linux-kernel - Re: [PATCH v4 net-next v4 2/6] net: add support for segmenting TCP fraglist GSO packets

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Date: Tue, 30 Apr 2024 12:57:33 +0200
From: Felix Fietkau <nbd@....name>
To: Paolo Abeni <pabeni@...hat.com>, 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>
Cc: willemdebruijn.kernel@...il.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 net-next v4 2/6] net: add support for segmenting TCP
 fraglist GSO packets

On 30.04.24 12:40, Paolo Abeni wrote:
> On Tue, 2024-04-30 at 12:27 +0200, Felix Fietkau wrote:
>> On 30.04.24 12:19, Paolo Abeni wrote:
>> > On Sat, 2024-04-27 at 20:22 +0200, 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   | 67 ++++++++++++++++++++++++++++++++++++++++
>> > >  net/ipv6/tcpv6_offload.c | 58 ++++++++++++++++++++++++++++++++++
>> > >  2 files changed, 125 insertions(+)
>> > > 
>> > > diff --git a/net/ipv4/tcp_offload.c b/net/ipv4/tcp_offload.c
>> > > index fab0973f995b..affd4ed28cfe 100644
>> > > --- a/net/ipv4/tcp_offload.c
>> > > +++ b/net/ipv4/tcp_offload.c
>> > > @@ -28,6 +28,70 @@ 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)
>> > > +{
>> > > +	const struct tcphdr *th;
>> > > +	const struct iphdr *iph;
>> > > +	struct sk_buff *seg;
>> > > +	struct tcphdr *th2;
>> > > +	struct iphdr *iph2;
>> > > +
>> > > +	seg = segs;
>> > > +	th = tcp_hdr(seg);
>> > > +	iph = ip_hdr(seg);
>> > > +	th2 = tcp_hdr(seg->next);
>> > > +	iph2 = ip_hdr(seg->next);
>> > > +
>> > > +	if (!(*(const u32 *)&th->source ^ *(const u32 *)&th2->source) &&
>> > > +	    iph->daddr == iph2->daddr && iph->saddr == iph2->saddr)
>> > > +		return segs;
>> > > +
>> > > +	while ((seg = seg->next)) {
>> > > +		th2 = tcp_hdr(seg);
>> > > +		iph2 = ip_hdr(seg);
>> > > +
>> > > +		__tcpv4_gso_segment_csum(seg,
>> > > +					 &iph2->saddr, iph->saddr,
>> > > +					 &th2->source, th->source);
>> > > +		__tcpv4_gso_segment_csum(seg,
>> > > +					 &iph2->daddr, iph->daddr,
>> > > +					 &th2->dest, th->dest);
>> > > +	}
>> > > +
>> > > +	return segs;
>> > > +}
>> > 
>> > AFAICS, all the above is really alike the UDP side, except for the
>> > transport header zero csum.
>> > 
>> > What about renaming the udp version of this helpers as 'tcpudpv4_...',
>> > move them in common code, add an explicit argument for
>> > 'zerocsum_allowed' and reuse such helper for both tcp and udp?
>> > 
>> > The same for the ipv6 variant.
>> 
>> Wouldn't that make it more convoluted when taking into account that the 
>> checksum field offset is different for tcp vs udp?
>> How would you handle that?
> 
> Probably making a common helper just for
> __tcpudpv{4,6}_gso_segment_csum and pass it the target l4 csum pointer
> as an additional argument. It would not be spectacularly nice, so no
> strong opinion either way.

I'd rather keep it duplicated but more straightforward and easier to read.

- Felix