netdev - [PATCH net-next v6 7/8] io_uring/zcrx: document area chunking parameter

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Message-ID: <a17cb10f4595458bc892015a40aa07798936841b.1764264798.git.asml.silence@gmail.com>
Date: Thu, 27 Nov 2025 20:44:20 +0000
From: Pavel Begunkov <asml.silence@...il.com>
To: netdev@...r.kernel.org
Cc: "David S . Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>,
	Paolo Abeni <pabeni@...hat.com>,
	Jonathan Corbet <corbet@....net>,
	Michael Chan <michael.chan@...adcom.com>,
	Pavan Chebbi <pavan.chebbi@...adcom.com>,
	Andrew Lunn <andrew+netdev@...n.ch>,
	Ilias Apalodimas <ilias.apalodimas@...aro.org>,
	Stanislav Fomichev <sdf@...ichev.me>,
	Pavel Begunkov <asml.silence@...il.com>,
	Jens Axboe <axboe@...nel.dk>,
	Simon Horman <horms@...nel.org>,
	linux-doc@...r.kernel.org,
	linux-kselftest@...r.kernel.org,
	io-uring@...r.kernel.org,
	dtatulea@...dia.com
Subject: [PATCH net-next v6 7/8] io_uring/zcrx: document area chunking parameter

struct io_uring_zcrx_ifq_reg::rx_buf_len is used as a hint specifying
the kernel what buffer size it should use. Document the API and
limitations.

Signed-off-by: Pavel Begunkov <asml.silence@...il.com>
---
 Documentation/networking/iou-zcrx.rst | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/Documentation/networking/iou-zcrx.rst b/Documentation/networking/iou-zcrx.rst
index 54a72e172bdc..7f3f4b2e6cf2 100644
--- a/Documentation/networking/iou-zcrx.rst
+++ b/Documentation/networking/iou-zcrx.rst
@@ -196,6 +196,26 @@ Return buffers back to the kernel to be used again::
   rqe->len = cqe->res;
   IO_URING_WRITE_ONCE(*refill_ring.ktail, ++refill_ring.rq_tail);
 
+Area chunking
+-------------
+
+zcrx splits the memory area into fixed-length physically contiguous chunks.
+This limits the maximum buffer size returned in a single io_uring CQE. Users
+can provide a hint to the kernel to use larger chunks by setting the
+``rx_buf_len`` field of ``struct io_uring_zcrx_ifq_reg`` to the desired length
+during registration. If this field is set to zero, the kernel defaults to
+the system page size.
+
+To use larger sizes, the memory area must be backed by physically contiguous
+ranges whose sizes are multiples of ``rx_buf_len``. It also requires kernel
+and hardware support. If registration fails, users are generally expected to
+fall back to defaults by setting ``rx_buf_len`` to zero.
+
+Larger chunks don't give any additional guarantees about buffer sizes returned
+in CQEs, and they can vary depending on many factors like traffic pattern,
+hardware offload, etc. It doesn't require any application changes beyond zcrx
+registration.
+
 Testing
 =======
 
-- 
2.52.0