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

From: John Fastabend <[email-masked]>
Date: Mon, 07 Dec 2015 23:33:48 -0800

On 15-12-02 04:15 PM, Tom Herbert wrote:
> On Wed, Dec 2, 2015 at 3:35 PM, John Fastabend <[email-masked]> wrote:
>> [...]
>>
>>>>
>>>> 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.
>>
> This might be relevant:
> http://richard.systems/research/pdf/IEEE_HPSR_BPF_OPENFLOW.pdf
> 

Thanks Tom interesting read but they seem to argue for a BPF engine in
hardware which I'm still not convinced is necessary and the numbers
provided are for a 1Gbps link where 10Gpbs/100Gbps+ would be more
valuable.

I am still leaning towards a fully programmable parse graph and a set
of basic actions push/pop/set/fwd/etc. This would be useful for other
features not just checksum offloads. I guess it doesn't necessarily
exclude also having 1s complement logic though.

.John