rte_atomic API compatibility & standard atomics

[Morten Brørup]

> From: Tyler Retzlaff [mailto:roretzla at linux.microsoft.com]
> Sent: Tuesday, 28 March 2023 20.46
> 
> On Mon, Mar 27, 2023 at 10:08:10PM +0200, Morten Brørup wrote:
> > > From: Tyler Retzlaff [mailto:roretzla at linux.microsoft.com]
> > > Sent: Monday, 27 March 2023 21.39
> > >
> > > Hi folks,
> > >
> > > I don't think we discussed it specifically but what is the expectation
> > > in relation to converting to standard atomics and compatibility of the
> > > legacy rte_atomic APIs?
> > >
> > > We can't really convert the inline function implementations of the
> > > rte_atomic APIs because doing so would break compatibility. This is
> > > because if the implementation uses standard atomics APIs then we are
> > > required to pass _Atomic types to the generic atomic intrinsics.
> > >
> > > We can choose to just leave the rte_atomic API implementations as they
> > > are using the GCC builtins and i'm fine with that, but I do need some
> > > help with what to do with msvc then since it doesn't have those
> > > builtins.
> > >
> > > The options seem to be as follows.
> > >
> > > 1.
> > > Just cast the non-atomic types in the rte_atomic APIs implementation
> > > to _Atomic which may work but i'm pretty sure is undefined behavior
> > > since
> > > you can't qualify a non _Atomic type to suddenly be _Atomic.

This could also be an option, wrapped in #ifdef MSVC, so they are still unchanged for other build environments.

That limits your concern about undefined behavior to specifically how MSVC behaves.

> > >
> > > 2.
> > > We could conditionally compile (hide) the legacy rte_atomic APIs when
> > > msvc is in use, this seems not bad since there technically aren't any
> > > Windows/MSVC consumers, but if someone wanted to port an existing
> > > application they would have to adapt the code to avoid use of
> > > rte_atomic.
> > >
> > > For now I think the safest option is to go with 2 since it doesn't
> > > impose any compatibility risk and conditional compilation only exists
> > > until we deprecate and remove the old rte_atomic APIs.
> > >
> > > Are there any other options i'm missing here?
> > >
> > > Thanks
> >
> > As a variant of your second option, you could make most of the legacy
> rte_atomic APIs available to MSVC by changing the atomic counter types from
> volatile to _Atomic. Then only the atomic cmpset() and exchange() functions
> are unavailable for the application. E.g. for the 32 bit atomic counter type:
> >
> > typedef struct {
> > -	volatile int32_t cnt; /**< An internal counter value. */
> > +	_Atomic int32_t cnt; /**< An internal counter value. */
> > } rte_atomic32_t;
> >
> 
> it's a good suggestion. but i'm not sure i want to get bogged down
> making an old api available that hopefully we will remove soon.
> 
> though i'm still torn because i would really like the path to use msvc
> for any application to be lower burden.
> 
> unless there are objections i think i'll do 2 as is. if good progress is
> made we can re-evaluate doing the extra work to make available the old apis
> as you suggest or potentially leave them unavailable forever subject to
> any plans to deprecate and remove them.

No objections from me, either way.

>From a high level perspective, I consider it perfectly reasonable to get up and running with very limited support. When MSVC gets more traction, and MSVC users want more of DPDK, I expect to see questions on the mailing list, or directly to you or the MSVC team. Then you can focus catching up on the features in demand.