netdev - Re: net/smc and the RDMA core

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Date:   Tue, 2 May 2017 18:39:20 +0000
From:   Bart Van Assche <Bart.VanAssche@...disk.com>
To:     "hch@....de" <hch@....de>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "ubraun@...ux.vnet.ibm.com" <ubraun@...ux.vnet.ibm.com>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>
Subject: Re: net/smc and the RDMA core

On Tue, 2017-05-02 at 14:25 +0200, Ursula Braun wrote:
> if you can point out specific issues, we will be happy to work with you
> to get them addressed!

Hello Ursula,

My list of issues that I would like to see addressed can be found below. Doug,
Christoph and others may have additional inputs. The issues that have not yet
been mentioned in other e-mails are:
- The SMC driver only supports one RDMA transport type (RoCE v1) but
  none of the other RDMA transport types (RoCE v2, IB and iWARP). New
  RDMA drivers should support all RDMA transport types transparently.
  The traditional approach to support multiple RDMA transport types is
  by using the RDMA/CM to establish connections.
- The implementation of the SMC driver only supports RoCEv1. This is
  a very unfortunate choice because:
  - RoCEv1 is not routable and hence is limited to a single Ethernet
    broadcast domain.
  - RoCEv1 packets escape a whole bunch of mechanisms that only work
    for IP packets. Firewalls pass all RoCEv1 packets and switches
    do not restrict RoCEv1 packets to a single VLAN. This means that
    if the network configuration is changed after an SMC connection
    has been set up such that IP communication between the endpoints
    of an SMC connection is blocked that the SMC RoCEv1 packets will
    not be blocked by the network equipment of which the configuration
    has just been changed.
- As already mentioned by Christoph, the SMC implementation uses RDMA
  calls that probably will be deprecated soon (ib_create_cq()) and
  should use the RDMA R/W API instead of building sge lists itself.

Bart.