netdev - Re: [PATCH net 1/1] openvswitch: Fix setting ipv6 fields causing hw csum failure

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Date:   Tue, 8 Feb 2022 20:11:55 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Paul Blakey <paulb@...dia.com>
Cc:     <dev@...nvswitch.org>, <netdev@...r.kernel.org>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        "Pravin B Shelar" <pshelar@....org>, <davem@...emloft.net>,
        Eelco Chaudron <echaudro@...hat.com>,
        Oz Shlomo <ozsh@...dia.com>, Vlad Buslov <vladbu@...dia.com>,
        Roi Dayan <roid@...dia.com>,
        Ariel Levkovich <lariel@...dia.com>
Subject: Re: [PATCH net 1/1] openvswitch: Fix setting ipv6 fields causing hw
 csum failure

On Mon, 7 Feb 2022 16:41:01 +0200 Paul Blakey wrote:
> Ipv6 ttl, label and tos fields are modified without first
> pulling/pushing the ipv6 header, which would have updated
> the hw csum (if available). This might cause csum validation
> when sending the packet to the stack, as can be seen in
> the trace below.
> 
> Fix this by calling postpush/postpull checksum calculation
> which will update the hw csum if needed.

> -static void set_ipv6_fl(struct ipv6hdr *nh, u32 fl, u32 mask)
> +static void set_ipv6_dsfield(struct sk_buff *skb, struct ipv6hdr *nh, __u8 ipv6_tclass, __u8 mask)
>  {
> +	skb_postpull_rcsum(skb, nh, 4);
> +
> +	ipv6_change_dsfield(nh, ~mask, ipv6_tclass);
> +
> +	skb_postpush_rcsum(skb, nh, 4);
> +}

The calls seem a little heavy for single byte replacements.
Can you instead add a helper based on csum_replace4() maybe?

BTW doesn't pedit have the same problem?