[RFC,v1,0/4] Direct re-arming of buffers on receive side

  Currently, the transmit side frees the buffers into the lcore cache and
the receive side allocates buffers from the lcore cache. The transmit
side typically frees 32 buffers resulting in 32*8=256B of stores to
lcore cache. The receive side allocates 32 buffers and stores them in
the receive side software ring, resulting in 32*8=256B of stores and
256B of load from the lcore cache.

This patch proposes a mechanism to avoid freeing to/allocating from
the lcore cache. i.e. the receive side will free the buffers from
transmit side directly into it's software ring. This will avoid the 256B
of loads and stores introduced by the lcore cache. It also frees up the
cache lines used by the lcore cache.

However, this solution poses several constraint:

1)The receive queue needs to know which transmit queue it should take
the buffers from. The application logic decides which transmit port to
use to send out the packets. In many use cases the NIC might have a
single port ([1], [2], [3]), in which case a given transmit queue is
always mapped to a single receive queue (1:1 Rx queue: Tx queue). This
is easy to configure.

If the NIC has 2 ports (there are several references), then we will have
1:2 (RX queue: TX queue) mapping which is still easy to configure.
However, if this is generalized to 'N' ports, the configuration can be
long. More over the PMD would have to scan a list of transmit queues to
pull the buffers from.

2)The other factor that needs to be considered is 'run-to-completion' vs
'pipeline' models. In the run-to-completion model, the receive side and
the transmit side are running on the same lcore serially. In the pipeline
model. The receive side and transmit side might be running on different
lcores in parallel. This requires locking. This is not supported at this
point.

3)Tx and Rx buffers must be from the same mempool. And we also must
ensure Tx buffer free number is equal to Rx buffer free number:
(txq->tx_rs_thresh == RTE_I40E_RXQ_REARM_THRESH)
Thus, 'tx_next_dd' can be updated correctly in direct-rearm mode. This
is due to tx_next_dd is a variable to compute tx sw-ring free location.
Its value will be one more round than the position where next time free
starts.

Current status in this RFC:
1)An API is added to allow for mapping a TX queue to a RX queue.
  Currently it supports 1:1 mapping.
2)The i40e driver is changed to do the direct re-arm of the receive
  side.
3)L3fwd application is hacked to do the mapping for the following command:
  one core two flows case:
  $./examples/dpdk-l3fwd -n 4 -l 1 -a 0001:01:00.0 -a 0001:01:00.1
  -- -p 0x3 -P --config='(0,0,1),(1,0,1)'
  where:
  Port 0 Rx queue 0 is mapped to Port 1 Tx queue 0
  Port 1 Rx queue 0 is mapped to Port 0 Tx queue 0

Testing status:
1)Tested L3fwd with the above command:
The testing results for L3fwd are as follows:
-------------------------------------------------------------------
N1SDP:
Base performance(with this patch)   with direct re-arm mode enabled
      0%                                  +14.1%

Ampere Altra:
Base performance(with this patch)   with direct re-arm mode enabled
      0%                                  +17.1%
-------------------------------------------------------------------
This patch can not affect performance of normal mode, and if enable
direct-rearm mode, performance can be improved by 14% - 17% in n1sdp
and ampera-altra.

Feedback requested:
1) Has anyone done any similar experiments, any lessons learnt?
2) Feedback on API

Next steps:
1) Update the code for supporting 1:N(Rx : TX) mapping
2) Automate the configuration in L3fwd sample application

Reference:
[1] https://store.nvidia.com/en-us/networking/store/product/MCX623105AN-CDAT/NVIDIAMCX623105ANCDATConnectX6DxENAdapterCard100GbECryptoDisabled/
[2] https://www.intel.com/content/www/us/en/products/sku/192561/intel-ethernet-network-adapter-e810cqda1/specifications.html
[3] https://www.broadcom.com/products/ethernet-connectivity/network-adapters/100gb-nic-ocp/n1100g

Feifei Wang (4):
  net/i40e: enable direct re-arm mode
  ethdev: add API for direct re-arm mode
  net/i40e: add direct re-arm mode internal API
  examples/l3fwd: give an example for direct rearm mode

 drivers/net/i40e/i40e_ethdev.c        |  34 ++++++
 drivers/net/i40e/i40e_rxtx.h          |   4 +
 drivers/net/i40e/i40e_rxtx_vec_neon.c | 149 +++++++++++++++++++++++++-
 examples/l3fwd/main.c                 |   3 +
 lib/ethdev/ethdev_driver.h            |  15 +++
 lib/ethdev/rte_ethdev.c               |  14 +++
 lib/ethdev/rte_ethdev.h               |  31 ++++++
 lib/ethdev/version.map                |   3 +
 8 files changed, 251 insertions(+), 2 deletions(-)