Re: [dpdk-users] DPDK 16.04 link changes cause PMD drivers to not be loaded

[Aurojit Panda <apanda@cs.berkeley.edu>]

[Cross-posting to dev]

Panu Matilainen wrote:
> On 04/20/2016 05:26 PM, Aurojit Panda wrote:
>> I am sorry that is a bit unintuitive considering:
>>
>> (a) This behavior differs between static and shared builds of DPDK.
>> - In fact this behavior was identical in 2.2, and even in mainline
>> before 948fd64befc3726 went in.
>> (b) You already know the EAL_PMD_PATH at build time, and it makes it
>> quite hard to ship scripts or anything to build DPDK, since now the
>> configuration file becomes dependent on location.
>
> The expectation is that shared libraries are installed to a shared, known location. Otherwise there's not that much
> point in using shared libraries.
>
> Note that you can also use the EAL option -d to load either single PMDs or entire directories at runtime (regardless of
> EAL_PMD_PATH) so you dont have to know the path at build time if that's what bothers you most.
>

I am actually bothered by needing to supply the path at all. You seem to assume that knowing an install path is 
essential for shared libraries, while you can in reality embed this in the rpath. Shared libraries are used for many 
reasons, including interop with some runtime, in which case they are not installed at a known place. I would prefer for 
those PMDs that are included with DPDK that either they be loaded from the "known" build path for them.

>>
>> I wonder if you would consider changing this, as it stands just building
>> DPDK after setting CONFIG_RTE_SHARED_LIB=y results in a testpmd that
>> cannot run.
>
> In shared library configuration, testpmd is not directly runnable regardless of the drivers since its missing all the
> other libraries too:
>
> [pmatilai@sopuli dpdk]$ build/app/testpmd
> build/app/testpmd: error while loading shared libraries: librte_distributor.so.1.1: cannot open shared object file: No
> such file or directory
> [pmatilai@sopuli dpdk]$
>
> You'll need to get those libraries into linkers path anyway, either by installing them to a common location or by
> extending LD_LIBRARY_PATH. In either case, you need to know where the libraries are anyway.
>
> All of which is not to say there might not be room for improvement, but the linking behavior is not going to change.
> Again, PMDs are *plugins* that are *meant* to be loaded at runtime. That allows for all sorts of flexibility especially
> for packaging and shipping, at some extra cost in setup complexity.

I am all for a plugin architecture, I was merely suggesting that you embed some path infromation at the beginning. Also 
please note:
(a) This behavior changed recently.
(b) This change is entirely undocumented, which is why I was reporting it in the first place.
(c) It is actually quite unintutive, because previously ensuring LD_LIBRARY_PATH was correct was all that was required 
to get any DPDK application to interact with ports.

>
> For your own purposes, you can of course tweak the linking settings as much as you like. Look for "plugins" in
> mk/rte.app.mk and change the shared lib condition on the line above to "y" and there you have it. But that's not the way
> plugins are meant to be used.

That is not a reasonable solution given that it makes it very hard to track future changes to DPDK without merges. My 
alternatives neither break people's abilities to use plugins, nor do they impact behavior.

>
> Oh and BTW, please don't top-post.
>
> - Panu -
>