netdev - Re: [net-next PATCH 3/4] net: sched: cls_u32 add bit to specify software only rules

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Date:	Thu, 25 Feb 2016 08:39:48 -0800
From:	John Fastabend <john.fastabend@...il.com>
To:	Jamal Hadi Salim <jhs@...atatu.com>, Jiri Benc <jbenc@...hat.com>
CC:	Jiri Pirko <jiri@...nulli.us>, "Amir Vadai\"" <amir@...ai.me>,
	daniel@...earbox.net, netdev@...r.kernel.org,
	alexei.starovoitov@...il.com, davem@...emloft.net
Subject: Re: [net-next PATCH 3/4] net: sched: cls_u32 add bit to specify software
 only rules

On 16-02-25 05:19 AM, Jamal Hadi Salim wrote:
> On 16-02-24 11:09 PM, John Fastabend wrote:
>> On 16-02-24 01:29 AM, Jiri Benc wrote:
>>> On Wed, 24 Feb 2016 00:55:55 -0800, John Fastabend wrote:
>>>> The flags however likely stays with with TCA_U32_FLAGS until there is
>>>> some better way to group common attributes in 'tc' framework.
>>>
>>> That's pretty bad, as this is uAPI and will need to be supported
>>> forever. And having a different attribute in every filter won't ease
>>> things for user space tools. I'd say we need the "better way" to be
>>> added before this patchset.
>>>
>>>   Jiri
>>>
>>
>> The 'tc' semantics seem to support this "pretty bad" API design
>> with many of the fields already duplicated.
> 
> Mostly this is a netlink-ism. Netlink has the same problem with
> command name spaces. The problem is mixing verbs and nouns together.
> I like the switchdev approach where you have very few verbs
> (SET, GET etc) and the content of the object or path describes
> the nouns. This is why initially i thought it was better to have
> this offload passing by switchdev. Could we leverage some of that?
> 

No I don't think so. Either way you need some flag at the 'tc' layer
to push this down to hardware offload ops.

>> I suppose we could
>> put the flags at the same level as the TCA_* attributes but this
>> also doesn't seem right to me as it isn't actually handled until
>> we get into the TCA_#CLASSIFIER#_* set of attributes.
>>
> 
> Could we not steal a couple of bits off netlink flags? Then it applies
> to all subssystems.

I did that in some original patch I never sent but I didn't like it.
Netlink is used for all sorts of things and those flags already have
a meaning. We've already started this trend of using flags inside the
specific messages for other things like the bridge code. And there are
only a few bits left in the rtnetlink flags field I would prefer to
save them for something necessary.

In the end I think the solution here is really not that bad the
userspace code to add it is trivial (~5 lines of code). Its not
how I would do it if I was writing the entire OS from scratch
but we have to maintain UAPI so not sure I have any better solutions.

> 
> cheers,
> jamal
>