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

[Magnus Karlsson]

On Mon, 26 Oct 2026 at 16:30, Rongzheng Wang <testwrzwrz@xxxxxxxxx> wrote:
>
> Hi Maciej,
>
> Thanks for the review.
>
> Yes. xsk_skb_metadata() is only called for the first descriptor when
> skb is NULL; continuation fragments do not call it. So
> skb->ip_summed == CHECKSUM_PARTIAL means the first fragment requested
> the checksum.
>
> I'll preserve unlikely() around pool->tx_sw_csum like this:
>
>     static int xsk_skb_sw_csum(struct sk_buff *skb,
>                                struct xsk_buff_pool *pool)
>     {
>             if (unlikely(pool->tx_sw_csum &&
>                          skb->ip_summed == CHECKSUM_PARTIAL))
>                     return skb_checksum_help(skb);
>
>             return 0;
>     }
>
> For the drop path: xsk_drop_skb() increments tx_invalid_descs and
> consumes the skb; sendto() returns the error from skb_checksum_help().
> Since the descriptors are already consumed, retrying is not useful. The
> current metadata validation makes the malformed-packet path effectively
> unreachable, so this is a defensive drop.
>
> I verified the propagation in a local QEMU test kernel. For the test
> only, I temporarily changed xsk_skb_sw_csum() to:
>
>     static int xsk_skb_sw_csum(struct sk_buff *skb,
>                                struct xsk_buff_pool *pool)
>     {
>             if (unlikely(pool->tx_sw_csum &&
>                          skb->ip_summed == CHECKSUM_PARTIAL))
>                     return -EINVAL;
>
>             return 0;
>     }
>
> and ran test_xdp_metadata. It failed at the first generate_packet()
> sendto:
>
>     generate_packet:FAIL:sendto unexpected sendto: actual -1 < expected 0
>     Summary: 0/0 PASSED, 0 SKIPPED, 1/0 FAILED
>     TEST_RC=1
>
> The normal kernel still passes. The test harness does not print errno,
> so I can only state that sendto() returned -1; the injected error was
> -EINVAL. I restored the source after the test.
>
> Magnus suggested a distinct counter for malformed packets. I agree that
> would be more precise than tx_invalid_descs, but I'd prefer to do that
> in a separate patch instead of extending the UAPI in this stable fix.

Correct. If we decide to do anything like that, it should be a
separate patch for net-next at some later point in time.

Thanks: Magnus

> I'll address the remaining feedback and send v3 as a new thread.
>
> Thanks,
> Rongzheng
>