Re: [PATCH rdma-next 1/6] IB/core: Enable QP creation which is associated to underlay QP

From: Jason Gunthorpe <jgunthorpe-ePGOBjL8dl3ta4EC/59zMFaTQe2KTcn/@public.gmane.org>
To: Alex Rosenbaum <rosenbaumalex-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
Cc: Yishai Hadas
	<yishaih-LDSdmyG8hGV8YrgS2mwiifqBs+8SCbDb@public.gmane.org>,
	Leon Romanovsky <leon-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>,
	Doug Ledford <dledford-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>,
	linux-rdma-u79uwXL29TY76Z2rM5mHXA@public.gmane.org,
	Yishai Hadas <yishaih-VPRAkNaXOzVWk0Htik3J/w@public.gmane.org>,
	"Alex @ Mellanox" <Alexr-VPRAkNaXOzVWk0Htik3J/w@public.gmane.org>,
	Majd Dibbiny <majd-VPRAkNaXOzVWk0Htik3J/w@public.gmane.org>
Subject: Re: [PATCH rdma-next 1/6] IB/core: Enable QP creation which is associated to underlay QP
Date: Tue, 6 Jun 2017 10:33:20 -0600	[thread overview]
Message-ID: <20170606163320.GC8671@obsidianresearch.com> (raw)
In-Reply-To: <CAFgAxU9c1SwehkCY3o0RZPO_CTHGJb2A1omjVvJvyabO0V57iQ-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>

On Tue, Jun 06, 2017 at 06:49:52PM +0300, Alex Rosenbaum wrote:
>    IPoIB is a tunneled protocol. The underlay part, a UD QP, has a set of
>    properties that define its L2-L4 characteristics. In this suggestion we
>    wanted to make sure the overlay will use the same QP so that there are
>    no mismatches between the IB layer properties. An example for such a
>    mistake can be if the overlay sets a PKey which is different than the
>    IPoIB UD QP. A mistake example on the receive flow; when setting
>    multiple ingress steering on using multiple different PKey's into a
>    single user space UD QP.

That seems like a strange thing to worry about, any user of this
already has to closely track all of the IB parameters as they are
required to issue path records.

So this underlay/overlay complexity really doesn't do much for
safety/usability, IMHO - it just introduces more complexity and more
failure modes.

>    Naming it "source_qpn" instead of associated, as you suggest, sound
>    more appropriate.

Simpler, certainly if you completely eliminate the notion of a linkage
to another QP. This is just creating a TX QP with a specified QPN that
can overlap an existing bidir QP's QPN.

This could then also be potentially used for applications that desire a
fixed QPN.

>    As noted in previous email, the overlay QP can work properly only when
>    the underlay QP is not in RTR or RTS. Meaning no egress or ingress
>    traffic will happen once underlay is not in these states. This is

Yuk, a hidden variable that causes undetectable failure is gross.

Jason
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