[PATCH 0/2] xennet: improve RX NBL batching and fix TX doorbell coalescing

[david ambu <david.preetham@xxxxxxxxxx>]

This series addresses two independent throughput issues in xennet.

Patch 1 — NBL batching on the receive path:

  The receive path previously called NdisMIndicateReceiveNetBufferLists
  once per NBL (batch size of 1), causing excessive NDIS stack overhead
  on high-throughput workloads. This patch raises NblBatchMax to 32
  (configurable up to 64), groups consecutive NBLs with the same
  EtherType into a single indicate call with
  NDIS_RECEIVE_FLAGS_SINGLE_ETHER_TYPE, and fixes __ReceiverPushPackets
  to move KeMemoryBarrier before the Returned/Indicated reads and return
  early when the queue is empty.

Patch 2 — TX doorbell coalescing across NBL boundaries:

  __TransmitterSendNetBufferList computed the More flag as
  (NET_BUFFER_NEXT_NB(NetBuffer) != NULL), which evaluates to FALSE for
  the last NB of every NBL even when the outer loop still has additional
  NBLs to submit. This caused a premature doorbell ring between
  back-to-back NBLs. Fixed by passing MoreNbls from the outer loop.

  Note: a companion fix in xenvif is also required —
  TransmitterQueuePacket unconditionally reset More = FALSE for
  non-RSS traffic, overriding the value passed in from xennet.

Performance (iperf2, 60 s, 3 runs, Windows PV guest):

  Single-stream TCP:  4,606 -> 6,056 Mbps  (+31.5%)
  8-stream TCP:      14,663 -> 18,679 Mbps  (+27.4%)
  8-stream UDP:       1,117 ->  1,230 Mbps  (+10.1%)
  Single-stream UDP:    327 ->    329 Mbps  (no change, expected)

david ambu (2):
  xennet: improve NBL batching receive path
  xennet: fix TX doorbell coalescing — More flag dropped at NBL
    boundaries

 src/xennet.inf           |   6 +++
 src/xennet/adapter.c     |  13 +++++
 src/xennet/adapter.h     |   8 +++
 src/xennet/receiver.c    | 108 +++++++++++++++++++++++++++++----------
 src/xennet/transmitter.c |   9 ++--
 5 files changed, 114 insertions(+), 30 deletions(-)

-- 
2.51.0.windows.1