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Date:	Fri, 04 Dec 2015 22:11:12 +0100
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Tom Herbert <tom@...bertland.com>
Cc:	David Miller <davem@...emloft.net>,
	John Linville <linville@...driver.com>,
	Jesse Gross <jesse@...nel.org>,
	Anjali Singhai Jain <anjali.singhai@...el.com>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	Kiran Patil <kiran.patil@...el.com>
Subject: Re: [PATCH v1 1/6] net: Generalize udp based tunnel offload

On Fri, Dec 4, 2015, at 21:43, Tom Herbert wrote:
> On Fri, Dec 4, 2015 at 12:26 PM, Hannes Frederic Sowa
> <hannes@...essinduktion.org> wrote:
> > 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!)
> >
> Yes, you are wrong. Port numbers are not used in transmit path to
> signal offload. To perform TSO on UDP encapsulated packets the skb is
> marked with SKB_GSO_UDP_TUNNEL or SKB_GSO_UDP_TUNNEL_CSUM and
> SKB_GSO_TCP, etc. The driver can use information along with the
> offsets of the inner and outer headers in the packet to set up the
> operation in the device.  Some devices only support TSO for VXLAN, but
> regardless SKB_GSO_UDP_TUNNEL is generic for all known UDP
> encapsulations. Protocol specific offload is not needed. Please start
> looking at
> http://people.netfilter.org/pablo/netdev0.1/papers/UDP-Encapsulation-in-Linux.pdf
> and the kernel code to see how things _actually_ work.

I am sorry. Of course we have _inner_ header pointers and the
corresponding gso_types in skb_shinfo to signal that already. Probably I
got confused by some driver sources and commit descriptions I looked
into lately. Thanks for the correction! :)

Bye,
Hannes
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