[ovs-dev] [RFC 0/2] dpdk: Add support for TSO

[Tiago Lam]

Enabling TSO offload allows a host stack to delegate the segmentation of
oversized TCP packets to the underlying physical NIC, if supported. In the case
of a VM this means that the segmentation of the packets is not performed by the
guest kernel, but by the host NIC itself. In turn, since the TSO calculations
and checksums are being performed in hardware, this alleviates the CPU load on
the host system. In inter VM communication this might account to significant
savings, and higher throughput, even more so if the VMs are running on the same
host.

Thus, although inter VM communication is already possible as is, there's a
sacrifice in terms of CPU, which may affect the overall throughput.

This series proposes to add support for TSO in OvS-DPDK, by making use of the
TSO offloading feature already supported by DPDK vhost backend, having the
following scenarios in mind:
- Inter VM communication on the same host;
- Inter VM communication on different hosts;
- The same two use cases above, but on a VLAN network.

The work is based on [1]; It has been rebased to run on top of the
multi-segment mbufs work (v7) [2] and re-worked to use the new Tx offload API
as per [3].

[1] https://patchwork.ozlabs.org/patch/749564/
[2] https://mail.openvswitch.org/pipermail/ovs-dev/2018-July/350081.html
[3] http://dpdk.readthedocs.io/en/v17.11/rel_notes/deprecation.html

Considerations:
- This series depends on the multi-segment mbuf series (v7) and can't be
  applied on master as is;
- Right now TSO is enabled by default when using multi-segment mbufs, or
  otherwise TSO is disabled and can't be set on its own. I'm open to opinions
  on this, though. My main idea to stick TSO behind multi-segment mbufs is:
  - The performance enhancements enabled by DPDK PMDs, such as vectorization,
    that are disabled when using multi-segment mbufs are also disabled when
    using some of the offload features;
  - By enabling the multi-segment mbuf flag, the packets have a default size of
    2048B. This might be higher if not using multi-segments mbufs, since the
    size of the packet will be ajusted to the MTU (such as 9000B). This might
    lead to a higher waste of memory when appending mbufs to each other as we
    will be increamenting larger ammounts.
- There's some initial documentation on patch 1/2 (which came form [1]), but
  needs improving.

Tiago Lam (2):
  netdev-dpdk: Consider packets marked for TSO.
  netdev-dpdk: Enable TSO when using multi-seg mbufs

 Documentation/topics/dpdk/phy.rst |  64 ++++++++++++++
 lib/dp-packet.c                   |   5 +-
 lib/netdev-dpdk.c                 | 172 ++++++++++++++++++++++++++++++--------
 3 files changed, 204 insertions(+), 37 deletions(-)

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