[dpdk-dev] Direct using of 'rte_eth_devices' in DPDK apps.

[Ananyev, Konstantin]

Hi everyone,

> 
> Hi,
> 
> 16/11/2018 09:42, Ilya Maximets:
> > Hi,
> > While discussing the ways to enable DPDK 18.11 new features in OVS
> > there was suggestions to use 'rte_eth_devices[]' array directly.
> > But this array is marked as '@internal' and also it located in
> > the internal header 'lib/librte_ethdev/rte_ethdev_core.h' with the
> > following disclaimer:
> >
> > /**
> >  * @file
> >  *
> >  * RTE Ethernet Device internal header.
> >  *
> >  * This header contains internal data types. But they are still part of the
> >  * public API because they are used by inline functions in the published API.
> >  *
> >  * Applications should not use these directly.
> >  *
> >  */
> >
> > From the other hand, test-pmd and some example apps in DPDK source
> > tree are using this array for various reasons.
> >
> > So, is it OK to use this array directly or not?
> 
> Good question :)
> Thanks for bringing this discussion.
> 
> As you said, it is public because of inline functions using it directly
> for performance purpose. The DPDK API is bad for separating public and
> internal stuff. And over time, there is not a lot of attention on trying
> to not use internal symbols in applications.
> 
> > In general we need to change the API, i.e. make 'rte_eth_devices' part
> > of a public API. Or change the test-pmd and example apps to stop
> > using it.
> 
> I agree we need to decide an option and make it clear.
> 
> We can try to make this variable private and add more public functions
> to use it (I'm thinking at more iterators like sibling ones).
> It would clarify the API.
> It can be evaluated what is the real cost after compiler optimization
> for Rx/Tx functions. It can also be evaluated to uninline functions.
> 
> On the other hand, we can wonder what is the real benefit of trying to
> hide access to internal resources. Should we make all public?

In that case every change in any of such structures will be an ABI breakage.
Even now any change in rte_eth_dev is quite problematic because of that.
I think we better keep them private as much as possible and cleanup
our examples and testpmd code.
Konstantin

> 
> > One more related question: Is it OK to access internal device
> > stuff using 'device' pointer obtained by 'rte_eth_dev_info'?
> > This looks really dangerous. It's unclear why pointers like this
> > exposed to user.
> 
> It's a lot easier to expose pointers than doing a good API for all uses.
> We need to question what is really dangerous and what we want to avoid?
> 
>