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

Wiles, Keith keith.wiles at intel.com
Fri Jun 7 20:21:21 CEST 2019

> On Jun 7, 2019, at 10:42 AM, Ray Kinsella <mdr at ashroe.eu> wrote:
> On 06/06/2019 16:03, Neil Horman wrote:
>> On Thu, Jun 06, 2019 at 02:02:03PM +0000, Jerin Jacob Kollanukkaran wrote:
>>>> -----Original Message-----
>>>> From: Neil Horman <nhorman at tuxdriver.com>
>>>> Sent: Thursday, June 6, 2019 7:05 PM
>>>> To: Jerin Jacob Kollanukkaran <jerinj at marvell.com>
>>>> Cc: Bruce Richardson <bruce.richardson at intel.com>; dev at dpdk.org;
>>>> Thomas Monjalon <thomas at 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 at tuxdriver.com>
>>>>>> Sent: Thursday, June 6, 2019 5:04 PM
>>>>>> To: Jerin Jacob Kollanukkaran <jerinj at marvell.com>
>>>>>> Cc: Bruce Richardson <bruce.richardson at intel.com>; dev at dpdk.org;
>>>>>> Thomas Monjalon <thomas at 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 at tuxdriver.com>
>>>>>>>> Sent: Wednesday, June 5, 2019 11:41 PM
>>>>>>>> To: Bruce Richardson <bruce.richardson at intel.com>
>>>>>>>> Cc: Jerin Jacob Kollanukkaran <jerinj at marvell.com>;
>>>>>>>> dev at dpdk.org; Thomas Monjalon <thomas at 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 at tuxdriver.com>
>>>>>>>>>>> Sent: Sunday, May 26, 2019 12:14 AM
>>>>>>>>>>> To: dev at dpdk.org
>>>>>>>>>>> Cc: Neil Horman <nhorman at tuxdriver.com>; Jerin Jacob
>>>>>>>>>>> Kollanukkaran <jerinj at marvell.com>; Bruce Richardson
>>>>>>>>>>> <bruce.richardson at intel.com>; Thomas Monjalon
>>>>>>>>>>> <thomas at 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
>> Because its more clear.  Implicitly deciding that any function not prefixed with
>> rte_ is internal only does nothing to prevent a developer from accidentally
>> naming a function incorrectly, exporting it, and allowing a user to call it. We
>> can move headers all you want, but we provide an ABI guarantee to end users, and
>> developers should have a way to clearly record that without having to check the
>> documentation for each function that an application developer wants to use.
>> The long and the short of it for me is that I want a way for developers to opt
>> their code into an internal only condition, not to just document it as such
>> and hope its up to date.  If they tag a function as __rte_internal then its
>> clearly marked as internal only, they have checks to ensure that its in the
>> INTERNAL section of the version map, and should that header somehow get
>> externally exported (see rte_mempool_check_cookies for an example of how thats
>> happened), users are prevented from using them at build time, rather than having
>> to ask questions on the list, or read documentation after an error to find out
>> "oops, shouldn't have done that".
>> I think you'll find that going through all the header files, and bifurcating
>> them to public and private headers is a much larger undertaking than just
>> tagging those functions accordingly.  a quick scan of all our header file for
>> the @internal tag shows about 260 instances of such functions, almost all of
>> which are published to applications.  All of those functions would have to be
>> moved to private headers, and their requisite C files would need to be updated
>> to include the new header.  with the use of __rte_internal, we just have tag the
>> functions as such, which can be handled with a cocinelle or awk script.
>> Neil
> This is good, I like alot about this, especially the build system
> complaining loudly when the developer does something they shouldn't - I
> think anything that we can add that promotes good behaviors is to be
> 100% welcomed.
> I also agree with the points made elsewhere that this is essentially
> trying to solve a header problem, the mixing of public and private
> symbols in what are public headers, with __rte_internal. Adding
> __rte_internal would essentially ratify that behavior, whereas I would
> argue that something inherently private, should never see the light of
> day in a public header.
> I completely get that it may be more work, however for me it is better
> way to fix this problem. It would also add completely clarity, all the
> extra hassle around does it have the rte_ prefix goes away - if it is in
> a "public header" it is part of the ABI/API, end of discussion.
> Finally, not opposed to also asking folks putting symbols in the private
> header to mark those symbols with __rte_internal.

+1 I think we need to do both split headers and __rte_internal for extra measure. I am still concerned we are adding more work for the developer, if not then at least we split the headers.
>>>>>> 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


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