netdev - Re: Designing a safe RX-zero-copy Memory Model for Networking

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Date:   Tue, 13 Dec 2016 10:36:55 -0600 (CST)
From:   Christoph Lameter <cl@...ux.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
cc:     John Fastabend <john.fastabend@...il.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Björn Töpel <bjorn.topel@...el.com>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>,
        Alexander Duyck <alexander.duyck@...il.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Tom Herbert <tom@...bertland.com>,
        Brenden Blanco <bblanco@...mgrid.com>,
        Tariq Toukan <tariqt@...lanox.com>,
        Saeed Mahameed <saeedm@...lanox.com>,
        Jesse Brandeburg <jesse.brandeburg@...el.com>,
        Kalman Meth <METH@...ibm.com>,
        Vladislav Yasevich <vyasevich@...il.com>
Subject: Re: Designing a safe RX-zero-copy Memory Model for Networking

On Tue, 13 Dec 2016, Jesper Dangaard Brouer wrote:

> This is the early demux problem.  With the push-mode of registering
> memory, you need hardware steering support, for zero-copy support, as
> the software step happens after DMA engine have written into the memory.

Right. But we could fall back to software. Transfer to a kernel buffer and
then move stuff over. Not much of an improvment but it will make things
work.

> > The discussion here is a bit amusing since these issues have been
> > resolved a long time ago with the design of the RDMA subsystem. Zero
> > copy is already in wide use. Memory registration is used to pin down
> > memory areas. Work requests can be filed with the RDMA subsystem that
> > then send and receive packets from the registered memory regions.
> > This is not strictly remote memory access but this is a basic mode of
> > operations supported  by the RDMA subsystem. The mlx5 driver quoted
> > here supports all of that.
>
> I hear what you are saying.  I will look into a push-model, as it might
> be a better solution.
>  I will read up on RDMA + verbs and learn more about their API model.  I
> even plan to write a small sample program to get a feeling for the API,
> and maybe we can use that as a baseline for the performance target we
> can obtain on the same HW. (Thanks to Björn for already giving me some
> pointer here)

Great.

> > What is bad about RDMA is that it is a separate kernel subsystem.
> > What I would like to see is a deeper integration with the network
> > stack so that memory regions can be registred with a network socket
> > and work requests then can be submitted and processed that directly
> > read and write in these regions. The network stack should provide the
> > services that the hardware of the NIC does not suppport as usual.
>
> Interesting.  So you even imagine sockets registering memory regions
> with the NIC.  If we had a proper NIC HW filter API across the drivers,
> to register the steering rule (like ibv_create_flow), this would be
> doable, but we don't (DPDK actually have an interesting proposal[1])

Well doing this would mean adding some features and that also would at
best allow general support for zero copy direct to user space with a
fallback to software if the hardware is missing some feature.

> > The RX/TX ring in user space should be an additional mode of
> > operation of the socket layer. Once that is in place the "Remote
> > memory acces" can be trivially implemented on top of that and the
> > ugly RDMA sidecar subsystem can go away.
>
> I cannot follow that 100%, but I guess you are saying we also need a
> more efficient mode of handing over pages/packet to userspace (than
> going through the normal socket API calls).

A work request contains the user space address of the data to be sent
and/or received. The address must be in a registered memory region. This
is different from copying the packet into kernel data structures.

I think this can easily be generalized. We need support for registering
memory regions, submissions of work request and the processing of
completion requets. QP (queue-pair) processing is probably the basis for
the whole scheme that is used in multiple context these days.