Re: [PATCH net-next v2] xsk: skip validating skb list in xmit path

[Stanislav Fomichev <stfomichev@gmail.com>]

On 07/16, Jason Xing wrote:
> From: Jason Xing <kernelxing@tencent.com>
> 
> This patch only does one thing that removes validate_xmit_skb_list()
> for xsk.
> 
> For xsk, it's not needed to validate and check the skb in
> validate_xmit_skb_list() in copy mode because xsk_build_skb() doesn't
> and doesn't need to prepare those requisites to validate. Xsk is just
> responsible for delivering raw data from userspace to the driver.
> 
> The __dev_direct_xmit was taken out of af_packet in commit 865b03f21162
> ("dev: packet: make packet_direct_xmit a common function"). And a call
> to validate_xmit_skb_list was added in commit 104ba78c9880 ("packet: on
> direct_xmit, limit tso and csum to supported devices") to support TSO.
> Since we don't support tso/vlan offloads in xsk_build_skb, we can remove
> validate_xmit_skb_list for xsk. Skipping numerous checks somehow
> contributes to the transmission especially in the extremely hot path.
> 
> Performance-wise, I used './xdpsock -i enp2s0f0np0 -t  -S -s 64' to verify
> the guess and then measured on the machine with ixgbe driver. It stably
> goes up by 5.48%, which can be seen in the shown below:
> Before:
>  sock0@enp2s0f0np0:0 txonly xdp-skb
>                    pps            pkts           1.00
> rx                 0              0
> tx                 1,187,410      3,513,536
> After:
>  sock0@enp2s0f0np0:0 txonly xdp-skb
>                    pps            pkts           1.00
> rx                 0              0
> tx                 1,252,590      2,459,456
> 
> This patch also removes total ~4% consumption which can be observed
> by perf:
> |--2.97%--validate_xmit_skb
> |          |
> |           --1.76%--netif_skb_features
> |                     |
> |                      --0.65%--skb_network_protocol
> |
> |--1.06%--validate_xmit_xfrm
> 
> Signed-off-by: Jason Xing <kernelxing@tencent.com>
> ---
> V2
> Link: https://lore.kernel.org/all/20250713025756.24601-1-kerneljasonxing@gmail.com/
> 1. avoid adding a new flag
> 2. add more descriptions from Stan

Acked-by: Stanislav Fomichev <sdf@fomichev.me>

LGTM, but would be nice if Willem or Magnus can chime in to confirm that
it's safe.