netdev - Re: [PATCH] net: Add ndo_gso

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Date:	Tue, 07 Oct 2014 14:47:52 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	alexei.starovoitov@...il.com
Cc:	therbert@...gle.com, jesse@...ira.com, gerlitz.or@...il.com,
	alexander.h.duyck@...el.com, john.r.fastabend@...el.com,
	jeffrey.t.kirsher@...el.com, netdev@...r.kernel.org, tgraf@...g.ch,
	pshelar@...ira.com, azhou@...ira.com
Subject: Re: [PATCH] net: Add ndo_gso_check

From: Alexei Starovoitov <alexei.starovoitov@...il.com>
Date: Tue, 7 Oct 2014 10:18:25 -0700

> On Tue, Oct 7, 2014 at 10:05 AM, David Miller <davem@...emloft.net> wrote:
>> From: Alexei Starovoitov <alexei.starovoitov@...il.com>
>> Date: Tue, 7 Oct 2014 09:50:50 -0700
>>
>>> CHECKSUM_COMPLETE is a burden on software.
>>
>> I totally disagree, it's the most software friendly checksumming
>> offload mechanism possible.  I wish every card did it.
>>
>> CHECKSUM_COMPLETE means that any sub-protocol or tunneling mechanism
>> can be trivially supported without any modifications to hardware, and
>> it therefore makes checksum offloading of new protocols require no
>> hardware changes whatsoever.
> 
> yes, of course. My point is that if HW can parse the packet and validate
> csum it should do that, since it's faster for the stack on top.
> HW can fall back to CHECKSUM_COMPLETE if it fails to parse, for example.
> I think some NICs do exactly that.

I am totally against boolean "yes/no" protocol specific checksum
validation by HW.

It's not faster.  You have to look at the pseudo-header and bring it into
the CPU cache _anyways_, so negating it and 2's complementing it into
the CHECKSUM_COMPLETE value for validation is free.

There is no performance advantage whatsoever to use another checksumming
scheme.
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