Re: [dpdk-dev] [v4 1/3] cryptodev: support enqueue callback functions

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

Hi Akhil,
 
> Hi Konstantin,
> 
> > > > Hi Tech board members,
> > > >
> > > > I have a doubt about the ABI breakage in below addition of field.
> > > > Could you please comment.
> > > >
> > > > >  /** The data structure associated with each crypto device. */  struct
> > > > > rte_cryptodev {
> > > > >  	dequeue_pkt_burst_t dequeue_burst;
> > > > > @@ -867,6 +922,10 @@ struct rte_cryptodev {
> > > > >  	__extension__
> > > > >  	uint8_t attached : 1;
> > > > >  	/**< Flag indicating the device is attached */
> > > > > +
> > > > > +	struct rte_cryptodev_enq_cb_rcu *enq_cbs;
> > > > > +	/**< User application callback for pre enqueue processing */
> > > > > +
> > > > >  } __rte_cache_aligned;
> > > >
> > > > Here rte_cryptodevs is defined in stable API list in map file which is a pointer
> > > > To all rte_cryptodev and the above change is changing the size of the
> > structure.
> >
> > While this patch adds new fields into rte_cryptodev structure,
> > it doesn't change the size of it.
> > struct rte_cryptodev is cache line aligned, so it's current size:
> > 128B for 64-bit systems, and 64B(/128B) for 32-bit systems.
> > So for 64-bit we have 47B implicitly reserved, and for 32-bit we have 19B
> > reserved.
> > That's enough to add two pointers without changing size of this struct.
> >
> 
> The structure is cache aligned, and if the cache line size in 32Byte and the compilation
> is done on 64bit machine, then we will be left with 15Bytes which is not sufficient for 2
> pointers.
> Do we have such systems? 

AFAIK - no, minimal supported cache-line size: 64B:
lib/librte_eal/include/rte_common.h:#define RTE_CACHE_LINE_MIN_SIZE 64

> Am I missing something?

> The reason I brought this into techboard is to have a consensus on such change
> As rte_cryptodev is a very popular and stable structure. Any changes to it may
> Have impacts which one person cannot judge all use cases.

+1 here.
I also think it would be good to get other TB members opinion about proposed changes.
 
> > > > IMO, it seems an ABI breakage, but not sure. So wanted to double check.
> > > > Now if it is an ABI breakage, then can we allow it? There was no deprecation
> > > > notice Prior to this release.
> >
> > Yes, there was no deprecation note in advance.
> > Though I think the risk is minimal - size of the struct will remain unchanged (see
> > above).
> > My vote to let it in for 20.11.
> >
> > > > Also I think if we are allowing the above change, then we should also add
> > > > another Field for deq_cbs also for post crypto processing in this patch only.
> >
> > +1 for this.
> > I think it was already addressed in v5.
> >
> > Konstantin