netdev - [PATCH net-next 18/20] netkit: Add io_uring zero-copy support for TCP

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Message-ID: <20250919213153.103606-19-daniel@iogearbox.net>
Date: Fri, 19 Sep 2025 23:31:51 +0200
From: Daniel Borkmann <daniel@...earbox.net>
To: netdev@...r.kernel.org
Cc: bpf@...r.kernel.org,
	kuba@...nel.org,
	davem@...emloft.net,
	razor@...ckwall.org,
	pabeni@...hat.com,
	willemb@...gle.com,
	sdf@...ichev.me,
	john.fastabend@...il.com,
	martin.lau@...nel.org,
	jordan@...fe.io,
	maciej.fijalkowski@...el.com,
	magnus.karlsson@...el.com,
	David Wei <dw@...idwei.uk>
Subject: [PATCH net-next 18/20] netkit: Add io_uring zero-copy support for TCP

From: David Wei <dw@...idwei.uk>

This adds the last missing bit to netkit for supporting io_uring with
zero-copy mode [0]. Up until this point it was not possible to consume
the latter out of containers or Kubernetes Pods where applications are
in their own network namespace.

Thus, as a last missing bit, implement ndo_queue_get_dma_dev() in netkit
to return the physical device of the real rxq for DMA. This allows memory
providers like io_uring zero-copy or devmem to bind to the physically
mapped rxq in netkit.

io_uring example with eth0 being a physical device with 16 queues where
netkit is bound to the last queue, iou-zcrx.c is binary from selftests.
Flow steering to that queue is based on the service VIP:port of the
server utilizing io_uring:

  # ethtool -X eth0 start 0 equal 15
  # ethtool -X eth0 start 15 equal 1 context new
  # ethtool --config-ntuple eth0 flow-type tcp4 dst-ip 1.2.3.4 dst-port 5000 action 15
  # ip netns add foo
  # ip link add numrxqueues 2 type netkit
  # ynl-bind eth0 15 nk0
  # ip link set nk0 netns foo
  # ip link set nk1 up
  # ip netns exec foo ip link set lo up
  # ip netns exec foo ip link set nk0 up
  # ip netns exec foo ip addr add 1.2.3.4/32 dev nk0
  [ ... setup routing etc to get external traffic into the netns ... ]
  # ip netns exec foo ./iou-zcrx -s -p 5000 -i nk0 -q 1

Remote io_uring client:

  # ./iou-zcrx -c -h 1.2.3.4 -p 5000 -l 12840 -z 65536

We have tested the above against a dual-port Nvidia ConnectX-6 (mlx5)
100G NIC as well as Broadcom BCM957504 (bnxt_en) 100G NIC, both
supporting TCP header/data split. For Cilium, the plan is to open
up support for io_uring in zero-copy mode for regular Kubernetes Pods
when Cilium is configured with netkit datapath mode.

Signed-off-by: David Wei <dw@...idwei.uk>
Co-developed-by: Daniel Borkmann <daniel@...earbox.net>
Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
Link: https://kernel-recipes.org/en/2024/schedule/efficient-zero-copy-networking-using-io_uring [0]
---
 drivers/net/netkit.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/drivers/net/netkit.c b/drivers/net/netkit.c
index 27ff84833f28..5129b27a7c3c 100644
--- a/drivers/net/netkit.c
+++ b/drivers/net/netkit.c
@@ -274,6 +274,21 @@ static const struct ethtool_ops netkit_ethtool_ops = {
 	.get_channels		= netkit_get_channels,
 };

+static struct device *netkit_queue_get_dma_dev(struct net_device *dev, int idx)
+{
+	struct netdev_rx_queue *rxq, *peer_rxq;
+	unsigned int peer_idx;
+
+	rxq = __netif_get_rx_queue(dev, idx);
+	if (!rxq->peer)
+		return NULL;
+
+	peer_rxq = rxq->peer;
+	peer_idx = get_netdev_rx_queue_index(peer_rxq);
+
+	return netdev_queue_get_dma_dev(peer_rxq->dev, peer_idx);
+}
+
 static int netkit_queue_create(struct net_device *dev)
 {
 	struct netkit *nk = netkit_priv(dev);
@@ -299,7 +314,8 @@ static int netkit_queue_create(struct net_device *dev)
 }

 static const struct netdev_queue_mgmt_ops netkit_queue_mgmt_ops = {
-	.ndo_queue_create = netkit_queue_create,
+	.ndo_queue_get_dma_dev		= netkit_queue_get_dma_dev,
+	.ndo_queue_create		= netkit_queue_create,
 };

 static struct net_device *netkit_alloc(struct nlattr *tb[],
-- 
2.43.0