[PATCH net 0/2] lib/dim: fix 32-bit overflow in dim_calc_stats()

[Shashank Mohan Jain <jain.sm@gmail.com>]

On 32-bit kernels dim_calc_stats() multiplies the u32 byte, packet and
completion counts of a DIM window by USEC_PER_MSEC (1000L) in 32-bit
long arithmetic. Once a window carries more than about 4.3 MB, bpms
wraps, and net_dim steers interrupt moderation on a meaningless
throughput value. At 1 Gbit/s line rate a 64-event window passes that
size when there are fewer than about 1,860 DIM events per second,
which is common while NAPI keeps the interrupt masked under load.
32-bit users of the library include mtk_eth_soc (MT7621, MT7623),
bcmgenet and bcmsysport on 32-bit ARM, xilinx_axienet on Zynq-7000 and
MicroBlaze, and virtio_net in 32-bit guests. The overflow goes back to
the mlx5e code the library was moved from.

Patch 1 does the multiplications in 64 bits and divides with
DIV_ROUND_UP_ULL(); the results on 64-bit are unchanged. Patch 2 adds a
KUnit suite for dim_calc_stats() whose large-window cases fail on
32-bit without patch 1. It is part of this series as described under
"Co-posting selftests" in maintainer-netdev.rst.

The series is based on net (a7bfaba4823e) and has no dependencies; both
patches also apply to mainline (fd179f8a05be) and net-next.

The bug was found and the patches were prepared with Claude Code
(Anthropic), model Claude Opus 5.5 (claude-opus-5-5).

Tested:
- KUnit (CONFIG_DIMLIB_KUNIT_TEST=y) on UML i386 (SUBARCH=i386):
  without patch 1, 4 of the 7 dim_calc_stats cases fail (many_bytes,
  bytes_32bit_limit, gigabit, many_packets); with it all pass. On UML
  x86_64 all cases pass with and without patch 1. Both were run on
  mainline and on net.
- W=1 builds of lib/dim/ for UML x86_64 and i386 without warnings;
  dim.o references no libgcc 64-bit division helpers. A native i386
  defconfig build (vmlinux and modules, DIMLIB=y) succeeds.
- checkpatch --strict. Its "does MAINTAINERS need updating?" warning
  on patch 2 does not apply: lib/dim/ is already covered by the
  DYNAMIC INTERRUPT MODERATION entry.

Not tested: 32-bit ARM or MIPS builds (no cross compiler was
available), and no run on a 32-bit NIC. The traffic levels at which
the drivers hit the overflow are derived from how they count DIM
events, not measured.

Shashank Mohan Jain (2):
  lib/dim: fix 32-bit overflow in dim_calc_stats() rates
  lib/dim: add KUnit test for dim_calc_stats()

 lib/Kconfig.debug   |  15 ++++++
 lib/dim/Makefile    |   2 +
 lib/dim/dim.c       |  10 ++--
 lib/dim/dim_kunit.c | 126 ++++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 150 insertions(+), 3 deletions(-)
 create mode 100644 lib/dim/dim_kunit.c


base-commit: a7bfaba4823e3c165bb2004c74eff7c096672bc7
-- 
2.43.0