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

From: John Fastabend <[email-masked]>
Date: Wed, 02 Dec 2015 15:35:53 -0800

[...]

>>
>> 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.

Just keying off the last statement there...

I think BPF programs are going to be hard to translate into hardware
for most devices. The problem is the BPF programs in general lack
structure. A parse graph would be much more friendly for hardware or
at minimum the BPF program would need to be a some sort of
well-structured program so a driver could turn that into a parse graph.

.John