Re: [dpdk-dev] [PATCH v3 3/6] mk: sort libraries when linking, move pmd libs to higher level

[Thomas Monjalon <thomas.monjalon@6wind.com>]

2016-06-13 11:04, Ferruh Yigit:
> On 6/13/2016 10:29 AM, Thomas Monjalon wrote:
> > 2016-06-10 19:32, Ferruh Yigit:
> >> As stated in the comment:
> >>     Order is important: from higher level to lower level
> >>
> >> This is an attempt to make the layering order better respected.
> >>
> >> Limit scope of --whole-archive to pmd libraries
> > 
> > Compared to the link order in the v2, you did two things:
> > 1/ move PMDs to the first position
> > 2/ restrict --whole-archive to PMDs only
> > 
> > Having the PMDs first, helps the linker to get all the PMD dependencies
> > in the static binary. Thus it seems we do not need --whole-archive
> > for the PMD dependencies (ethdev, mbuf, etc).
> > But, if an external PMD is loaded via dlopen, it is possible that it
> > needs a symbol which was not used by the internal PMDs. So it will not
> > be found in the statically linked binary.
> > Let's take another example: if we disable the internal PMDs with their
> > config options from the static build, and we decide to build them
> > separately as DSOs. We won't be able to load them as plugins because
> > they depend on symbols which won't be found in the static binary.
> 
> So you want to keep all objects in final binary (used or unused) because
> of the possibility that any plugin may use them.

Yes

> But what is the list to include here, whole dpdk?, -since we may not
> know what API will plugin call.

The list of the libraries candidates to be called from a driver can be
discussed. The top layers like lpm or distributor should not be in this
list I think.

> What I am confused is --whole-archive only used when dpdk compiled as
> static, if dpdk compiled as shared, each PMD should have proper
> dependencies [1], and if external PMD compiled properly there shouldn't
> be a problem. So do we have a case that dpdk compiled statically into
> final binary but we still want to load some plugins dynamically?

Yes a plugin can be loaded from a static binary.
Breaking this feature would need a separate discussion and notices.

> > To make it short, the PMDs must be considered as plugins. Therefore, we
> > must not rely on their availability to link the required symbols in
> > a static binary.
> > To make sure the plugin loading will be always well achieved, we must
> > link the static PMDs at the last position.
> 
> I think this is not the issue of linking PMD's first or last, but
> expanding --whole-archive to cover other libraries other than PMDs.

Yes, linking PMD at the end is a way to force us to keep some libraries
in --whole-archive.

> > If you agree, I vote for the v2 of this patchset.
> 
> If this is breaking something and best way to fix is not in external PMD
> but in here, please feel free to go with v2.

I don't see any other solution.
But I'm sure we could discuss it more and/or improve it in the future.