Date:	Wed, 2 Dec 2015 11:15:28 -0800
From:	Tom Herbert <tom@...bertland.com>
To:	Hannes Frederic Sowa <hannes@...essinduktion.org>
Cc:	"John W. Linville" <linville@...driver.com>,
	Jesse Gross <jesse@...nel.org>,
	David Miller <davem@...emloft.net>,
	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 Wed, Dec 2, 2015 at 8:35 AM, Hannes Frederic Sowa
<hannes@...essinduktion.org> wrote:
> On Wed, Dec 2, 2015, at 04:50, Tom Herbert wrote:
>> On Tue, Dec 1, 2015 at 7:49 AM, Hannes Frederic Sowa
>> <hannes@...essinduktion.org> wrote:
>> > On Tue, Dec 1, 2015, at 16:44, John W. Linville wrote:
>> >> On Mon, Nov 30, 2015 at 09:26:51PM -0800, Tom Herbert wrote:
>> >> > On Mon, Nov 30, 2015 at 5:28 PM, Jesse Gross <jesse@...nel.org> wrote:
>> >>
>> >> > > Based on what we can do today, I see only two real choices: do some
>> >> > > refactoring to clean up the stack a bit or remove the existing VXLAN
>> >> > > offloading altogether. I think this series is trying to do the former
>> >> > > and the result is that the stack is cleaner after than before. That
>> >> > > seems like a good thing.
>> >> >
>> >> > There is a third choice which is to do nothing. Creating an
>> >> > infrastructure that claims to "Generalize udp based tunnel offload"
>> >> > but actually doesn't generalize the mechanism is nothing more than
>> >> > window dressing-- this does nothing to help with the VXLAN to
>> >> > VXLAN-GPE transition for instance. If geneve specific offload is
>> >> > really needed now then that can be should with another ndo function,
>> >> > or alternatively ntuple filter with a device specific action would at
>> >> > least get the stack out of needing to be concerned with that.
>> >> > Regardless, we will work optimize the rest of the stack for devices
>> >> > that implement protocol agnostic mechanisms.
>> >>
>> >> Is there no concern about NDO proliferation? Does the size of the
>> >> netdev_ops structure matter? Beyond that, I can see how a single
>> >> entry point with an enum specifying the offload type isn't really any
>> >> different in the grand scheme of things than having multiple NDOs,
>> >> one per offload.
>> >>
>> >> Given the need to live with existing hardware offloads, I would lean
>> >> toward a consolidated NDO. But if a different NDO per tunnel type is
>> >> preferred, I can be satisified with that.
>> >
>> > Having per-offloading NDOs helps the stack to gather further information
>> > what kind of offloads the driver has even maybe without trying to call
>> > down into the layer (just by comparing to NULL). Checking this inside
>> > the driver offload function clearly does not have this feature. So we
>> > finally can have "ip tunnel please-recommend-type" feature. :)
>> >
>> That completely misses the whole point of the rest of this thread.
>> Protocol specific offloads are what we are trying to discourage not
>> encourage. Adding any more ndo functions for this purpose should be an
>> exception, not the norm. The bar should be naturally high considering
>> the cost of exposing this to ndo.
>
> Why?
>
> I wonder why we need protocol generic offloads? I know there are
> currently a lot of overlay encapsulation protocols. Are there many more
> coming?
>
Yes, and assume that there are more coming with an unbounded limit
(for instance I just noticed today that there is a netdev1.1 talk on
supporting GTP in the kernel). Besides, this problem space not just
limited to offload of encapsulation protocols, but how to generalize
offload of any transport, IPv[46], application protocols, protocol
implemented in user space, security protocols, etc.

> Besides, this offload is about TSO and RSS and they do need to parse the
> packet to get the information where the inner header starts. It is not
> only about checksum offloading.
>
RSS does not require the device to parse the inner header. All the UDP
encapsulations protocols being defined set the source port to entropy
flow value and most devices already support RSS+UDP (just needs to be
enabled) so this works just fine with dumb NICs. In fact, this is one
of the main motivations of encapsulating UDP in the first place, to
leverage existing RSS and ECMP mechanisms. The more general solution
is to use IPv6 flow label (RFC6438). We need HW support to include the
flow label into the hash for ECMP and RSS, but once we have that much
of the motivation for using UDP goes away and we can get back to just
doing GRE/IP, IPIP, MPLS/IP, etc. (hence eliminate overhead and
complexity of UDP encap).

> Please provide a sketch up for a protocol generic api that can tell
> hardware where a inner protocol header starts that supports vxlan,
> vxlan-gpe, geneve and ipv6 extension headers and knows which protocol is
> starting at that point.
>
BPF. Implementing protocol generic offloads are not just a HW concern
either, adding kernel GRO code for every possible protocol that comes
along doesn't scale well. This becomes especially obvious when we
consider how to provide offloads for applications protocols. If the
kernel provides a programmable framework for the offloads then
application protocols, such as QUIC, could use use that without
needing to hack the kernel to support the specific protocol (which no
one wants!). Application protocol parsing in KCM and some other use
cases of BPF have already foreshadowed this, and we are working on a
prototype for a BPF programmable engine in the kernel. Presumably,
this same model could eventually be applied as the HW API to
programmable offload.
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