Re: [PATCH net v3 3/3] net: stmmac: document oversized AF_XDP frame handling

On 08/20, Maciej Fijalkowski wrote:

On Wed, Aug 19, 2026 at 09:05:35AM -0700, Stanislav Fomichev wrote:

quoted

stmmac drops AF_XDP zero-copy frames that exceed taprio's queueMaxSDU
after xsk_tx_peek_desc() has reserved their completion entries.

Completing a rejected descriptor is unsafe because AF_XDP completions are
ordered: xsk_tx_completed(pool, 1) would complete the oldest outstanding
descriptor, which may still be owned by hardware. Instead, leave the
completion pending so the ring eventually wedges and increment the drop
counter to expose the application error without risking hardware
misbehavior.

Document this intentional ring imbalance at the check.

Signed-off-by: Stanislav Fomichev <sdf@fomichev.me>
---
 drivers/net/ethernet/stmicro/stmmac/stmmac_main.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
index 62de03e65a90..6a532747c039 100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c

@@ -2713,6 +2713,10 @@ static bool stmmac_xdp_xmit_zc(struct stmmac_priv *priv, u32 queue, u32 budget)
 		if (priv->est && priv->est->enable &&
 		    priv->est->max_sdu[queue] &&
 		    xdp_desc.len > priv->est->max_sdu[queue]) {
+			/* Completions are ordered, so this descriptor cannot
+			 * be completed safely. Wedge the ring to expose the
+			 * application error instead.
+			 */
 			priv->xstats.max_sdu_txq_drop[queue]++;
 			continue;

Hmm. I read the discussion on v2. Maybe we could cancel cq entry here in
this branch? Also it feels like something achievable at bind time when
taprio is configured and vice versa?

Otherwise we over-commit cq entries.

What do you want to achieve with the cancel here? IIUC it will make it look
as if some (if the user has posted many) tx descriptor has not been consumed
by the kernel?

I do agree that a better idea is to probably do these checks during control
paths, but it's a bit more involved (and not sure if it's possible? if we
have a bunch of xsk sockets and we change that max_sdu, do we go over all
sockets on the system somehow?). My main motivation with this patch was
to make our LLM reviewers less chatty about preexisting issues.