Re: [PATCH net v2 1/2] xsk: defer software checksum until the skb is complete

[Jason Xing]

On Sat, Oct 24, 2026 at 10:12 PM Rongzheng Wang <testwrzwrz@xxxxxxxxx> wrote:
>
> When XDP_UMEM_TX_SW_CSUM is used with multi-buffer TX, xsk_skb_metadata()
> calls skb_checksum_help() while only the first descriptor has been added to
> the skb. The checksum therefore covers just the first fragment, and
> skb_checksum_help() sets ip_summed to CHECKSUM_NONE, so the remaining
> fragments are never included in the checksum.
>
> Move the software checksum calculation to the point where the last
> descriptor has been added to the skb.
>
> On a checksum failure the packet is now dropped instead of being returned
> to the application for a retry. This path is not reachable with the
> existing metadata validation, but the behavior change is intentional.
>
> Fixes: 11614723af26 ("xsk: Add option to calculate TX checksum in SW")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Rongzheng Wang <testwrzwrz@xxxxxxxxx>

Please do not post patches within 24 hours; we need more time for
other maintainers to give feedback, especially for weekend.

Do not reply to the old version patch if you're trying to submit a new one.

You didn't respond to my questions. At least, discussions there are
obviously welcome and needed.

Thanks,
Jason

> ---
>  net/xdp/xsk.c | 22 ++++++++++++++--------
>  1 file changed, 14 insertions(+), 8 deletions(-)
>
> diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
> index 33475b180ea6..65be2a368325 100644
> --- a/net/xdp/xsk.c
> +++ b/net/xdp/xsk.c
> @@ -1011,14 +1011,6 @@ static int xsk_skb_metadata(struct sk_buff *skb, void *buffer,
>                 skb->csum_start = hr + csum_start;
>                 skb->csum_offset = csum_offset;
>                 skb->ip_summed = CHECKSUM_PARTIAL;
> -
> -               if (unlikely(pool->tx_sw_csum)) {
> -                       int err;
> -
> -                       err = skb_checksum_help(skb);
> -                       if (err)
> -                               return err;
> -               }
>         }
>
>         if (flags & XDP_TXMD_FLAGS_LAUNCH_TIME)
> @@ -1030,6 +1022,14 @@ static int xsk_skb_metadata(struct sk_buff *skb, void *buffer,
>         return 0;
>  }
>
> +static int xsk_skb_sw_csum(struct sk_buff *skb, struct xsk_buff_pool *pool)
> +{
> +       if (!pool->tx_sw_csum || skb->ip_summed != CHECKSUM_PARTIAL)
> +               return 0;
> +
> +       return skb_checksum_help(skb);
> +}
> +
>  static struct sk_buff *xsk_build_skb_zerocopy(struct xdp_sock *xs,
>                                               struct xdp_desc *desc)
>  {
> @@ -1275,6 +1275,12 @@ static int __xsk_generic_xmit(struct sock *sk)
>                 if (xp_mb_desc(&desc)) {
>                         xs->skb = skb;
>                         continue;
> +               } else {
> +                       err = xsk_skb_sw_csum(skb, xs->pool);
> +                       if (unlikely(err)) {
> +                               xsk_drop_skb(skb);
> +                               goto out;
> +                       }
>                 }
>
>                 err = __dev_direct_xmit(skb, xs->queue_id);
> --
> 2.39.5 (Apple Git-154)
>