[dpdk-dev] [EXT] Re: [PATCH v11 2/4] eal: add legacy kni option

[Jerin Jacob]

On Wed, Oct 23, 2019 at 8:17 PM Olivier Matz <olivier.matz at 6wind.com> wrote:
>
> Hi,
>
> On Wed, Oct 23, 2019 at 03:42:39PM +0530, Jerin Jacob wrote:
> > On Tue, Oct 22, 2019 at 7:01 PM Vamsi Krishna Attunuru
> > <vattunuru at marvell.com> wrote:
> > >
> > > Hi Ferruh,
> > >
> > > Can you please explain the problems in using kni dedicated mbuf alloc routines while enabling kni iova=va mode. Please see the below discussion with Andrew. He wanted to know the problems in having newer APIs.
> >
> >
> > While waiting  for the Ferruh reply, I would like to summarise the
> > current status
> >
> > # In order to make KNI work with IOVA as VA, We need to make sure
> > mempool pool _object_ should not span across two huge pages
> >
> > # This problem can be fixed by, either of:
> >
> > a) Introduce a flag in mempool to define this constraint, so that,
> > when only needed, this constraint enforced and this is in line
> > with existing semantics of addressing such problems in mempool
> >
> > b) Instead of creating a flag, Make this behavior by default in
> > mempool for IOVA as VA case
> >
> > Upside:
> > b1) There is no need for specific mempool_create for KNI.
> >
> > Downside:
> > b2) Not align with existing mempool API semantics
> > b3) There will be a trivial amount of memory waste as we can not
> > allocate from the edge. Considering the normal huge
> > page memory size is 1G or 512MB this not a real issue.
> >
> > c) Make IOVA as PA when KNI kernel module is loaded
> >
> > Upside:
> > c1) Doing option (a) would call for new KNI specific mempool create
> > API i.e existing KNI applications need a one-line change in
> > application to make it work with release 19.11 or later.
> >
> > Downslide:
> > c2) Driver which needs RTE_PCI_DRV_NEED_IOVA_AS_VA can not work with KNI
> > c3) Need root privilege to run KNI as IOVA as PA need root privilege
> >
> > For the next year, we expect applications to work 19.11 without any
> > code change. My personal opinion to make go with option (a)
> > and update the release notes to document the change any it simple
> > one-line change.
> >
> > The selection of (a) vs (b) is between KNI and Mempool maintainers.
> > Could we please reach a consensus? Or can we discuss this TB meeting?
> >
> > We are going back and forth on this feature on for the last 3
> > releases. Now that, we solved all the technical problems, please help
> > us
> > to decide (a) vs (b) to make forward progress.
>
> Thank you for the summary.
> What is not clear to me is if (a) or (b) may break an existing
> application, and if yes, in which case.

Thanks for the reply.

To be clear we are talking about out of tree KNI tree application.
Which they don't want to
change rte_pktmbuf_pool_create() to rte_kni_pktmbuf_pool_create() and
build for v19.11

So in case (b) there is no issue as It will be using rte_pktmbuf_pool_create ().
But in case of (a) it will create an issue if out of tree KNI
application is using rte_pktmbuf_pool_create() which is not using the
NEW flag.