Re: [PATCH V1 rdma-core 1/3] verbs: Enable creating QP with a given source QP number

[Jason Gunthorpe <jgunthorpe-ePGOBjL8dl3ta4EC/59zMFaTQe2KTcn/@public.gmane.org>]

On Sun, Sep 10, 2017 at 09:54:45AM +0300, Alex Rosenbaum wrote:
> On Thu, Sep 7, 2017 at 10:29 PM, Jason Gunthorpe
> <jgunthorpe-ePGOBjL8dl3ta4EC/59zMFaTQe2KTcn/@public.gmane.org> wrote:
> > On Thu, Sep 07, 2017 at 09:46:39PM +0300, Alex Rosenbaum wrote:
> >> On Thu, Sep 7, 2017 at 6:16 PM, Jason Gunthorpe
> >> <jgunthorpe-ePGOBjL8dl3ta4EC/59zMFaTQe2KTcn/@public.gmane.org> wrote:
> >> > On Thu, Sep 07, 2017 at 03:12:05PM +0300, Yishai Hadas wrote:
> >> >> +uint32_t                source_qpn;     /* Source QP number, creation flag IBV_QP_CREATE_SOURCE_QPN should be set */
> >> >
> >> > I think more discussion in the man page is required for this
> >> > feature. Eg that it is a SEND ONLY QP and only works for UD.
> >>
> >> Verbs UD QP can handle receive of ingress packets sent with
> >> BTH.DestQPN = ibv_qp->qp_num. This is applicable also in this case.
> >>
> >> Also, future implementation might include RC source qpn use case. This
> >> can be accomplished by setting the HCA's QPN to be the same as the
> >> 'source qpn' value, and returning that to the user.
> >>
> >> We prefer not to limit the API definition in the man page.
> >
> > Well, you have to define what the API does, specifically.
> >
> > If you expect it to RX then you need to specify that.
> 
> Like any other QP, this too can have a max_recv_wr defined by user. I
> don't think we need to specifically mention it, because it is not a
> limition.

You need to mention what impact source_qpn has on the RX, assuming the
user does nothing else.

>From what I can gather, without flow steering, RX should not be
possible.

This needs to be described.

> > There is nothing wrong with specifying the API limit that exist today,
> > it helps people understand what the feature is and how to use it.
> 
> RC is currently not supported. Do you prefer we update the man page
> with the UD limitation? and once RC or other limitation are removed we
> update it again?

Yes. Describe what it does. The man page is not a spec, it documents
what the software actually does.

Jason
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