linux-kernel - Re: [PATCH v2 net-next v2 2/5] net: add support for segmenting TCP fraglist GSO packets

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Date: Thu, 25 Apr 2024 11:17:19 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Felix Fietkau <nbd@....name>, 
 netdev@...r.kernel.org, 
 Eric Dumazet <edumazet@...gle.com>, 
 "David S. Miller" <davem@...emloft.net>, 
 David Ahern <dsahern@...nel.org>, 
 Jakub Kicinski <kuba@...nel.org>, 
 Paolo Abeni <pabeni@...hat.com>
Cc: willemdebruijn.kernel@...il.com, 
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 net-next v2 2/5] net: add support for segmenting TCP
 fraglist GSO packets

Felix Fietkau wrote:
> Preparation for adding TCP fraglist GRO support. It expects packets to be
> combined in a similar way as UDP fraglist GSO packets.
> For IPv4 packets, NAT is handled in the same way as UDP fraglist GSO.
> 
> Signed-off-by: Felix Fietkau <nbd@....name>
> ---
>  net/ipv4/tcp_offload.c   | 69 ++++++++++++++++++++++++++++++++++++++++
>  net/ipv6/tcpv6_offload.c |  3 ++
>  2 files changed, 72 insertions(+)
> 
> diff --git a/net/ipv4/tcp_offload.c b/net/ipv4/tcp_offload.c
> index fab0973f995b..e455f884190c 100644
> --- a/net/ipv4/tcp_offload.c
> +++ b/net/ipv4/tcp_offload.c
> @@ -28,6 +28,72 @@ static void tcp_gso_tstamp(struct sk_buff *skb, unsigned int ts_seq,
>  	}
>  }
>  
> +static void __tcpv4_gso_segment_csum(struct sk_buff *seg,
> +				     __be32 *oldip, __be32 *newip,
> +				     __be16 *oldport, __be16 *newport)
> +{
> +	struct tcphdr *th;
> +	struct iphdr *iph;
> +
> +	if (*oldip == *newip && *oldport == *newport)
> +		return;
> +
> +	th = tcp_hdr(seg);
> +	iph = ip_hdr(seg);
> +
> +	inet_proto_csum_replace4(&th->check, seg, *oldip, *newip, true);
> +	inet_proto_csum_replace2(&th->check, seg, *oldport, *newport, false);
> +	*oldport = *newport;
> +
> +	csum_replace4(&iph->check, *oldip, *newip);
> +	*oldip = *newip;
> +}
> +
> +static struct sk_buff *__tcpv4_gso_segment_list_csum(struct sk_buff *segs)
> +{
> +	struct sk_buff *seg;
> +	struct tcphdr *th, *th2;
> +	struct iphdr *iph, *iph2;
> +	__be32 flags, flags2;
> +
> +	seg = segs;
> +	th = tcp_hdr(seg);
> +	iph = ip_hdr(seg);
> +	flags = tcp_flag_word(th);
> +	flags2 = tcp_flag_word(tcp_hdr(seg->next));

Vestigial, now that flag overwrite is removed in v2?

All this code is very similar to __udpv4_gso_segment_list_csum. But
the zero checksum handling in __udpv4_gso_segment_csum makes it just
different enough that I also do not immediately see a straightforward
way to avoid duplicating.