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

[Neil Horman]

On Tue, Apr 26, 2016 at 01:39:47PM -0400, Neil Horman wrote:
> Hey-
> 	So a few days ago we were reviewing Davids patch series to introduce the
> abiilty to dump hardware support from pmd DSO's in a human readable format.
> That effort encountered some problems, most notably the fact that stripping a
> DSO removed the required information that the proposed tool keyed off, as well
> as the need to dead reckon offsets between symbols that may not be constant
> (dependent on architecture).
> 
> 	I was going to start looking into the possibility of creating a modinfo
> section in a simmilar fashion to what kernel modules do in linux or BSD.  I
> decided to propose this solution instead though, because the kernel style
> solution requires a significant amount of infrastructure that I think we can
> maybe avoid maintaining, if we accept some minor caviats
> 
> To do this We emit a set of well known marker symbols for each DSO that an
> external application can search for (in this case I called them
> this_pmd_driver<n>, where n is a counter macro).  These marker symbols are
> n is a counter macro).  These marker symbols are exported by PMDs for
> external access.  External tools can then access these symbols via the
> dlopen/dlsym api (or via elfutils libraries)
> 
> The symbols above alias the rte_driver struct for each PMD, and the external
> application can then interrogate the registered driver information.  
> 
> I also add a pointer to the pci id table struct for each PMD so that we can
> export hardware support.
> 
> 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.
> 
> 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.
> 
> But regardless, I thought I would propose this to see what you all thought of
> it.
> 
> FWIW, heres sample output of the pmdinfo tool from this series probing the
> librte_pmd_ena.so module:
> 
> [nhorman at hmsreliant dpdk]$ ./build/app/pmdinfo
> ~/git/dpdk/build/lib/librte_pmd_ena.so
> PMD 0 Information:
> Driver Name: ena_driver
> Driver Type: PCI
> |====================PCI Table========================|
> | VENDOR ID | DEVICE ID | SUBVENDOR ID | SUBDEVICE ID |
> |-----------------------------------------------------|
> |       1d0f|       ec20|          ffff|          ffff|
> |       1d0f|       ec21|          ffff|          ffff|
> |-----------------------------------------------------|
> 
> 
> 
> 
Ping, thoughts here?

Neil