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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-49fbd6f24c7sm23288715e9.1.2026.09.17.03.11.28 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 17 Sep 2026 03:11:28 -0700 (PDT) Date: Thu, 17 Sep 2026 11:11:27 +0100 From: David Laight To: vladimir.oltean@nxp.com Cc: netdev@vger.kernel.org, Zefir Kurtisi , Claudiu Manoil , Wei Fang , Clark Wang , Andrew Lunn , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Alexei Starovoitov , Daniel Borkmann , Jesper Dangaard Brouer , John Fastabend , Stanislav Fomichev , Simon Horman , Richard Cochran , Yangbo Lu , Ioana Ciornei , imx@lists.linux.dev, linux-kernel@vger.kernel.org, bpf@vger.kernel.org Subject: Re: [PATCH v3 net 4/7] net: enetc: pad short frames in software Message-ID: <20260917111127.577d344b@pumpkin> In-Reply-To: <20260915222735.1016937-5-vladimir.oltean@nxp.com> References: <20260915222735.1016937-1-vladimir.oltean@nxp.com> <20260915222735.1016937-5-vladimir.oltean@nxp.com> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Wed, 16 Sep 2026 01:27:31 +0300 vladimir.oltean@nxp.com wrote: > The ENETC does not support BUF_LEN or FRM_LEN in TX buffer descriptors > less than 16. This is written in the reference manual of all SoCs > supported by the driver: LS1028A, i.MX943, i.MX95 etc. > > Frames must not have a FRM_LEN that is less than 16 bytes. Frames of > 0-15 bytes are not supported. > (...) > The first descriptor in a chain must not have a BUFF_LEN that is less > than 16 bytes. > > I don't think proper attention was paid to this during development, we > found the text at the end of a bug investigation. Therefore, the driver > does not enforce this. > > But the frame length is out of the driver's control, and the network > stack can actually send packets with skb->len smaller than that. The > result is unpleasant, as will be explained below, so for simplicity > sake, we just pad anything shorter than ETH_ZLEN. > > Zefir Kurtisi found a case where transmitting L2 WNM keep-alive frames > through ENETC would soft-lockup the host through an IRQ storm. He later > distilled this into a small enetc-killer.c user space program which > sends a packet with MAC DA, MAC SA and EtherType IPv4 (14 octets in > length) through an AF_PACKET raw socket. > > The IRQ storm is actually a curious effect of a chain of events. > > The hardware behaviour, when an invalid BD is put in its TX ring, is > that it would transmit the packet as normal, update counters, raise > completion interrupt as normal, but it would just not advance the > consumer index of the ring (TBaCIR) to signify that the BD has been > consumed and is available for software to free. The ring will also get > its TBaSR[BUSY] bit persistently set to 1 afterwards. > > It deserves an explanation why the behaviour above would lead to an > IRQ storm, since ENETC interrupts are message-based (MSI-X), and an > unhandled interrupt would typically just be lost rather than retrigger > itself as a wired interrupt would. > > NAPI processing in ENETC has 3 steps: > > I. the enetc_msix() hardirq handler disables RBaIER, TBaIER and sets > softirq processing to the 'pending' state. > > II. the enetc_poll() softirq handler for the IRQ vector walks through > the TX rings affine to that vector, checks which ones have a TBCIR > updated since last time - enetc_bd_ready_count() - processes those > completed frames, and clears pending interrupts in these updated TX > rings by writing to TBaIDR. (I've excluded RX processing due to it > being irrelevant). > > III. After the softirq handler does its round of checking all RX and TX > rings for updates, it re-enables all interrupts in RBaIER and > TBaIER that were previously disabled by the hardirq handler, and > exits. > > Because the TX ring with the short frame is skipped at step II (TBCIR > wasn't updated as part of HW malfunction), its pending IRQ is not > cleared in TBaIDR by enetc_clean_tx_ring(). > > But because enetc_msix() disables TBaIER at step I and re-enables it at > step III, another MSI will be fired upon re-enabling it. This is what > completes the cycle and the driver goes back to step I. > > So the driver misinterprets the mixed signals it's getting from the > hardware, and ends up causing a software-amplified IRQ storm. > > Fixes: d4fd0404c1c9 ("enetc: Introduce basic PF and VF ENETC ethernet drivers") > Reported-by: Zefir Kurtisi > Closes: https://lore.kernel.org/netdev/b3d9136c-2803-4203-b1ea-1f9e62de80a1@gmail.com/ > Tested-by: Zefir Kurtisi > Reviewed-by: Wei Fang > Signed-off-by: Vladimir Oltean > --- > v1->v3: none > --- > drivers/net/ethernet/freescale/enetc/enetc.c | 13 +++++++++++++ > drivers/net/ethernet/freescale/enetc/enetc.h | 2 ++ > 2 files changed, 15 insertions(+) > > diff --git a/drivers/net/ethernet/freescale/enetc/enetc.c b/drivers/net/ethernet/freescale/enetc/enetc.c > index 0216f7d08e19..bbad942041f5 100644 > --- a/drivers/net/ethernet/freescale/enetc/enetc.c > +++ b/drivers/net/ethernet/freescale/enetc/enetc.c > @@ -1077,6 +1077,19 @@ netdev_tx_t enetc_xmit(struct sk_buff *skb, struct net_device *ndev) > u8 udp, msgtype, twostep; > u16 offset1, offset2; > > + /* Hardware does not support transmit buffer descriptors with a total > + * length of less than 16 bytes, or a first buffer size of less than > + * 16 bytes. > + */ > + if (unlikely(skb_headlen(skb) < ENETC_MIN_BUFF_SIZE && > + skb_linearize(skb))) { Isn't it only necessary to pull a few bytes into the linear region? > + dev_kfree_skb_any(skb); > + return NETDEV_TX_OK; > + } > + > + if (eth_skb_pad(skb)) > + return NETDEV_TX_OK; That could be inside the (skb_headlen(skb) < ENETC_MIN_BUFF_SIZE) test. David > + > /* Mark tx timestamp type on enetc_cb->flag if requires */ > if ((skb_shinfo(skb)->tx_flags & SKBTX_HW_TSTAMP) && > (priv->active_offloads & ENETC_F_TX_TSTAMP_MASK)) > diff --git a/drivers/net/ethernet/freescale/enetc/enetc.h b/drivers/net/ethernet/freescale/enetc/enetc.h > index d1e9d9130057..a4e76060e94b 100644 > --- a/drivers/net/ethernet/freescale/enetc/enetc.h > +++ b/drivers/net/ethernet/freescale/enetc/enetc.h > @@ -22,6 +22,8 @@ > #define ENETC_MAX_MTU (ENETC_MAC_MAXFRM_SIZE - \ > (ETH_FCS_LEN + ETH_HLEN + VLAN_HLEN)) > > +#define ENETC_MIN_BUFF_SIZE 16 > + > #define ENETC_CBD_DATA_MEM_ALIGN 64 > > #define ENETC_MADDR_HASH_TBL_SZ 64