netdev - Re: [RFC PATCH net] net: flow_offload: simplify hw stats check handling

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Date:   Mon, 11 May 2020 07:33:59 +0200
From:   Jiri Pirko <jiri@...nulli.us>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Pablo Neira Ayuso <pablo@...filter.org>,
        Edward Cree <ecree@...arflare.com>, netdev@...r.kernel.org,
        davem@...emloft.net, netfilter-devel@...r.kernel.org
Subject: Re: [RFC PATCH net] net: flow_offload: simplify hw stats check
 handling

Fri, May 08, 2020 at 01:48:20AM CEST, kuba@...nel.org wrote:
>On Thu, 7 May 2020 18:46:43 +0200 Pablo Neira Ayuso wrote:
>> On Thu, May 07, 2020 at 04:49:15PM +0100, Edward Cree wrote:
>> > On 07/05/2020 16:32, Pablo Neira Ayuso wrote:  
>> > > On Thu, May 07, 2020 at 03:59:09PM +0100, Edward Cree wrote:  
>> > >> Make FLOW_ACTION_HW_STATS_DONT_CARE be all bits, rather than none, so that
>> > >>  drivers and __flow_action_hw_stats_check can use simple bitwise checks.  
>> > > 
>> > > You have have to explain why this makes sense in terms of semantics.
>> > > 
>> > > _DISABLED and _ANY are contradicting each other.  
>> > No, they aren't.  The DISABLED bit means "I will accept disabled", it doesn't
>> >  mean "I insist on disabled".  What _does_ mean "I insist on disabled" is if
>> >  the DISABLED bit is set and no other bits are.
>> > So DISABLED | ANY means "I accept disabled; I also accept immediate or
>> >  delayed".  A.k.a. "I don't care, do what you like".  
>> 
>> Jiri said Disabled means: bail out if you cannot disable it.
>
>That's in TC uAPI Jiri chose... doesn't mean we have to do the same
>internally.

Yeah, but if TC user says "disabled", please don't assign counter or
fail.


>
>> If the driver cannot disable, then it will have to check if the
>> frontend is asking for Disabled (hence, report error to the frontend)
>> or if it is actually asking for Don't care.
>> 
>> What you propose is a context-based interpretation of the bits. So
>> semantics depend on how you accumulate/combine bits.
>> 
>> I really think bits semantics should be interpreted on the bit alone
>> itself.
>
>These 3 paragraphs sound to me like you were arguing for Ed's approach..
>
>> There is one exception though, that is _ANY case, where you let the
>> driver pick between delayed or immediate. But if the driver does not
>> support for counters, it bails out in any case, so the outcome in both
>> request is basically the same.
>> 
>> You are asking for different outcome depending on how bits are
>> combined, which can be done, but it sounds innecessarily complicated
>> to me.
>
>No, quite the opposite, the code as committed to net has magic values
>which drivers have to check.
>
>The counter-proposal is that each bit represents a configuration, and
>if more than one bit is set the driver gets to choose which it prefers. 
>What could be simpler?
>
>netfilter just has to explicitly set the field to DONT_CARE rather than 
>depending on 0 form zalloc() coinciding with the correct value.