Re: [dpdk-dev] [EXT] [RFC PATCH 0/2] introduce __rte_internal tag

[Jerin Jacob Kollanukkaran <jerinj@marvell.com>]

> -----Original Message-----
> From: Neil Horman <nhorman@tuxdriver.com>
> Sent: Thursday, June 6, 2019 7:05 PM
> To: Jerin Jacob Kollanukkaran <jerinj@marvell.com>
> Cc: Bruce Richardson <bruce.richardson@intel.com>; dev@dpdk.org;
> Thomas Monjalon <thomas@monjalon.net>
> Subject: Re: [EXT] [RFC PATCH 0/2] introduce __rte_internal tag
> 
> On Thu, Jun 06, 2019 at 12:04:57PM +0000, Jerin Jacob Kollanukkaran wrote:
> > > -----Original Message-----
> > > From: Neil Horman <nhorman@tuxdriver.com>
> > > Sent: Thursday, June 6, 2019 5:04 PM
> > > To: Jerin Jacob Kollanukkaran <jerinj@marvell.com>
> > > Cc: Bruce Richardson <bruce.richardson@intel.com>; dev@dpdk.org;
> > > Thomas Monjalon <thomas@monjalon.net>
> > > Subject: Re: [EXT] [RFC PATCH 0/2] introduce __rte_internal tag
> > >
> > > On Thu, Jun 06, 2019 at 09:44:52AM +0000, Jerin Jacob Kollanukkaran
> wrote:
> > > > > -----Original Message-----
> > > > > From: Neil Horman <nhorman@tuxdriver.com>
> > > > > Sent: Wednesday, June 5, 2019 11:41 PM
> > > > > To: Bruce Richardson <bruce.richardson@intel.com>
> > > > > Cc: Jerin Jacob Kollanukkaran <jerinj@marvell.com>;
> > > > > dev@dpdk.org; Thomas Monjalon <thomas@monjalon.net>
> > > > > Subject: Re: [EXT] [RFC PATCH 0/2] introduce __rte_internal tag
> > > > >
> > > > > On Wed, Jun 05, 2019 at 05:45:41PM +0100, Bruce Richardson wrote:
> > > > > > On Wed, Jun 05, 2019 at 04:24:09PM +0000, Jerin Jacob
> > > > > > Kollanukkaran
> > > > > wrote:
> > > > > > > > -----Original Message-----
> > > > > > > > From: Neil Horman <nhorman@tuxdriver.com>
> > > > > > > > Sent: Sunday, May 26, 2019 12:14 AM
> > > > > > > > To: dev@dpdk.org
> > > > > > > > Cc: Neil Horman <nhorman@tuxdriver.com>; Jerin Jacob
> > > > > > > > Kollanukkaran <jerinj@marvell.com>; Bruce Richardson
> > > > > > > > <bruce.richardson@intel.com>; Thomas Monjalon
> > > > > > > > <thomas@monjalon.net>
> > > > > > > > Subject: [EXT] [RFC PATCH 0/2] introduce __rte_internal
> > > > > > > > tag
> > > > > > > >
> > > > > > > > Hey-
> > > > > > > > 	Based on our recent conversations regarding the use of
> > > > > > > > symbols only meant for internal dpdk consumption (between
> > > > > > > > dpdk libraries), this is an idea that I've come up with
> > > > > > > > that I'd like to get some feedback on
> > > > > > > >
> > > > > > > > Summary:
> > > > > > > > 1) We have symbols in the DPDK that are meant to be used
> > > > > > > > between DPDK libraries, but not by applications linking to
> > > > > > > > them
> > > > > > > > 2) We would like to document those symbols in the code, so
> > > > > > > > as to note them clearly as for being meant for internal
> > > > > > > > use only
> > > > > > > > 3) Linker symbol visibility is a very coarse grained tool,
> > > > > > > > and so there is no good way in a single library to mark
> > > > > > > > items as being meant for use only by other DPDK libraries,
> > > > > > > > at least not without some extensive runtime checking
> > > > > > > >
> > > > > > > >
> > > > > > > > Proposal:
> > > > > > > > I'm proposing that we introduce the __rte_internal tag.
> > > > > > > > From a coding standpoint it works a great deal like the
> > > > > > > > __rte_experimental tag in that it expempts the tagged
> > > > > > > > symbol from ABI constraints (as the only users should be
> > > > > > > > represented in the DPDK build environment).  Additionally,
> > > > > > > > the __rte_internal macro resolves differently based on the
> > > > > > > > definition of the BUILDING_RTE_SDK flag (working under the
> > > > > > > > assumption that said flag should only ever be set if we
> > > > > > > > are actually building DPDK libraries which will make use
> > > > > > > > of internal calls).  If the BUILDING_RTE_SDK flag is set
> > > > > > > > __rte_internal resolves to __attribute__((section
> > > > > > > > "text.internal)), placing it in a special text section
> > > > > > > > which is then used to validate that the the symbol appears
> > > > > > > > in the INTERNAL section of the corresponding library version
> map).
> > > > > > > > If BUILDING_RTE_SDK is not set, then __rte_internal
> > > > > > > > resolves to
> > > > > __attribute__((error("..."))), which causes any caller of the
> > > > > tagged function to throw an error at compile time, indicating
> > > > > that the symbol is not available for external use.
> > > > > > > >
> > > > > > > > This isn't a perfect solution, as applications can still
> > > > > > > > hack around it of course,
> > > > > > >
> > > > > > > I think, one way to, avoid, hack around could be to,
> > > > > > >
> > > > > > > 1) at config stage, create  a random number for the build
> > > > > > > 2) introduce RTE_CALL_INTERNAL macro for calling internal
> > > > > > > function, compare the generated random number for allowing
> > > > > > > the calls to make within the library. i.e leverage the fact
> > > > > > > that external library would never know the random number
> > > > > > > generated for the DPDK build
> > > > > and internal driver code does.
> > > > > > >
> > > > > > Do we really need to care about this. If have some determined
> > > > > > enough to hack around our limitations, then they surely know
> > > > > > that they have an unsupported configuration. We just need to
> > > > > > protect against inadvertent use of internals, IMHO.
> > > > > >
> > > > > I agree, I too had thought about doing some sort of internal
> > > > > runtime checking to match internal only symbols, such that they
> > > > > were only accessable by internally approved users, but it
> > > > > started to feel like a great
> > > deal of overhead.
> > > > > Its a good idea for a general mechanism I think, but I believe
> > > > > the value here is more to internally document which apis we want
> > > > > to mark as being for internal use only, and create a lightweight
> > > > > roadblock at build time to catch users inadvertently using them.
> > > > > Determined users will get around anything, and theres not much
> > > > > we can do to stop
> > > them.
> > > >
> > > > I agree too. IMHO, Simply having following items would be enough
> > > >
> > > > 1) Avoid exposing the internal function prototype through public
> > > > header files
> > > > 2) Add @internal to API documentation
> > > > 3) Just decide the name space for internal API for tooling(i.e not
> > > > start with rte_ or so) Using objdump scheme to detect internal
> > > > functions
> > > requires the the library to build prior to run the checkpatch.
> > > >
> > >
> > > No, I'm not comfortable with that approach, and I've stated why:
> > > 1) Not exposing the functions via header files is a fine start
> > >
> > > 2) Adding internal documentation is also fine, but does nothing to
> > > correlate the code implementing those functions to the
> > > documentation.  Its valuable to have a tag on a function identifying it as
> internal only.
> > >
> > > 3) Using naming conventions to separate internal only from
> > > non-internal functions is a vague approach, requiring future
> > > developers to be cogniscent of the convention and make the
> > > appropriate naming choices.  It also implicitly restricts the
> > > abliity for future developers to make naming changes in conflict
> > > with that convention
> >
> > Enforcing the naming convention can be achieved through tooling as well.
> >
> Sure, but why enforce any function naming at all, when you don't have to.

May I ask,  why to  enforce __rte_internal, when you don't have to

> 
> > >
> > > 4) Adding a tag like __rte_internal creates an interlock whereby,
> > > not only are internal functions excused from ABI constraints, but
> > > forces developers to intentionally mark their internal functions as
> > > being internal in the code, which is beneficial to clarlity of understanding
> during the development process.
> >
> > No issues in adding __rte_internal. But, I am against current
> > implementaion, Ie. adding objdump dependency
> That dependency already exists for the __rte_external flag

Sorry, I could not see the dependency.

[master][dpdk.org] $ grep -ri "objdump" devtools/
[master][dpdk.org] $ grep -ri "objdump" usertools/
[master][dpdk.org] $ grep -ri "__rte_external" *

> 
> > to checkpatch i.e developer has to build the library first so  that
> > checkpatch can can know, Is it belongs to internal section or not?
> >
> What developer is running checkpatch/posting patches without first building
> their changes?

# it is not developer, The CI/CD tools can quicky check the sanity of patches
before the build itself. Why to add unnecessary dependency?
# If some PMD is not building if the requirements are not meet(say AES NI PMD for crypto)
then how do take care of the dependency.


> 
> 
> > >
> > > 5) Adding a tag like __rte_internal is explicit, and allows
> > > developers to use a single header file instead of multiple header
> > > files if they so choose
> > >
> > > We went through this with experimental symbols as well[1], and it
> > > just makes more sense to me to clearly document in the code what
> > > constitutes an internal symbol rather than relying on naming
> > > conventions and hoping that developers read the documentation before
> > > exporting a symbol publically.
> > >
> > >
> > > [1] https://mails.dpdk.org/archives/dev/2017-December/083828.html
> > > > >
> > > > > If we really wanted to go down that road, we could use a
> > > > > mechainsm simmilar to the EXPORT_SYMBOL / EXPORT_SYMBOL_GPL
> > > > > infrastructure that the kernel uses, but that would required
> > > > > building our own custom linker script, which seems like overkill here.
> > > > >
> > > > > Best
> > > > > Neil
> > > > >
> > > > > > /Bruce
> > > > > >
> > > >
> >