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

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

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.