Re: win-pv-devel Digest, Vol 140, Issue 14

[Mounah Honore Cesaire <cesaire.mounah@xxxxxxxxxx>]

On 9/17/26 5:45 PM, win-pv-devel-request@xxxxxxxxxxxxxxxxxxxx wrote:
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> Today's Topics:
>
>     1. [PATCH 0/2] xennet: improve RX NBL batching and fix TX
>        doorbell coalescing (david ambu)
>     2. [PATCH 1/2] xennet: improve NBL batching receive path (david ambu)
>     3. [PATCH 2/2] xennet: fix TX doorbell coalescing ? More flag
>        dropped at NBL boundaries (david ambu)
>     4. [PATCH] xenvif: fix TX doorbell coalescing - More overridden
>        for non-RSS traffic (david ambu)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Thu, 17 Sep 2026 16:41:28 +0100
> From: david ambu<david.preetham@xxxxxxxxxx>
> To:win-pv-devel@xxxxxxxxxxxxxxxxxxxx
> Cc: david ambu<david.preetham@xxxxxxxxxx>
> Subject: [PATCH 0/2] xennet: improve RX NBL batching and fix TX
>       doorbell coalescing
> Message-ID:<20260917154131.2040-1-david.preetham@xxxxxxxxxx>
> Content-Type: text/plain; charset=UTF-8
>
> 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.

Hello,

Patch 1 looks fine for me.
Just a remark: moving KeMemoryBarrier before Returned read seems to 
break the ordering as Returned needs to be read before Indicated. Or 
maybe there is no need for an ordering here?

> 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)
Batching packets can also potentially add latency costs. A latency 
benchmark should also be done.
> 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(-)
PS: For now I can only respond to the digest, sorry. This should be 
fixed in future mails.

-- 

Césaire Mounah | Vates Windows Guest Tools Engineer

XCP-ng & Xen Orchestra - Vates solutions

web: https://vates.tech



--
Césaire Mounah | Vates Windows Guest Tools Engineer

XCP-ng & Xen Orchestra - Vates solutions

web: https://vates.tech