linux-kernel - Re: [PATCH net-next v5 1/2] net: mana: Handle SKB if TX SGEs exceed hardware limit

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Message-ID: <20251117194618.33af8e98@kernel.org>
Date: Mon, 17 Nov 2025 19:46:18 -0800
From: Jakub Kicinski <kuba@...nel.org>
To: Aditya Garg <gargaditya@...ux.microsoft.com>
Cc: kys@...rosoft.com, haiyangz@...rosoft.com, wei.liu@...nel.org,
 decui@...rosoft.com, andrew+netdev@...n.ch, davem@...emloft.net,
 edumazet@...gle.com, pabeni@...hat.com, longli@...rosoft.com,
 kotaranov@...rosoft.com, horms@...nel.org,
 shradhagupta@...ux.microsoft.com, ssengar@...ux.microsoft.com,
 ernis@...ux.microsoft.com, dipayanroy@...ux.microsoft.com,
 shirazsaleem@...rosoft.com, leon@...nel.org, mlevitsk@...hat.com,
 yury.norov@...il.com, sbhatta@...vell.com, linux-hyperv@...r.kernel.org,
 netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-rdma@...r.kernel.org, gargaditya@...rosoft.com
Subject: Re: [PATCH net-next v5 1/2] net: mana: Handle SKB if TX SGEs exceed
 hardware limit

On Fri, 14 Nov 2025 13:16:42 -0800 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.

> +	BUILD_BUG_ON(MAX_TX_WQE_SGL_ENTRIES != MANA_MAX_TX_WQE_SGL_ENTRIES);
> +#if (MAX_SKB_FRAGS + 2 > MANA_MAX_TX_WQE_SGL_ENTRIES)
> +	if (skb_shinfo(skb)->nr_frags + 2 > MAX_TX_WQE_SGL_ENTRIES) {

nit: please try to avoid the use of ifdef if you can. This helps to
avoid build breakage sneaking in as this code will be compiled out
on default config on all platforms.

Instead you should be able to simply add the static condition to the
if statement:

	if (MAX_SKB_FRAGS + 2 > MANA_MAX_TX_WQE_SGL_ENTRIES &&
	    skb_shinfo(skb)->nr_frags + 2 > MAX_TX_WQE_SGL_ENTRIES) {

and let the compiler (rather than preprocessor) eliminate this if ()
block.

> +		/* GSO skb with Hardware SGE limit exceeded is not expected here
> +		 * as they are handled in mana_features_check() callback
> +		 */
> +		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;
> +		}
> +		apc->eth_stats.linear_pkt_tx_cnt++;
> +	}
> +#endif
-- 
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