Date:	Tue, 23 Feb 2016 16:34:56 +0100
From:	Daniel Borkmann <daniel@...earbox.net>
To:	Jamal Hadi Salim <jhs@...atatu.com>, davem@...emloft.net
CC:	netdev@...r.kernel.org, xiyou.wangcong@...il.com,
	alexei.starovoitov@...il.com
Subject: Re: [net-next PATCH 0/5] net_sched: Add support for IFE action

On 02/23/2016 03:28 PM, Jamal Hadi Salim wrote:
> On 16-02-23 08:20 AM, Daniel Borkmann wrote:
>> On 02/23/2016 01:09 PM, Jamal Hadi Salim wrote:
>>> On 16-02-22 11:47 AM, Daniel Borkmann wrote:
>> [...]
>>>> So, basically this is a L2 encap with TLVs, right?
>>>>
>>>> And as TLVs you have skb->mark, skb->priority, skb->hash,
>>>> skb->queue_mapping
>>>> that you transfer from one machine to another, where on the destination,
>>>> you
>>>> are applying the above meta data to the skb itself. And, configuration
>>>> is via
>>>> tc.
>>>>
>>>> I couldn't parse from the commit log what the real world use case is,
>>>> resp.
>>>> who is going to use this infrastructure?
>>>>
>>>> Do you have some typical setup, where the above needs to be transferred
>>>> in the
>>>> encap and restored?
>>>
>>> I am assuming you are asking this for the sake of people who dont
>>> have context (and not for yourself)?
>>> I added a pointer to the paper. It is 6 pages and simple to read.
>>> Isnt that sufficient? I dont want to write a novel here. Some could
>>> argue that in fact i am already writing a novel in commit 1/5.
>>
>> Ok, the paper talks about, quote:
>>
>>   - Pipeline-stage Indexing.
>>   - OAM information - example turn on some packet debug information on a
>> need basis.
>>   - Exception handling information - example VXLAN service handling.
>>   - Authentication and authorization information.
>>   - Versioning information.
>>   - Compliance information.
>>   - Service Identifiers.
>
> Which is now added to the cover letter.
>
>> As your primary examples, you provide skb->mark, skb->priority, skb->hash,
>> skb->queue_mapping for encapsulating, f.e. how do you use skb>hash in this
>> scenario? What's the use-case?
>
> These are basic metadata. The question to ask is what could one use
> skb->hash for. Today it is used to select a cpu to balance to.

Right, but that happens before you decode that information from your TLV
on ingress qdisc. And any subsequent skb_get_hash() to read out skb->hash
will effectively overwrite what you set there and call into flow dissector.

> I dont understand what your concern is: I could pass a string
> as metadata if i wanted to. I chose to pass skb metadata;
> some of which i actually use. Only one i dont use is queue
> map - but because it is still an skb metadata so i wrote code
> to pass it.

But if it's not used by anyone currently, why add it? ;) Better to only
add if there's a real demand for usage, otherwise it's code that no-one
uses and distros needlessly ship it to everyone.

>>> They need to be separated to make them unique. These are basic
>>> metadatum; I have a few others lined up - but i just wanted to start
>>> with these because they are obvious to see. What i mulled over is
>>> to send one big patch or several. In the end it seemed cleaner to
>>> send separate patches.
>>
>> But just to make them unique, you don't need to add extra modules for
>> this ... just having a module for encoding one skb member seems like
>> total overdesign to me. If you really want them to be separate ops,
>> you can still include them into act_ife.c itself.
>
> These things will evolve. I would rather leave them the way they
> are for that reason and they serve as good examples of how one
> would new metadatum.

My concern is we add 20 new modules like this that only do trivial things,
where instead they could have been consolidated and reduce maintenance. Or
is this hard module requirement related to the IFE_META_* module parameter?

> Example: skb->mark may end up evolving to include masks.

Thanks,
Daniel

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