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

[david ambu <david.preetham@xxxxxxxxxx>]

These two patches address TX doorbell coalescing correctness in xenvif
and RX receive-path NBL batching efficiency in xennet.

Background
----------
NDIS passes packets to the miniport as chains of Net Buffer Lists (NBLs),
each NBL containing one or more Net Buffers (NBs). The xennet transmitter
signals xenvif's TX ring via a "More" flag: More=TRUE defers the ring
doorbell (kick), More=FALSE fires it.

The intent is to batch all NBs within one NBL with a single kick at the
NBL boundary. Each NBL carries a single hash, so all its NBs map to the
same TX ring - making per-NBL kicking both correct and sufficient.

Patch 1 - xenvif (TX fix)
--------------------------
TransmitterQueuePacket unconditionally overrode More=FALSE in the
XENVIF_PACKET_HASH_ALGORITHM_NONE case, causing a ring kick on every
individual NB instead of once per NBL. Removing the override lets the
More flag from xennet flow through correctly, giving ALGORITHM_NONE the
same per-NBL kick behaviour as ALGORITHM_TOEPLITZ.

Patch 2 - xennet (RX batching)
-------------------------------
Three improvements to the RX receive-indicate path:

  - Raise NblBatchMax default to 32 (configurable 1-64 via registry)
    so NDIS receives larger batches in a single
    NdisMIndicateReceiveNetBufferLists call, reducing per-indication
    overhead.

  - Split batches at EtherType boundaries and set
    NDIS_RECEIVE_FLAGS_SINGLE_ETHER_TYPE per batch, enabling NDIS to
    apply protocol-layer optimisations.

  - In __ReceiverPushPackets: sample Returned before the InterlockedAdd
    so it is read prior to the implicit full memory barrier that
    InterlockedAdd provides; use the return value directly as Indicated
    and remove the now-redundant KeMemoryBarrier.

Performance (inter-host, Windows desktop VM, 10G, 3 iterations x 60s)
-----------------------------------------------------------------------

                     Without patch    With patch (32 NBL default)
Single-stream TCP     4525 Mbits/s     5550 Mbits/s   (+23%)
Single-stream UDP      372 Mbits/s      441 Mbits/s   (+19%)
8-stream TCP         11931 Mbits/s    17017 Mbits/s   (+43%)
8-stream UDP            87 Mbits/s      120 Mbits/s   (+38%)
TCP latency (psping)   0.8 ms           0.9 ms        (no regression)
ICMP latency (ping)    <1 ms            <1 ms         (no regression)

Tested at NblBatchMax = 8, 16, 32, 64. Peak throughput at 8-16;
larger values show marginal returns. Default of 32 is a conservative
mid-point. Latency is unaffected across all batch sizes.

david ambu (2):
  xenvif: fix TX doorbell coalescing for ALGORITHM_NONE traffic
  xennet: improve NBL batching receive path

 src/xennet.inf           |   6 +++
 src/xennet/adapter.c     |  13 ++++++
 src/xennet/adapter.h     |   8 ++++
 src/xennet/receiver.c    | 110 +++++++++++++++++++++++++++++++++++----
 src/xenvif/transmitter.c |   1 -
 5 files changed, 107 insertions(+), 31 deletions(-)

--
2.51.0.windows.1