netdev - Re: [PATCH v1 1/6] net: Generalize udp based tunnel offload

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Message-Id: <1449260785.11089.458370265.3954BCFA@webmail.messagingengine.com>
Date:	Fri, 04 Dec 2015 21:26:25 +0100
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	David Miller <davem@...emloft.net>
Cc:	tom@...bertland.com, linville@...driver.com, jesse@...nel.org,
	anjali.singhai@...el.com, netdev@...r.kernel.org,
	kiran.patil@...el.com
Subject: Re: [PATCH v1 1/6] net: Generalize udp based tunnel offload

Hi Dave,

On Fri, Dec 4, 2015, at 21:06, David Miller wrote:
> From: Hannes Frederic Sowa <hannes@...essinduktion.org>
> Date: Fri, 04 Dec 2015 20:59:05 +0100
> 
> > Yes, I agree, I am totally with you here. If generic offloading can be
> > realized by NICs I am totally with you that this should be the way to
> > go. I don't see that coming in the next (small number of) years, so I
> > don't see a reason to stop this patchset.
> 
> If I just apply this and say "yeah ok", the message is completely lost
> and your prediction about "small number of years" indeed will occur.
> 
> However if I push back hard on this, as I will, then the message has
> some chance of seeping back to the people designing these chips.
> 
> So that's what I'm going to do, like it or not.
> 
> Or can someone convince me that someone who understand this stuff
> is telling the hardware guys to universally put 2's complement
> checksums into the descriptors?
> 
> Who is doing that at each and every prominent ethernet hardware
> verndor?
> 
> Who?
> 
> If I get silence, or some vague non-specific response, then I'm going
> to hold my ground and keep pushing back on this stuff.

This is not only about 1's checksumming but also about TSO (and to some
smaller degree about RSS, as I tried to explain): if we attach a geneve
header in front of a skb we expect the hardware to recognize it and
duplicate it while doing the hardware segmentation. The hardware can
only do so if it is in knowledge of the specific port (in this case UDP
port used for geneve) which is in use for this particular tunneling
transport protocol. We currently cannot describe this in a generic way,
thus this patchset. (Please correct me if I am wrong!)

The other way to do it would probably be to enlarge the skb and push the
structure of the packet into it, so hardware has more semantic knowledge
about the frames structure. I guess(!!!) DPDK does it like that?

If it would only be about checksuming I probably would agree.

Bye,
Hannes
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