[PATCH net-next 0/4] net: devmem: allow rx-buf-size > PAGE_SIZE per binding

[Bobby Eshleman <bobbyeshleman@gmail.com>]

Every devmem dmabuf binding hands the page_pool PAGE_SIZE niovs today.
On NICs that consume one descriptor per netmem, this caps a single RX
descriptor at PAGE_SIZE and burns CPU on buffer churn.

In this series, we add a bind-time netlink attribute,
NETDEV_A_DMABUF_RX_BUF_SIZE, that lets userspace request a larger niov size
(power of two >= PAGE_SIZE). Drivers must opt in via
queue_mgmt_ops.QCFG_RX_PAGE_SIZE.

Selftests use udmabuf, but udmabuf sgtables were previously hardcoded to
PAGE_SIZE. This series modifies udmabuf to respect folio sizes in its exported
sgtable. The result is that when backing udmabuf with MFD_HUGETLB 2MB pages,
the sgtable is populated with 2MB entries, allowing devmem's gen_pool to carve
out large (eg. 64K) niovs.

Measurements
------------

Setup: kperf devmem RX/TX cuda, 4 flows, 64 MB messages, 60s, dctcp,
num-rx-queues=4, dmabuf-rx/tx-size-mb=2048, 10 runs per niov size,
mlx5.

   niov       RX dev Gbps   RX flow avg Gbps         app sys %
  -----  ----------------  -----------------  ----------------
     4K  300.63 +/- 53.21    75.16 +/- 13.30   54.15 +/- 10.23
    16K  321.35 +/- 28.20    80.34 +/-  7.05   41.05 +/-  8.87
    32K  347.63 +/-  2.20    86.91 +/-  0.55   44.54 +/-  3.51
    64K  332.11 +/- 14.26    83.03 +/-  3.56   35.47 +/-  3.11

RX app sys % drops ~19% from 4K to 64K.

kperf support (not yet merged):
https://github.com/facebookexperimental/kperf/commit/8837577f920876bce6986ec18869ac04439ebcd2

Signed-off-by: Bobby Eshleman <bobbyeshleman@meta.com>
---
Bobby Eshleman (4):
      net: devmem: allow rx-buf-size > PAGE_SIZE per dmabuf binding
      udmabuf: emit one sg entry per pinned folio
      selftests/net: ncdevmem: add -b option to set rx-buf-size on bind
      selftests/net: devmem.py: add check_rx_large_niov

 Documentation/netlink/specs/netdev.yaml            |  8 ++++
 drivers/dma-buf/udmabuf.c                          | 47 ++++++++++++++++---
 include/uapi/linux/netdev.h                        |  1 +
 net/core/devmem.c                                  | 52 +++++++++++++---------
 net/core/devmem.h                                  | 13 ++++--
 net/core/netdev-genl-gen.c                         |  5 ++-
 net/core/netdev-genl.c                             | 18 +++++++-
 tools/include/uapi/linux/netdev.h                  |  1 +
 tools/testing/selftests/drivers/net/hw/config      |  1 +
 tools/testing/selftests/drivers/net/hw/devmem.py   | 12 ++++-
 .../testing/selftests/drivers/net/hw/devmem_lib.py | 46 ++++++++++++++++++-
 tools/testing/selftests/drivers/net/hw/ncdevmem.c  | 49 ++++++++++++++++++--
 .../testing/selftests/drivers/net/hw/nk_devmem.py  | 11 ++++-
 13 files changed, 220 insertions(+), 44 deletions(-)
---
base-commit: dfcc2ff12925d99e858eaf539eaa4aaaf81fe2a6
change-id: 20260602-tcpdm-large-niovs-56523a3a1077

Best regards,
-- 
Bobby Eshleman <bobbyeshleman@meta.com>