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Fri, 21 Aug 2026 10:32:00 +0000 Date: Fri, 21 Aug 2026 12:31:42 +0200 From: Maciej Fijalkowski To: Stanislav Fomichev CC: , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , Subject: Re: [PATCH net v3 3/3] net: stmmac: document oversized AF_XDP frame handling Message-ID: References: <20260819160535.1472459-1-sdf@fomichev.me> <20260819160535.1472459-4-sdf@fomichev.me> Content-Type: text/plain; charset="us-ascii" Content-Disposition: inline In-Reply-To: X-ClientProxiedBy: TL2P290CA0029.ISRP290.PROD.OUTLOOK.COM (2603:1096:950:3::19) To DM4PR11MB6117.namprd11.prod.outlook.com (2603:10b6:8:b3::19) Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DM4PR11MB6117:EE_|IA1PR11MB7870:EE_ X-MS-Office365-Filtering-Correlation-Id: b56b599a-8cb8-43b5-7a30-08deff6f6ecf X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|376014|23010399003|7416014|1800799024|366016|6133799003|56012099006|10067099003|11063799006|4143699003|18002099003|22082099003; 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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 > > > --- > > > 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? Oof. My bad. I meant completely different thing :D Right now the semantics are that we post invalid/dropped addrs to cq (the rationale was that dropped descs are gone and unreachable which might eventually lead to dying traffic). We should submit xdp_desc's addr to cq. Regarding the comment included in code I must disagree. CQ entries no longer imply that 'this particular descriptor has been successfully sent by HW'. > > 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. I hear you, however I feel like we do not know this driver too much and probably we don't have a HW to test out such changes, so maybe let us try to fix existing behavior?