Re: [dpdk-dev] [PATCH 0/8] Improve build process

["Gonzalez Monroy, Sergio" <sergio.gonzalez.monroy@intel.com>]

On 13/02/2015 10:14, Panu Matilainen wrote:
> On 02/12/2015 05:52 PM, Neil Horman wrote:
>> On Thu, Feb 12, 2015 at 04:07:50PM +0200, Panu Matilainen wrote:
>>> On 02/12/2015 02:23 PM, Neil Horman wrote:
> [...snip...]
>>>>>>>>
>>>>>>> So I just realized that I was not having into account a possible
>>>>>>> scenario, where
>>>>>>> we have an app built with static dpdk libs then loading a dso 
>>>>>>> with -d
>>>>>>> option.
>>>>>>>
>>>>>>> In such case, because the pmd would have DT_NEEDED entries, 
>>>>>>> dlopen will
>>>>>>> fail.
>>>>>>> So to enable such scenario we would need to build PMDs without 
>>>>>>> DT_NEEDED
>>>>>>> entries.
>>>>>>
>>>>>> Hmm, for that to be a problem you'd need to have the PMD built 
>>>>>> against
>>>>>> shared dpdk libs and while the application is built against 
>>>>>> static dpdk
>>>>>> libs. I dont think that's a supportable scenario in any case.
>>>>>>
>>>>>> Or is there some other scenario that I'm not seeing?
>>>>>>
>>>>>>     - Panu -
>>>>>>
>>>>> I agree with you. I suppose it comes down to, do we want to 
>>>>> support such
>>>>> scenario?
>>>>>
>>>>>  From what I can see, it seems that we do currently support such 
>>>>> scenario by
>>>>> building dpdk apps against all static dpdk libs using 
>>>>> --whole-archive (all
>>>>> libs and not only PMDs).
>>>>> http://dpdk.org/browse/dpdk/commit/?id=20afd76a504155e947c770783ef5023e87136ad8 
>>>>>
>>>>>
>>>>> Am I misunderstanding this?
>>>>>
>>>> Shoot, you're right, I missed the static build aspect to this.  
>>>> Yes, if we do the following:
>>>>
>>>> 1) Build the DPDK as a static library
>>>> 2) Link an application against (1)
>>>> 3) Use the dlopen mechanism to load a PMD built as a DSO
>>>>
>>>> Then the DT_NEEDED entries in the DSO will go unsatisfied, because 
>>>> the shared
>>>> objects on which it (the PMD) depends will not exist in the file 
>>>> system.
>>>
>>> I think its even more twisty:
>>>
>>> 1) Build the DPDK as a static library
>>> 2) Link an application against (1)
>>> 3) Do another build of DPDK as a shared library
>>> 4) In app 2), use the dlopen mechanism to load a PMD built as a part 
>>> of or
>>> against 3)
>>>
>>> Somehow I doubt this would work very well.
>>>
>> Ideally it should, presuming the ABI is preserved between (1) and 
>> (3), though I
>> agree, up until recently, that was an assumption that was unreliable.
>
> Versioning is a big and important step towards reliability but there 
> are more issues to solve. This of course getting pretty far from the 
> original topic, but at least one such issue is that there are some 
> cases where a config value affects what are apparently public structs 
> (rte_mbuf wrt RTE_MBUF_REFCNT for example), which really is a no-go.
>
Agree, the RTE_MBUF_REFCNT is something that needs to be dealt with 
asap. I'll look into it.

>>>>
>>>> I think the problem is a little bit orthogonal to the libdpdk_core 
>>>> problem you
>>>> were initially addressing.  That is to say, this problem of 
>>>> dlopen-ed PMD's
>>>> exists regardless of weather you build the DPDK as part of a static 
>>>> or dynamic
>>>> library.  The problems just happen to intersect in their 
>>>> manipulation of the
>>>> DT_NEEDED entries.
>>>>
>>>> Ok, so, given the above, I would say your approach is likely 
>>>> correct, just
>>>> prevent DT_NEEDED entries from getting added to PMD's. Doing so 
>>>> will sidestep
>>>> loading issue for libraries that may not exist in the filesystem, 
>>>> but thats ok,
>>>> because by all rights, the symbols codified in those needed 
>>>> libraries should
>>>> already be present in the running application (either made 
>>>> available by the
>>>> application having statically linked them, or having the linker 
>>>> load them from
>>>> the proper libraries at run time).
>>>
>>> My 5c is that I'd much rather see the common case (all static or all 
>>> shared)
>>> be simple and reliable, which in case of DSOs includes no lying 
>>> (whether by
>>> omission or otherwise) about DT_NEEDED, ever. That way the issue is 
>>> dealt
>>> once where it belongs. If somebody wants to go down the rabbit hole 
>>> of mixed
>>> shared + static linkage, let them dig the hole by themselves :)
>>>
>> This is a fair point.  Can DT_NEEDED sections be stripped via tools 
>> like objcopy
>> after the build is complete?  If so, end users can hack this corner 
>> case to work
>> as needed.
>
> Patchelf (http://nixos.org/patchelf.html) appears to support that, but 
> given that source is available it'd be easier to just modify the 
> makefiles if that's really needed.
>
I think we agree on the issue.

So I'll be sending a patch to add DT_NEEDED entries to all libraries and 
PMDs. The only exception would be librte_eal, which would not have 
proper NEEDED entries.
Do we bother adding a linker script for librte_eal that would include 
dependent libraries?

Sergio