  2 files changed, 6 insertions(+), 1 deletion(-)
> > 
> > diff --git a/app/meson.build b/app/meson.build
> > index eb74f215a3..92479c7729 100644
> > --- a/app/meson.build
> > +++ b/app/meson.build
> > @@ -25,6 +25,7 @@ apps = [
> >  lib_execinfo = cc.find_library('execinfo', required: false)
> >  
> >  default_cflags = machine_args + ['-DALLOW_EXPERIMENTAL_API']
> > +default_ldflags = ['-Wl,--export-dynamic']
> >  
> >  foreach app:apps
> >  	build = true
> > @@ -32,6 +33,7 @@ foreach app:apps
> >  	sources = []
> >  	includes = []
> >  	cflags = default_cflags
> > +	ldflags = default_ldflags
> >  	objs = [] # other object files to link against, used e.g. for
> >  	          # instruction-set optimized versions of code
> >  
> > @@ -58,6 +60,7 @@ foreach app:apps
> >  		executable('dpdk-' + name,
> >  				sources,
> >  				c_args: cflags,
> > +				link_args: ldflags,
> >  				link_whole: link_libs,
> >  				dependencies: dep_objs,
> >  				install_rpath: join_paths(get_option('prefix'),
> > diff --git a/examples/meson.build b/examples/meson.build
> > index 46ec80919e..def4540e8f 100644
> > --- a/examples/meson.build
> > +++ b/examples/meson.build
> > @@ -63,6 +63,7 @@ default_cflags = machine_args
> >  if cc.has_argument('-Wno-format-truncation')
> >  	default_cflags += '-Wno-format-truncation'
> >  endif
> > +default_ldflags = ['-Wl,--export-dynamic'] + dpdk_extra_ldflags
> >  
> >  foreach example: examples
> >  	name = example.split('/')[-1]
> > @@ -70,6 +71,7 @@ foreach example: examples
> >  	sources = []
> >  	allow_experimental_apis = false
> >  	cflags = default_cflags
> > +	ldflags = default_ldflags
> >  
> >  	ext_deps = [execinfo]
> >  	includes = [include_directories(example)]
> > @@ -91,7 +93,7 @@ foreach example: examples
> >  		executable('dpdk-' + name, sources,
> >  			include_directories: includes,
> >  			link_whole: link_whole_libs,
> > -			link_args: dpdk_extra_ldflags,
> > +			link_args: ldflags,
> >  			c_args: cflags,
> >  			dependencies: dep_objs)
> >  	elif not allow_skips
> > -- 
> > 2.25.1
> > 
> 
> Patch looks reasonable to me. In terms of other approaches, we have:
> 
> 1. Add "add_project_link_arguments('-Wl,--export-dynamic', language: 'c')"
> to "config/meson.build". This has the advantage of being a lot shorter, but
> it would apply to all .so's too, so not sure if there is any impact there.

I will check if there is any impact on .so with this approach.

> 2. If there is no reason why any particular app would want to not provide
> this flag, you can skip defining a new ldflags variable and just add the
> flag into the existing link_args line for each app and example. However,
> your approach here, though longer, is more in keeping with the style of
> what is already there.