Re: [PATCH net v2 1/2] xsk: defer software checksum until the skb is complete
flat view
From: Jason Xing <hidden>
Date: 2026-10-25 02:51:53
Also in:
bpf, lkml, stable
On Sat, Oct 24, 2026 at 10:12 PM Rongzheng Wang [off-list ref] 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: <redacted>
Signed-off-by: Rongzheng Wang <redacted>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
quoted hunk ↗ jump to hunk
--- 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)