netdev - Re: [net-next PATCH v2 4/8] udp: Do not pass checksum as a parameter to GSO segmentation

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Date:   Fri, 4 May 2018 13:19:15 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Alexander Duyck <alexander.duyck@...il.com>,
        netdev@...r.kernel.org, willemb@...gle.com, davem@...emloft.net
Subject: Re: [net-next PATCH v2 4/8] udp: Do not pass checksum as a parameter
 to GSO segmentation



On 05/04/2018 11:30 AM, Alexander Duyck wrote:
> From: Alexander Duyck <alexander.h.duyck@...el.com>
> 
> This patch is meant to allow us to avoid having to recompute the checksum
> from scratch and have it passed as a parameter.
> 
> Instead of taking that approach we can take advantage of the fact that the
> length that was used to compute the existing checksum is included in the
> UDP header. If we cancel that out by adding the value XOR with 0xFFFF we
> can then just add the new length in and fold that into the new result.
> 

>  
> +	uh = udp_hdr(segs);
> +
> +	/* compute checksum adjustment based on old length versus new */
> +	newlen = htons(sizeof(*uh) + mss);
> +	check = ~csum_fold((__force __wsum)((__force u32)uh->check +
> +					    ((__force u32)uh->len ^ 0xFFFF) +
> +					    (__force u32)newlen));
> +


Can't this use csum_sub() instead of this ^ 0xFFFF trick ?