netdev - Re: [PATCH net-next 1/7] net: sched: Add a trap-and-forward action

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Message-ID: <b60df78a-1aba-ba27-6508-4c67b0496020@mojatatu.com>
Date:   Thu, 8 Apr 2021 10:05:07 -0400
From:   Jamal Hadi Salim <jhs@...atatu.com>
To:     Petr Machata <petrm@...dia.com>, netdev@...r.kernel.org
Cc:     Jiri Pirko <jiri@...dia.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Ido Schimmel <idosch@...dia.com>,
        Cong Wang <xiyou.wangcong@...il.com>
Subject: Re: [PATCH net-next 1/7] net: sched: Add a trap-and-forward action

Hi Petr,

On 2021-04-08 9:38 a.m., Petr Machata wrote:
> The TC action "trap" is used to instruct the HW datapath to drop the
> matched packet and transfer it for processing in the SW pipeline. If
> instead it is desirable to forward the packet and transferring a _copy_ to
> the SW pipeline, there is no practical way to achieve that.
> 
> To that end add a new generic action, trap_fwd. In the software pipeline,
> it is equivalent to an OK. When offloading, it should forward the packet to
> the host, but unlike trap it should not drop the packet.
> 

I am concerned about adding new opcodes which only make sense if you
offload (or make sense only if you are running in s/w).

Those opcodes are intended to be generic abstractions so the dispatcher
can decide what to do next. Adding things that are specific only
to scenarios of hardware offload removes that opaqueness.
I must have missed the discussion on ACT_TRAP because it is the
same issue there i.e shouldnt be an opcode. For details see:
https://people.netfilter.org/pablo/netdev0.1/papers/Linux-Traffic-Control-Classifier-Action-Subsystem-Architecture.pdf

IMO:
It seems to me there are two actions here encapsulated in one.
The first is to "trap" and the second is to "drop".
This is no different semantically than say "mirror and drop"
offload being enunciated by "skip_sw".

Does the spectrum not support multiple actions?
e.g with a policy like:
  match blah action trap action drop skip_sw

cheers,
jamal