Re: [dpdk-dev] [PATCH v4 1/2] mbuf: support attaching external buffer to mbuf

[Thomas Monjalon <thomas@monjalon.net>]

24/04/2018 23:53, Yongseok Koh:
> On Tue, Apr 24, 2018 at 10:22:45PM +0200, Thomas Monjalon wrote:
> > 24/04/2018 21:15, Olivier Matz:
> > > On Tue, Apr 24, 2018 at 09:21:00PM +0300, Andrew Rybchenko wrote:
> > > > On 04/24/2018 07:02 PM, Olivier Matz wrote:
> > > > > On Tue, Apr 24, 2018 at 03:28:33PM +0300, Andrew Rybchenko wrote:
> > > > > > On 04/24/2018 04:38 AM, Yongseok Koh wrote:
> > > > > > > + * Returns TRUE if given mbuf is cloned by mbuf indirection, or FALSE
> > > > > > > + * otherwise.
> > > > > > > + *
> > > > > > > + * If a mbuf has its data in another mbuf and references it by mbuf
> > > > > > > + * indirection, this mbuf can be defined as a cloned mbuf.
> > > > > > > + */
> > > > > > > +#define RTE_MBUF_CLONED(mb)     ((mb)->ol_flags & IND_ATTACHED_MBUF)
> > > > > > > +
> > > > > > > +/**
> > > > > > >     * Returns TRUE if given mbuf is indirect, or FALSE otherwise.
> > > > > > >     */
> > > > > > > -#define RTE_MBUF_INDIRECT(mb)   ((mb)->ol_flags & IND_ATTACHED_MBUF)
> > > > > > > +#define RTE_MBUF_INDIRECT(mb)   RTE_MBUF_CLONED(mb)
> > > > > > It is still confusing that INDIRECT != !DIRECT.
> > > > > > May be we have no good options right now, but I'd suggest to at least
> > > > > > deprecate
> > > > > > RTE_MBUF_INDIRECT() and completely remove it in the next release.
> > > > > Agree. I may have missed something, but is my previous suggestion
> > > > > not doable?
> > > > > 
> > > > > - direct = embeds its own data      (and indirect = !direct)
> > > > > - clone (or another name) = data is another mbuf
> > > > > - extbuf = data is in an external buffer
> > > > 
> > > > I guess the problem that it changes INDIRECT semantics since EXTBUF
> > > > is added as well. I think strictly speaking it is an API change.
> > > > Is it OK to make it without announcement?
> > > 
> > > In any case, there will be an ABI change, because an application
> > > compiled for 18.02 will not be able to handle these new kind of
> > > mbuf.
> > > 
> > > So unfortunatly yes, I think this kind of changes should first be
> > > announced.
> > > 
> > > Thomas, what do you think?
> > 
> > What is the impact for the application developer?
> > Is there something to change in the application after this patch?
> 
> Let me address two concerns discussed here.
> 
> 1) API breakage of RTE_MBUF_DIRECT()
> Previously, direct == !indirect but now direct == !indirect && !extbuf. But to
> set the new flag (EXT_ATTACHED_MBUF), the new API, rte_pktmbuf_attach_extbuf()
> should be used and it is experimental. If application isn't compiled without
> allowing experimental API or application doesn't use the new API, it is always
> true that direct == !indirect. It looks logically okay to me. And FYI, it passed
> the mbuf_autotest.
> 
> 2) ABI breakage of mlx5's new Multi-Packet RQ (a.k.a MPRQ) feature
> It's right that it could breadk ABI if the PMD delivers packets with external
> buffer attached. But, the MPRQ feature is disabled by default and it can be
> enabled only by the newly introduced PMD parameter (mprq_en). So, there's no
> possibility that 18.02-based application receives a mbuf having an external
> buffer. And, like Olivier mentioned, there's another ABI breakage by removing
> control mbuf anyway.
> 
> So, I don't think there's need for developers to change their application after
> this patch unless they want to use the new feature.

To summarize, this a feature addition, there is no breakage.
So I don't see what should be announced.

I think it could be integrated as experimental with a first
PMD implementation in 18.05. It will allow to test the feature
in the field, and have more feedbacks about how to improve the API.