Re: net/smc and the RDMA core

On 05/01/2017 07:29 PM, Bart Van Assche wrote:

On Mon, 2017-05-01 at 18:33 +0200, Christoph Hellwig wrote:

quoted

Hi Ursual, hi netdev reviewers,

how did the smc protocol manage to get merged without any review 
on linux-rdma at all?  As the results it seems it's very substandard
in terms of RDMA API usage, e.g. it neither uses the proper CQ API
nor the RDMA R/W API, and other will probably find additional issues
as well.

Hello Dave and Ursula,

It seems very rude to me to have merged the SMC protocol driver without
having involved the linux-rdma community. Anyway, I have the following
questions for Dave and Ursula:
* Since the Linux kernel is standards based: where can we find the standard
  that defines the SMC wire protocol? If this protocol has not been
  standardized yet: in what file (other than *.[ch]) in the Linux kernel
  tree has this protocol been documented?

Hello Bart,

The protocol is standardized, see: http://www.rfc-editor.org/info/rfc7609.
I described this and some more protocol details in my patch series
overview mail, see for instance:
     http://marc.info/?l=linux-s390&m=148397751211964&w=2

This description explains the reasons to come up with SMC-R.

* What are the differences between the SMC protocol, the SDP protocol and
  the rsockets protocol? How do existing implementations for these protocols
  compare to each other from a performance point of view? If no performance
  comparison between these protocols is available, shouldn't the performance
  of these protocols have been compared with each other before a review of
  the SMC driver even started?
* What are the reasons why the SDP driver was never accepted upstream? Do
  the arguments why SDP was not accepted upstream also apply to the SMC
  driver (SDP = Sockets Direct Protocol)?
* Since SMC has to be selected by specifying AF_SMC, how are users expected
  to specify whether AF_INET, AF_INET6 or yet another address family should
  be used to set up a connection between SMC
endpoints?

The IPv6 support in SMC-R is on our todo-list.

* Is the SMC driver limited to RoCE? Are you aware that the rsockets library
  supports multiple transport layers (RoCE, IB and iWARP)?

For now, only RoCE is supported. Other transports might be added in the future.

* Since functionality that is similar what the SMC driver provides already
  exists in user space (rsockets), why has this functionality been
  reimplemented as a kernel driver (SMC)?

Bart.

Regards, Ursula

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