netdev - [RFC] xdp, multiple rss objects

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Date:   Fri, 29 Mar 2019 16:14:57 +0000
From:   Jonathan Lemon <bsd@...com>
To:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>
CC:     Magnus Karlsson <magnus.karlsson@...el.com>,
        Björn Töpel <bjorn.topel@...il.com>,
        "jakub.kicinski@...ronome.com" <jakub.kicinski@...ronome.com>,
        Saeed Mahameed <saeedm@...lanox.com>
Subject: [RFC] xdp, multiple rss objects

Background:

With AF_XDP, packets are placed on queues which are serviced directly from
an application running in user space, which promises greatly increased
performance.  However, not all incoming packets should be delivered to the
application; there may be management traffic, or packets which should be
handled by normal Linux networking applications.

Having the AF_XDP application handle the packets and return them to the
kernel for processing is not ideal, nor is adding a bpf filter at the kernel
level which performs the redirection.  It would be nicer if the redirection
was done in the NIC itself.  Fortunately, there is a facility for this: the
RX network flow classification.

Unfortunately, the target of the classification is currently a single queue.

Traffic arriving at a NIC which is not steered to a specific queue can be
distributed across a set of queues through the RX flow hash redirection
table (RSS steering).  But being able to target a separate RSS object would
allow arbitrary distribution of previously classified traffic.

Ask:

It would be nice to have more than one RSS table, which would be the target
of a classifier or default packet stream.

Queues would be attached to an RSS object, and flow classifiers could target
the RSS object for further distribution.  For example, something like:

  ethtool -X eth0 rss 0 queue 0-13
  ethtool -X eth0 rss 1 queue 14-55
  ethtool -N eth0 flow-type udp6 dst-port 4242 rss 1

This would set up a default RSS object (rss 0), which sprays traffic to 14
queues; this would be equivalent to "ethtool -X eth0 equal 14".  At the same
time, a second object (rss 1) is set up, distributing traffic across another
set of queues.

Is there interest in adding this type of feature?  Having the ability to
express this concept (not necessarily in ethtool) should facilitate having
HW vendors add this to their products.  I believe that some vendors have
hardware which may be capable of this already.
-- 
Jonathan