netdev - Re: [PATCH] net: Add ndo_gso

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Date:	Tue, 7 Oct 2014 09:50:50 -0700
From:	Alexei Starovoitov <alexei.starovoitov@...il.com>
To:	Tom Herbert <therbert@...gle.com>
Cc:	Jesse Gross <jesse@...ira.com>, Or Gerlitz <gerlitz.or@...il.com>,
	Alexander Duyck <alexander.h.duyck@...el.com>,
	John Fastabend <john.r.fastabend@...el.com>,
	Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
	David Miller <davem@...emloft.net>,
	Linux Netdev List <netdev@...r.kernel.org>,
	Thomas Graf <tgraf@...g.ch>,
	Pravin Shelar <pshelar@...ira.com>,
	Andy Zhou <azhou@...ira.com>
Subject: Re: [PATCH] net: Add ndo_gso_check

On Mon, Oct 6, 2014 at 5:17 PM, Tom Herbert <therbert@...gle.com> wrote:
> On Mon, Oct 6, 2014 at 3:33 PM, Jesse Gross <jesse@...ira.com> wrote:
>
>> I have no disagreement with trying to be generic across protocols. I'm
>> just not convinced that it is a realistic plan. It's obvious that it
>> is not doable today nor will be it be in the next generation of NICs
>> (which are guaranteed to add support for new protocols). Furthermore,
>> there will be more advanced stuff coming in the future that I think
>> will be difficult or impossible to make protocol agnostic. Rather than
>> pretending that this doesn't exist or will never happen, it's better
>> focus on how to integrating it cleanly.
>
> Sorry, but I don't understand how supporting a new protocols in a
> device for the purposes of returning CHECKSUM_UNNECESSARY is better or
> easier to implement than just returning CHECKSUM_COMPLETE. Same thing
> for trying to use NETIF_F_IP_CSUM with encapsulation rather than
> NETIF_F_HW_CSUM. I'm not a hardware guy, so it's possible I'm missing
> something obvious...

it's definitely more difficult to properly implement
CHECKSUM_UNNECESSARY in HW, but it's worth it.
CHECKSUM_COMPLETE is a burden on software. Old NICs
used to do that, but overhead of recomputing csum for every
step of packet parsing and header modifications is too high.
sw routers, bridges and < L4 networking devices are
simpler and faster with CHECKSUM_UNNECESSARY.
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