Re: [PATCH net-next v4 0/5] net: devmem: size TX net_iovs from the dmabuf layout

[Nikolay Aleksandrov <razor@blackwall.org>]

On 12/11/2026 00:47, Stanislav Fomichev wrote:
> 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.
> 
> This doesn't make skbs larger. TCP still sizes them with size_goal.
> Only the frags get larger, and a single frag can now cover the whole
> skb.
> 
> An skb stays under 64KB unless BIG TCP is enabled. With BIG TCP,
> MSG_ZEROCOPY from huge pages can already create frags of 64KB or
> more. So drivers already have to handle them:
> - mlx5 has a 31-bit length per data segment
> - gve splits frags into 16KB descriptors
> - fbnic doesn't enable BIG TCP
> - bnxt has a 16-bit BD length. It is broken for such page frags today
>    and will be fixed separately.
> 
> 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 patch was tested on two 50Gb/s bnxt hosts with
> CONFIG_MAX_SKB_FRAGS=45. With the default of 17 its check compiles
> out. 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: pick the TX net_iov size from the sg table
> - 5: selftest for TX from a hugepage-backed dmabuf
> 
> v4:
> - drop the per-device frag size limit and the bnxt, fbnic and mlx5
>    patches. Drivers already have to handle large page frags (Jakub).
> 
> v3: https://lore.kernel.org/netdev/20261105222058.944293-1-sdf@fomichev.me/
> - default the netmem TX frag size limit to PAGE_SIZE, opt in bnxt,
>    fbnic and mlx5 (Mina)
> 
> v2: https://lore.kernel.org/netdev/20261103235152.1904727-1-sdf@fomichev.me/
> - reword the TX truesize commit message (Mina)
> - explain why the TX net_iov size ignores the device limit (Mina)
> - use 1 flow results in the cover letter (Mina)
> 
> v1: https://lore.kernel.org/netdev/20261102191438.1059771-1-sdf@fomichev.me/
> 
> Stanislav Fomichev (5):
>    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: size TX net_iovs from the dmabuf sg table
>    selftests: drv-net: devmem: TX from a hugepage-backed dmabuf
> 
>   drivers/net/ethernet/broadcom/bnxt/bnxt.c     |  5 +++
>   net/core/datagram.c                           |  2 +-
>   net/core/devmem.c                             | 36 +++++++++++++---
>   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 ++++-
>   8 files changed, 101 insertions(+), 16 deletions(-)
> 

Nice, I also tested it with netkit and TXed a 2MB net_iov size.
For the set:

Tested-by: Nikolay Aleksandrov <razor@blackwall.org>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>

Thanks,
  Nik