[ovs-dev] [RFC v5 0/8] Support multi-segment mbufs

[Tiago Lam]

Overview
========
This patchset introduces support for multi-segment mbufs to OvS-DPDK.
Multi-segment mbufs are typically used when the size of an mbuf is
insufficient to contain the entirety of a packet's data. Instead, the
data is split across numerous mbufs, each carrying a portion, or
'segment', of the packet data. mbufs are chained via their 'next'
attribute (an mbuf pointer).

Use Cases
=========
i.  Handling oversized (guest-originated) frames, which are marked
    for hardware accelration/offload (TSO, for example).

    Packets which originate from a non-DPDK source may be marked for
    offload; as such, they may be larger than the permitted ingress
    interface's MTU, and may be stored in an oversized dp-packet. In
    order to transmit such packets over a DPDK port, their contents
    must be copied to a DPDK mbuf (via dpdk_do_tx_copy). However, in
    its current implementation, that function only copies data into
    a single mbuf; if the space available in the mbuf is exhausted,
    but not all packet data has been copied, then it is lost.
    Similarly, when cloning a DPDK mbuf, it must be considered
    whether that mbuf contains multiple segments. Both issues are
    resolved within this patchset.

ii. Handling jumbo frames.

    While OvS already supports jumbo frames, it does so by increasing
    mbuf size, such that the entirety of a jumbo frame may be handled
    in a single mbuf. This is certainly the preferred, and most
    performant approach (and remains the default). However, it places
    high demands on system memory; multi-segment mbufs may be
    prefereable for systems which are memory-constrained.

Enabling multi-segment mbufs
============================
Multi-segment and single-segment mbufs are mutually exclusive, and the
user must decide on which approach to adopt on init. The introduction
of a new OVSDB field, 'dpdk-multi-seg-mbufs', facilitates this.

This is a global boolean value, which determines how jumbo frames are
represented across all DPDK ports. In the absence of a user-supplied
value, 'dpdk-multi-seg-mbufs' defaults to false, i.e. multi-segment
mbufs must be explicitly enabled / single-segment mbufs remain the
default.

Setting the field is identical to setting existing DPDK-specific OVSDB
fields:

    ovs-vsctl set Open_vSwitch . other_config:dpdk-init=true
    ovs-vsctl set Open_vSwitch . other_config:dpdk-lcore-mask=0x10
    ovs-vsctl set Open_vSwitch . other_config:dpdk-socket-mem=4096,0
==> ovs-vsctl set Open_vSwitch . other_config:dpdk-multi-seg-mbufs=true

---

v5: - Rebased on master e5e22dc ("datapath-windows: Prevent ct-counters
      from getting redundantly incremented");
    - Sugesh's comments have been addressed:
      - Changed dp_packet_set_data() and dp_packet_set_size() logic to
        make them independent of each other;
      - Dropped patch 3 now that dp_packet_set_data() and dp_packet_set_
        size() are independent;
      - dp_packet_clone_with_headroom() now has split functions for
        handling DPDK sourced packets and non-DPDK packets;
    - Modified various functions in dp-packet.h to account for multi-seg
      mbufs - dp_packet_put_uninit(), dp_packet_tail(), dp_packet_tail()
      and dp_packet_at();
    - Added support for shifting packet data in multi-seg mbufs, using
      dp_packet_shift();
    - Fixed some minor inconsistencies.

    Note that some of the changes in v5 have been contributed by Mark
    Kavanaugh as well.

v4: - restructure patchset
    - account for 128B ARM cacheline when sizing mbufs

Mark Kavanagh (5):
  netdev-dpdk: fix mbuf sizing
  dp-packet: init specific mbuf fields to 0
  dp-packet: Add support for multi-seg mbufs
  netdev-dpdk: copy large packet to multi-seg. mbufs
  netdev-dpdk: support multi-segment jumbo frames

Michael Qiu (2):
  dp-packet: copy mbuf info for packet copy
  dp-packet: copy data from multi-seg. DPDK mbuf

Tiago Lam (1):
  dp-packet: Fix data_len issue with multi-seg mbufs

 NEWS                 |   1 +
 lib/dp-packet.c      | 118 ++++++++++++++++++++++++++++----
 lib/dp-packet.h      | 189 ++++++++++++++++++++++++++++++++++++---------------
 lib/dpdk.c           |   7 ++
 lib/netdev-dpdk.c    | 183 +++++++++++++++++++++++++++++++++++++++----------
 lib/netdev-dpdk.h    |   1 +
 vswitchd/vswitch.xml |  20 ++++++
 7 files changed, 415 insertions(+), 104 deletions(-)

-- 
2.7.4