[PATCH net-next v2 0/7] net: devmem: size TX net_iovs from the dmabuf layout

[Stanislav Fomichev]

TX bindings use PAGE_SIZE net_iovs, so a frag covers at most a page.
A 64KB skb needs 16 frags, and MAX_SKB_FRAGS caps devmem skbs at about
68KB, which makes BIG TCP ineffective for devmem TX.

This series picks the TX net_iov size from the dmabuf sg table instead:
the largest power of 2, up to 2MB, which aligns every DMA segment. With
a 2MB hugepage-backed udmabuf a BIG TCP skb takes a few frags. There's
no UAPI change since TX addresses are dmabuf offsets.

TCP still bounds skbs by size_goal, so frags get larger, skbs don't.
A single frag can now be as large as the skb, which matters for
NETMEM_TX drivers with tso_max_size above 64KB:
- mlx5: 32-bit length per data segment
- gve: splits frags into 16KB descriptors
- fbnic: keeps the default 64KB tso_max_size
- bnxt: 16-bit BD length, devmem TX frags are capped at 32KB by a new
  per-device limit

Tested on two mlx5 400Gb/s hosts with kperf (udmabuf, 32KB RX
net_iovs, 2MB hugepage-backed TX buffer), series backported to a 7.1
kernel on the sender only. 1 flow, MTU 4200, receiver tcp_rmem capped
at 16MB so it doesn't drop, median of 5 x 60s runs:

                              before    after
  frags per skb (sender)       16-17      1-2
  throughput (Gb/s)            208.6    211.2
  sender sendmsg CPU (sys)     40.8%    19.5%   (-52%)

The receiver's CPU is the bottleneck, so the gain is CPU, not
throughput. More of the transmit runs from the ACK path, so the
sender's softirq grows (52% -> 59%), but sys + sirq still drops by 16%.

kperf output (sender side) of runs near the median, before:

  client: == Source <sender>
  client:   Tx213.826 Gbps (1603901849600 bytes in 60007723 usec)
  client:   Rx  0.000 Gbps (0 bytes in 60007723 usec)
  client:   TCP retrans reord rtt rttvar d_ce snd_wnd cwnd
  client:             0     0 633      2    0 15556608 7657
  client:   net CPU 49: usr: 0.01% sys: 0.10% idle:44.13% iow: 0.00% irq: 3.21% sirq:52.53%
  client:   app CPU 50: usr: 0.05% sys:41.55% idle:53.88% iow: 0.00% irq: 1.40% sirq: 3.10%

after:

  client: == Source <sender>
  client:   Tx211.242 Gbps (1584440279040 bytes in 60004741 usec)
  client:   Rx  0.000 Gbps (0 bytes in 60004741 usec)
  client:   TCP retrans reord rtt rttvar d_ce snd_wnd cwnd
  client:             0     0 556      3    0 15499264 7650
  client:   net CPU 13: usr: 0.01% sys: 0.08% idle:37.75% iow: 0.00% irq: 3.87% sirq:58.26%
  client:   app CPU 14: usr: 0.06% sys:19.48% idle:77.23% iow: 0.00% irq: 1.19% sirq: 2.02%

The bnxt patches (1 and 5) were tested on two 50Gb/s bnxt hosts with
CONFIG_MAX_SKB_FRAGS=45 (with 17 the check in patch 1 compiles out).
kperf devmem TX from a 2MB hugepage-backed udmabuf runs at line rate
(46.4Gb/s, payload validated) with 2-3 frags per skb, none above 32KB.
ncdevmem sending 1KB chunks builds skbs with up to 45 frags, the stack
segments them and the data arrives intact.

Patches:
- 1: bnxt: segment GSO skbs with too many frags in the stack
- 2-3: devmem TX prep (truesize, niov_shift in the TX lookup)
- 4-5: per-device netmem TX frag size limit, 32KB for bnxt
- 6: pick the TX net_iov size from the sg table
- 7: selftest for TX from a hugepage-backed dmabuf

v2:
- reword the TX truesize commit message (Mina)
- use 1 flow results in the cover letter (Mina)

v1: https://lore.kernel.org/netdev/20261102191438.1059771-1-sdf@xxxxxxxxxxx/

Stanislav Fomichev (7):
  bnxt_en: let the stack segment GSO skbs with too many frags
  net: devmem: charge the frag length to TX skb truesize
  net: devmem: use niov_shift in net_devmem_get_niov_at()
  net: devmem: add a per-device netmem TX frag size limit
  bnxt_en: limit netmem TX frags to 32KB
  net: devmem: size TX net_iovs from the dmabuf sg table
  selftests: drv-net: devmem: TX from a hugepage-backed dmabuf

 .../networking/net_cachelines/net_device.rst  |  1 +
 Documentation/networking/netmem.rst           |  6 +++
 drivers/net/ethernet/broadcom/bnxt/bnxt.c     |  6 +++
 include/linux/netdevice.h                     |  2 +
 net/core/datagram.c                           |  3 +-
 net/core/dev.c                                |  1 +
 net/core/devmem.c                             | 37 +++++++++++++---
 net/core/devmem.h                             |  1 +
 net/core/netdev-genl.c                        |  2 +-
 .../selftests/drivers/net/hw/devmem.py        | 10 ++++-
 .../selftests/drivers/net/hw/devmem_lib.py    | 43 +++++++++++++++++--
 .../selftests/drivers/net/hw/ncdevmem.c       |  8 +++-
 .../selftests/drivers/net/hw/nk_devmem.py     | 11 ++++-
 13 files changed, 115 insertions(+), 16 deletions(-)

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2.53.0-Meta