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Date: Thu, 30 Oct 2025 09:04:44 +0000
From: Simon Horman <horms@kernel.org>
To: Aditya Garg <gargaditya@linux.microsoft.com>
Cc: kys@microsoft.com, haiyangz@microsoft.com, wei.liu@kernel.org,
	decui@microsoft.com, andrew+netdev@lunn.ch, davem@davemloft.net,
	edumazet@google.com, kuba@kernel.org, pabeni@redhat.com,
	longli@microsoft.com, kotaranov@microsoft.com,
	shradhagupta@linux.microsoft.com, ssengar@linux.microsoft.com,
	ernis@linux.microsoft.com, dipayanroy@linux.microsoft.com,
	shirazsaleem@microsoft.com, linux-hyperv@vger.kernel.org,
	netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-rdma@vger.kernel.org, gargaditya@microsoft.com
Subject: Re: [PATCH net-next v2] net: mana: Handle SKB if TX SGEs exceed
 hardware limit
Message-ID: <aQMqLN0FRmNU3_ke@horms.kernel.org>
References: <20251029131235.GA3903@linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.internal.cloudapp.net>
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On Wed, Oct 29, 2025 at 06:12:35AM -0700, Aditya Garg wrote:
> The MANA hardware supports a maximum of 30 scatter-gather entries (SGEs)
> per TX WQE. Exceeding this limit can cause TX failures.
> Add ndo_features_check() callback to validate SKB layout before
> transmission. For GSO SKBs that would exceed the hardware SGE limit, clear
> NETIF_F_GSO_MASK to enforce software segmentation in the stack.
> Add a fallback in mana_start_xmit() to linearize non-GSO SKBs that still
> exceed the SGE limit.
> 
> Return NETDEV_TX_BUSY only for -ENOSPC from mana_gd_post_work_request(),
> send other errors to free_sgl_ptr to free resources and record the tx
> drop.
> 
> Co-developed-by: Dipayaan Roy <dipayanroy@linux.microsoft.com>
> Signed-off-by: Dipayaan Roy <dipayanroy@linux.microsoft.com>
> Signed-off-by: Aditya Garg <gargaditya@linux.microsoft.com>

...

> @@ -289,6 +290,21 @@ netdev_tx_t mana_start_xmit(struct sk_buff *skb, struct net_device *ndev)
>  	cq = &apc->tx_qp[txq_idx].tx_cq;
>  	tx_stats = &txq->stats;
>  
> +	if (MAX_SKB_FRAGS + 2 > MAX_TX_WQE_SGL_ENTRIES &&
> +	    skb_shinfo(skb)->nr_frags + 2 > MAX_TX_WQE_SGL_ENTRIES) {
> +		/* GSO skb with Hardware SGE limit exceeded is not expected here
> +		 * as they are handled in mana_features_check() callback
> +		 */

Hi,

I'm curious to know if we actually need this code.
Are there cases where the mana_features_check() doesn't
handle things and the kernel will reach this line?

> +		if (skb_is_gso(skb))
> +			netdev_warn_once(ndev, "GSO enabled skb exceeds max SGE limit\n");
> +		if (skb_linearize(skb)) {
> +			netdev_warn_once(ndev, "Failed to linearize skb with nr_frags=%d and is_gso=%d\n",
> +					 skb_shinfo(skb)->nr_frags,
> +					 skb_is_gso(skb));
> +			goto tx_drop_count;
> +		}
> +	}
> +
>  	pkg.tx_oob.s_oob.vcq_num = cq->gdma_id;
>  	pkg.tx_oob.s_oob.vsq_frame = txq->vsq_frame;
>  

...