Re: [dpdk-dev] [RFC PATCH 0/4]: Implement module information export

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

On Wed, May 04, 2016 at 11:16:42PM +0200, Thomas Monjalon wrote:
> This discussion requires more opinions.
> Please everybody, READ and COMMENT. Thanks
> 
> If it is not enough visible, a new thread could be started later.
> 
> 2016-05-04 07:43, Neil Horman:
> > On Wed, May 04, 2016 at 10:24:18AM +0200, David Marchand wrote:
> > > On Tue, May 3, 2016 at 1:57 PM, Neil Horman <nhorman@tuxdriver.com> wrote:
> > > >> This approach has a few pros and cons:
> > > >>
> > > >> pros:
> > > >> 1) Its simple, and doesn't require extra infrastructure to implement.  E.g. we
> > > >> don't need a new tool to extract driver information and emit the C code to build
> > > >> the binary data for the special section, nor do we need a custom linker script
> > > >> to link said special section in place
> > > >>
> > > >> 2) Its stable.  Because the marker symbols are explicitly exported, this
> > > >> approach is resilient against stripping.
> 
> It is a good point. We need something resilient against stripping.
> 
> > > >> cons:
> > > >> 1) It creates an artifact in that PMD_REGISTER_DRIVER has to be used in one
> > > >> compilation unit per DSO.  As an example em and igb effectively merge two
> > > >> drivers into one DSO, and the uses of PMD_REGISTER_DRIVER occur in two separate
> > > >> C files for the same single linked DSO.  Because of the use of the __COUNTER__
> > > >> macro we get multiple definitions of the same marker symbols.
> > > >>
> > > >> I would make the argument that the downside of the above artifact isn't that big
> > > >> a deal.  Traditionally in other projects a unit like a module (or DSO in our
> > > >> case) only ever codifies a single driver (e.g. module_init() in the linux kernel
> > > >> is only ever used once per built module).  If we have code like igb/em that
> > > >> shares some core code, we should build the shared code to object files and link
> > > >> them twice, once to an em.so pmd and again to an igb.so pmd.
> 
> It is also a problem for compilation units having PF and VF drivers.
> 
> > > >> But regardless, I thought I would propose this to see what you all thought of
> > > >> it.
> 
> Thanks for proposing.
> 
> > > - This implementation does not support binaries, so it is not suitable
> > > for people who don't want dso, this is partially why I used bfd rather
> > > than just dlopen.
> > 
> > If you're statically linking an application, you should know what hardware you
> > support already.  Its going to be very hard, if not impossible to develop a
> > robust solution that works with static binaries (the prior solutions don't work
> > consistently with static binaries either).  I really think the static solution
> > needs to just be built into the application (i.e. the application needs to add a
> > command line option to dump out the pci id's that are registered).
> 
> No, we need a tool to know what are the supported devices before running
> the application (e.g. for binding).
> This tool should not behave differently depending of how DPDK was compiled
> (static or shared).
> 
> [...]
> > > - How does it behave if we strip the dsos ?
> > 
> > I described this above, its invariant to stripping, because the symbols for each
> > pmd are explicitly exported, so strip doesn't touch the symbols that pmdinfo
> > keys off of.
> > 
> [...]
> > > - The tool output format is not script friendly from my pov.
> > 
> > Don't think it really needs to be script friendly, it was meant to provide human
> > readable output, but script friendly output can be added easily enough if you
> > want.
> 
> Yes it needs to be script friendly.
> 
> It appears that we must agree on a set of requirements first.
> Please let's forget the implementation until we have collected enough
> feedbacks on the needs.
> I suggest these items to start the list:
> 
> - query all drivers in static binary or shared library
> - stripping resiliency
> - human friendly
> - script friendly
> - show driver name
> - list supported device id / name
> - list driver options
> - show driver version if available
> - show dpdk version
> - show kernel dependencies (vfio/uio_pci_generic/etc)
> - room for extra information?
> 
> Please ack or comment items of this list, thanks.

That's quite a laundry list of requirements! I would view the following as core
requirements:
 - query all drivers in static binary or shared library
 - stripping resiliency
 - human friendly
 - script friendly
 - show driver name
 - list supported device id / name

and the rest as nice-to-have's that are not needed for a first version.
That being said, I would expect those nice-to-have's to be fairly easy to add on
once the base solution is in place.

On a semi-related note, I assume this discussion and a solution here is not going
to block the merging of the other clean-up patches in the driver/pci area?

http://dpdk.org/ml/archives/dev/2016-April/037686.html
http://dpdk.org/ml/archives/dev/2016-April/037708.html (Patches 1-10)


Regards,
/Bruce