Merge branch 'net-hw-counters-for-soft-devices'

Ido Schimmel says:

====================
HW counters for soft devices

Petr says:

Offloading switch device drivers may be able to collect statistics of the
traffic taking place in the HW datapath that pertains to a certain soft
netdevice, such as a VLAN. In this patch set, add the necessary
infrastructure to allow exposing these statistics to the offloaded
netdevice in question, and add mlxsw offload.

Across HW platforms, the counter itself very likely constitutes a limited
resource, and the act of counting may have a performance impact. Therefore
this patch set makes the HW statistics collection opt-in and togglable from
userspace on a per-netdevice basis.

Additionally, HW devices may have various limiting conditions under which
they can realize the counter. Therefore it is also possible to query
whether the requested counter is realized by any driver. In TC parlance,
which is to a degree reused in this patch set, two values are recognized:
"request" tracks whether the user enabled collecting HW statistics, and
"used" tracks whether any HW statistics are actually collected.

In the past, this author has expressed the opinion that `a typical user
doing "ip -s l sh", including various scripts, wants to see the full
picture and not worry what's going on where'. While that would be nice,
unfortunately it cannot work:

- Packets that trap from the HW datapath to the SW datapath would be
  double counted.

  For a given netdevice, some traffic can be purely a SW artifact, and some
  may flow through the HW object corresponding to the netdevice. But some
  traffic can also get trapped to the SW datapath after bumping the HW
  counter. It is not clear how to make sure double-counting does not occur
  in the SW datapath in that case, while still making sure that possibly
  divergent SW forwarding path gets bumped as appropriate.

  So simply adding HW and SW stats may work roughly, most of the time, but
  there are scenarios where the result is nonsensical.

- HW devices will have limitations as to what type of traffic they can
  count.

  In case of mlxsw, which is part of this patch set, there is no reasonable
  way to count all traffic going through a certain netdevice, such as a
  VLAN netdevice enslaved to a bridge. It is however very simple to count
  traffic flowing through an L3 object, such as a VLAN netdevice with an IP
  address.

  Similarly for physical netdevices, the L3 object at which the counter is
  installed is the subport carrying untagged traffic.

  These are not "just counters". It is important that the user understands
  what is being counted. It would be incorrect to conflate these statistics
  with another existing statistics suite.

To that end, this patch set introduces a statistics suite called "L3
stats". This label should make it easy to understand what is being counted,
and to decide whether a given device can or cannot implement this suite for
some type of netdevice. At the same time, the code is written to make
future extensions easy, should a device pop up that can implement a
different flavor of statistics suite (say L2, or an address-family-specific
suite).

For example, using a work-in-progress iproute2[1], to turn on and then list
the counters on a VLAN netdevice:

    # ip stats set dev swp1.200 l3_stats on
    # ip stats show dev swp1.200 group offload subgroup l3_stats
    56: swp1.200: group offload subgroup l3_stats on used on
	RX:  bytes packets errors dropped  missed   mcast
		0       0      0       0       0       0
	TX:  bytes packets errors dropped carrier collsns
		0       0      0       0       0       0

The patchset progresses as follows:

- Patch #1 is a cleanup.

- In patch #2, remove the assumption that all LINK_OFFLOAD_XSTATS are
  dev-backed.

  The only attribute defined under the nest is currently
  IFLA_OFFLOAD_XSTATS_CPU_HIT. L3_STATS differs from CPU_HIT in that the
  driver that supplies the statistics is not the same as the driver that
  implements the netdevice. Make the code compatible with this in patch #2.

- In patch #3, add the possibility to filter inside nests.

  The filter_mask field of RTM_GETSTATS header determines which
  top-level attributes should be included in the netlink response. This
  saves processing time by only including the bits that the user cares
  about instead of always dumping everything. This is doubly important
  for HW-backed statistics that would typically require a trip to the
  device to fetch the stats. In this patch, the UAPI is extended to
  allow filtering inside IFLA_STATS_LINK_OFFLOAD_XSTATS in particular,
  but the scheme is easily extensible to other nests as well.

- In patch #4, propagate extack where we need it.
  In patch #5, make it possible to propagate errors from drivers to the
  user.

- In patch #6, add the in-kernel APIs for keeping track of the new stats
  suite, and the notifiers that the core uses to communicate with the
  drivers.

- In patch #7, add UAPI for obtaining the new stats suite.

- In patch #8, add a new UAPI message, RTM_SETSTATS, which will carry
  the message to toggle the newly-added stats suite.
  In patch #9, add the toggle itself.

At this point the core is ready for drivers to add support for the new
stats suite.

- In patches #10, #11 and #12, apply small tweaks to mlxsw code.

- In patch #13, add support for L3 stats, which are realized as RIF
  counters.

- Finally in patch #14, a selftest is added to the net/forwarding
  directory. Technically this is a HW-specific test, in that without a HW
  implementing the counters, it just will not pass. But devices that
  support L3 statistics at all are likely to be able to reuse this
  selftest, so it seems appropriate to put it in the general forwarding
  directory.

We also have a netdevsim implementation, and a corresponding selftest that
verifies specifically some of the core code. We intend to contribute these
later. Interested parties can take a look at the raw code at [2].

[1] https://github.com/pmachata/iproute2/commits/soft_counters
[2] https://github.com/pmachata/linux_mlxsw/commits/petrm_soft_counters_2

v2:
- Patch #3:
    - Do not declare strict_start_type at the new policies, since they are
      used with nla_parse_nested() (sans _deprecated).
    - Use NLA_POLICY_NESTED to declare what the nest contents should be
    - Use NLA_POLICY_MASK instead of BITFIELD32 for the filtering
      attribute.
- Patch #6:
    - s/monotonous/monotonic/ in commit message
    - Use a newly-added struct rtnl_hw_stats64 for stats transfer
- Patch #7:
    - Use a newly-added struct rtnl_hw_stats64 for stats transfer
- Patch #8:
    - Do not declare strict_start_type at the new policies, since they are
      used with nla_parse_nested() (sans _deprecated).
- Patch #13:
    - Use a newly-added struct rtnl_hw_stats64 for stats transfer
====================

Signed-off-by: David S. Miller <davem@davemloft.net>