From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail.tipi-net.de (mail.tipi-net.de [194.13.80.246]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 3F18F39478D; Mon, 12 Oct 2026 21:04:17 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=194.13.80.246 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791839059; cv=none; b=HYaAPUxVv0pMIgGqxcJZX5Kzzl0b4LREPR2GMo4b30XwZzogx6qt6GwV6ZTXs870seR+e/c/8niS1197mmW9WDaOr42J5SCOtzhK8abL4GigGMFfh0/xW4we6Gj57EHmEqwRgEFCoGP8gsRTrU+aqHh+SAlgKxj/1trssWylZoA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791839059; c=relaxed/simple; bh=BGk7NAr2tTPjGgJaR62RozGUhRW+14yYSRcXHRoh4U8=; h=MIME-Version:Date:From:To:Cc:Subject:In-Reply-To:References: Message-ID:Content-Type; b=lNotmWSpgLkW0AEPgdGTJ+Uqmre7ng9OQnXXI5b/tzpSY0OEX0Fbu9kJ5i5Lj/u+3FgocJa4mWQTnb39zzObwsOQNqDp6jYcerI1cBZ+lcVcEHbtp9u+xK4c2Me9tBgtl08Jb9NVl2AyEPTykegmr8L1i6YWCag+XRDiU9C23/A= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=tipi-net.de; spf=pass smtp.mailfrom=tipi-net.de; dkim=pass (2048-bit key) header.d=tipi-net.de header.i=@tipi-net.de header.b=k9r/oLnD; arc=none smtp.client-ip=194.13.80.246 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=tipi-net.de Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=tipi-net.de Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=tipi-net.de header.i=@tipi-net.de header.b="k9r/oLnD" Received: from [127.0.0.1] (localhost [127.0.0.1]) by localhost (Mailerdaemon) with ESMTPSA id D0919A1310; Mon, 12 Oct 2026 23:04:11 +0200 (CEST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=tipi-net.de; s=dkim; t=1791839054; h=from:subject:date:message-id:to:cc:mime-version:content-type: content-transfer-encoding:in-reply-to:references; bh=ATDlnVkVpmBsbQi4Z42Xo1+upTwPMCBzGpQyazaRSrc=; b=k9r/oLnDGvjr3FmAMuH3CPaV+y2P+c3qben785WrqLhJh6pNaTakZQNJ2ky9wt6Fs+bsOj 3WSgg2NAc5OKhJX23eXNt1jiABziDNN2fCCMwcKeqME3fBwIqAJt440qMlGsOU+rpzeAH7 72lLMvjBBi1UQli8/iGnDKkwSWxF9EaTsEf90YetDK4Z4v07vpGCsTx0cIggwISc/JSVRH IYn3lkSC16pe8NfQCQ5/N024S+AMypBUpNCWml7VEbP06nEbjB/JCOittk0yd1IxPN9CZb CeNkg8TM8nk2zYFMHSgeS3CamxRvBPKWzjCjMl6V2M/99r7QGb8ry4VxOKAINA== Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Date: Mon, 12 Oct 2026 23:04:11 +0200 From: Nicolai Buchwitz To: Lorenzo Bianconi Cc: Maxime Chevallier , Andrew Lunn , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Maxime Coquelin , Alexandre Torgue , Alexei Starovoitov , Daniel Borkmann , Jesper Dangaard Brouer , John Fastabend , Stanislav Fomichev , Jose Abreu , Ong Boon Leong , netdev@vger.kernel.org, linux-arm-kernel@lists.infradead.org, bpf@vger.kernel.org Subject: Re: [PATCH net-next v6 3/4] net: stmmac: drop multi-buffer frames in XDP mode In-Reply-To: <20261012-stmmac-rx-mb-v6-3-2225050915c1@oss.qualcomm.com> References: <20261012-stmmac-rx-mb-v6-0-2225050915c1@oss.qualcomm.com> <20261012-stmmac-rx-mb-v6-3-2225050915c1@oss.qualcomm.com> Message-ID: X-Sender: nb@tipi-net.de Content-Type: text/plain; charset=US-ASCII; format=flowed Content-Transfer-Encoding: 7bit X-Last-TLS-Session-Version: TLSv1.3 Hi Lorenzo On 12.10.2026 18:44, Lorenzo Bianconi wrote: > stmmac_rx() runs the XDP program once per descriptor buffer and uses > skb == NULL as the only marker for the start of a frame. A frame larger > than one buffer is split by the hardware across several descriptors; > GMAC4 enables jumbo frames unconditionally (GMAC_CONFIG_JE in > GMAC_CORE_INIT), so an on-link peer can deliver such a frame even when > the interface MTU is left at 1500. > > When an XDP verdict ends a non-last descriptor, skb is cleared and the > next descriptor of the same hardware frame is parsed as a new frame. > For XDP_CONSUMED this happens via read_again with len preserved, so > stmmac_rx_buf1_len() then returns min(dma_buf_sz, plen - len) for the > last descriptor, which can be as small as 0-3 bytes. buf1_len -= > ETH_FCS_LEN underflows and xdp_prepare_buff() puts data_end far past > data; on XDP_PASS this produces an skb whose length is close to 4G that > reaches the network stack (skb->tail and skb->len are 32-bit and wrap > below skb->end, so skb_over_panic() does not fire). A 4+ byte tail is > instead offered to the XDP program and delivered as a standalone > mid-payload Ethernet frame on XDP_PASS. XDP_TX/XDP_REDIRECT instead > falls through to the next iteration, resetting the running length, so > the tail is re-parsed as a full-size new frame. > > XDP does not support scatter-gather frames until the multi-buffer > rework, so drop the whole frame instead: mark it as errored as soon as > a non-last descriptor is seen while an XDP program is attached, drain > and release every descriptor - including the last - and account the > frame once in rx_dropped. Removing the re-parse lets both XDP verdict > branches simply complete the packet via next. > > Note: the subsequent "net: stmmac: rework stmmac_rx to support XDP rx > multi-buff" accumulates the whole frame into a single xdp_buff and > supersedes this patch; the fix is kept so the net portion of the series > stands on its own. > > Fixes: 5fabb01207a2 ("net: stmmac: Add initial XDP support") > Signed-off-by: Lorenzo Bianconi > --- > drivers/net/ethernet/stmicro/stmmac/stmmac_main.c | 28 > ++++++++++------------- > 1 file changed, 12 insertions(+), 16 deletions(-) > > diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c > b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c > index 426bb68c9bbc..8c08d44fdbd3 100644 > --- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c > +++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c > @@ -5893,18 +5893,24 @@ static int stmmac_rx(struct stmmac_priv *priv, > int limit, u32 queue) > > if (priv->extend_desc) > stmmac_rx_extended_status(priv, &priv->xstats, rx_q->dma_erx + > entry); > - if (unlikely(status == discard_frame)) { > - page_pool_put_page(rx_q->page_pool, buf->page, 0, true); > - buf->page = NULL; > + > + if (unlikely(!error && status == discard_frame)) { > error = 1; > if (!priv->hwts_rx_en) > rx_errors++; > } > > - if (unlikely(error && (status & rx_not_ls))) > - goto read_again; > + /* We do not support SG in XDP mode */ > + if (unlikely(!error && (status & rx_not_ls) && > + stmmac_xdp_is_enabled(priv))) { > + rx_dropped++; > + error = 1; > + } AFAIU GMAC4 and XGMAC always use the second buffer of a descriptor. If so, at MTU 1500 a frame of up to 3072 bytes can fit in one descriptor, and this check would not catch it? The XDP program would then only see the first buffer. With XDP_TX the frame is cut and sent too short. With XDP_PASS the second buffer is added after the program already decided. Patch 4 fixes this. But if this patch should work on its own, should it also drop the frame when buf2_len is not zero? Or am I missing something? > [...] Thanks, Nicolai