linux-kernel - Re: [RFC] net: add TCP fraglist GRO support

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Date: Tue, 23 Apr 2024 18:55:32 +0200
From: Felix Fietkau <nbd@....name>
To: Paolo Abeni <pabeni@...hat.com>, Eric Dumazet <edumazet@...gle.com>
Cc: netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
 Jakub Kicinski <kuba@...nel.org>, David Ahern <dsahern@...nel.org>,
 linux-kernel@...r.kernel.org
Subject: Re: [RFC] net: add TCP fraglist GRO support

On 23.04.24 16:34, Paolo Abeni wrote:
> On Tue, 2024-04-23 at 14:23 +0200, Felix Fietkau wrote:
>> On 23.04.24 14:11, Eric Dumazet wrote:
>> > On Tue, Apr 23, 2024 at 1:55 PM Felix Fietkau <nbd@....name> wrote:
>> > > 
>> > > In the world of consumer-grade WiFi devices, there are a lot of chipsets
>> > > with limited or nonexistent SG support, and very limited checksum
>> > > offload capabilities on Ethernet. The WiFi side of these devices is
>> > > often even worse. I think fraglist GRO is a decent fallback for the
>> > > inevitable corner cases.
>> > 
>> > What about netfilter and NAT ? Are they okay with NETIF_F_FRAGLIST_GRO already ?
>> > 
>> > Many of these devices are probably using NAT.
>> 
>> In my tests, nftables NAT works just fine, both with and without 
>> flowtable offloading. I didn't see anything in netfilter that would have 
>> a problem with this.
> 
> I see you handle explicitly NAT changes in __tcpv4_gso_segment_csum(),
> like the current UDP code.
> 
> The TCP header has many other fields that could be updated affecting
> the TCP csum.
> Handling every possible mutation looks cumbersome and will likely
> reduce the performance benefits.
> 
> What is your plan WRT other TCP header fields update?

I think that should be easy enough to handle. My patch already only 
combines packets where tcp_flag_word(th) is identical. So when 
segmenting, I could handle all flags changes with a single 
inet_proto_csum_replace4 call.

> Strictly WRT the patch, I guess it deserves to be split in series,
> moving UDP helpers in common code and possibly factoring out more
> helpers with separate patches.
Will do.

> e.g. in __tcpv4_gso_segment_csum() is quite similar
> __udpv4_gso_segment_csum() - even too much, as the tcp csum should be
> always be updated when the ports or addresses change ;)

Will fix that.

Thanks,

- Felix