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io_uring/zcrx: document area chunking parameter

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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@gmail.com>
This commit is contained in:
Pavel Begunkov 2025-11-19 15:53:55 +00:00
parent a32bb32d01
commit d1de61db15
1 changed files with 20 additions and 0 deletions
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20
Documentation/networking/iou-zcrx.rst
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@ -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
=======