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

[Maciej Fijalkowski <maciej.fijalkowski@intel.com>]

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

>  		}
> -- 
> 2.53.0-Meta
>