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

On Mon, 26 Oct 2026 at 13:32, Maciej Fijalkowski
[off-list ref] wrote:

On Sat, Oct 24, 2026 at 10:11:56PM +0800, Rongzheng Wang wrote:

quoted

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>
---
 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)

If i am reading this right check against ip_summed is a way to tell that
first frag had XDP_TXMD_FLAGS_CHECKSUM flag set.

Agreed with Jason to preserve unlikely() when checking pool->tx_sw_csum.

Regarding skb drop, I am leaning towards this, I don't see much sense in
retry when packet is malformed? Not sure how this could be communicated up
to user space.

Do we need a new error statistics counter, e.g. tx_malformed_packet or
tx_invalid_packet? We do have one for invalid descriptors, but that
does not really apply here.

quoted

+             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)