[dpdk-dev,v3,0/5] net/virtio: Tx path selection and offload improvements

  Since v2, the series introduce a new devarg to disable the
Tx simple path default. Indeed, as reported by Tiwei, simple
Tx path does not comply with the Virtio spec, and so should
not be selected automatically as it might break with some
Virtio device implementations. In the default setup, it
has an impact on performance when mergeable buffers are
disabled, as the standard path will now be used.

This series initially addressed a problem seen when running
benchmarks, where we see a big difference in Tx performance
depending on whether Rx mergeable is enabled or not.
With patch 2, Tx simple path is selected even when Rx-mrg is
negotiated.

Digging a bit further, I found that Tx simple path could be
selected even when offload features had been negotiated.
With patch 3, guest does not try to negotiate Tx offload
features that haven't been selected by the application.
It means that without restarting the guest, we can now switch
from Tx simple path when application do no request offloads,
to Tx standard path when the application request offloads.
Note that to do so, one must stop the port first [0].
Another advantage of doing this than using Tx simple path
when guest application does not use offload features, is
that on host side, the virtio net header parsing is skipped
in dequeue function.

Patch 4 fixes an issue with Rx offload, as simple path was
be used if application requested TCP LRO only.

Finally, patch 5 aims at improving the offload feature checks
in the standard paths.

Below are benchmarks results when offloads are enabled at
device level and simple Tx is disabled (default).

Rx-mrg=off benchmarks:
+------------+-------+-------------+-------------+----------+
|    Run     |  PVP  | Guest->Host | Host->Guest | Loopback |
+------------+-------+-------------+-------------+----------+
| v18.05     | 14.47 |       17.02 |       17.57 |    13.15 |
| + series   | 11.73 |       13.90 |       17.50 |    13.19 |
+------------+-------+-------------+-------------+----------+

Rx-mrg=on benchmarks:
+------------+-------+-------------+-------------+----------+
|    Run     |  PVP  | Guest->Host | Host->Guest | Loopback |
+------------+-------+-------------+-------------+----------+
| v18.05     |  9.53 |       13.80 |       16.70 |    13.11 |
| + series   |  9.58 |       13.93 |       16.70 |    13.11 |
+------------+-------+-------------+-------------+----------+

Below are benchmarks results when offloads are enabled at
device level and simple Tx is unlocked (with passing
'simple_tx_support=1' as Virtio device devarg).

Rx-mrg=off benchmarks:
+------------+-------+-------------+-------------+----------+
|    Run     |  PVP  | Guest->Host | Host->Guest | Loopback |
+------------+-------+-------------+-------------+----------+
| v18.05     | 14.47 |       17.02 |       17.57 |    13.15 |
| + series   | 14.88 |       19.64 |       17.50 |    13.14 |
+------------+-------+-------------+-------------+----------+

Rx-mrg=on benchmarks:
+------------+-------+-------------+-------------+----------+
|    Run     |  PVP  | Guest->Host | Host->Guest | Loopback |
+------------+-------+-------------+-------------+----------+
| v18.05     |  9.53 |       13.80 |       16.70 |    13.11 |
| + series   | 12.62 |       19.69 |       16.70 |    13.11 |
+------------+-------+-------------+-------------+----------+


[0]:
testpmd> port start 0
EAL: Error disabling MSI-X interrupts for fd 17
set_rxtx_funcs(): virtio: using mergeable buffer Rx path on port 0
set_rxtx_funcs(): virtio: using simple Tx path on port 0
Port 0: 52:54:00:58:D7:01
Checking link statuses...
Done
testpmd> show port 0 tx_offload capabilities 
Tx Offloading Capabilities of port 0 :
  Per Queue :
  Per Port  : VLAN_INSERT UDP_CKSUM TCP_CKSUM TCP_TSO MULTI_SEGS
         
testpmd> show port 0 tx_offload configuration
Tx Offloading Configuration of port 0 :
  Port :
  Queue[ 0] :

testpmd> port stop 0
Stopping ports...
Checking link statuses...
Done                
testpmd> csum set tcp hw 0
Parse tunnel is off
IP checksum offload is sw
UDP checksum offload is sw
TCP checksum offload is hw
SCTP checksum offload is sw
Outer-Ip checksum offload is sw
testpmd> port start 0
Configuring Port 0 (socket 0)
EAL: Error disabling MSI-X interrupts for fd 17
set_rxtx_funcs(): virtio: using mergeable buffer Rx path on port 0
set_rxtx_funcs(): virtio: using standard Tx path on port 0
Port 0: 52:54:00:58:D7:01
Checking link statuses...
Done                                         
testpmd> show port 0 tx_offload configuration
Tx Offloading Configuration of port 0 :
  Port : TCP_CKSUM
  Queue[ 0] :

Changes in v3:
==============
- Fix devargs parsing not to break if other args are passed (Tiwei)
- Various comestic fixes (Tiwei)

Changes in v2:
==============
- Introduce a devarg to disable simple Tx path by default (Tiwei)
- Use standard PATH if VLAN strip or insert offloads are requested (Tiwei)
- Use boolean instead of int/uint8_t for has_tx/rx_offload (Tiwei)

Maxime Coquelin (5):
  net/virtio: forbid simple Tx path by default
  net/virtio: use simple path for Tx even if Rx mergeable
  net/vhost: improve Tx path selection
  net/virtio: don't use simple Rx if TCP LRO or VLAN strip requested
  net/virtio: improve offload check performance

 doc/guides/nics/virtio.rst         |   9 +++
 drivers/net/virtio/virtio_ethdev.c | 121 ++++++++++++++++++++++++++++++++++---
 drivers/net/virtio/virtio_ethdev.h |   3 -
 drivers/net/virtio/virtio_pci.h    |   4 ++
 drivers/net/virtio/virtio_rxtx.c   |  30 ++-------
 5 files changed, 132 insertions(+), 35 deletions(-)