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

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Date:   Thu, 8 Apr 2021 14:25:45 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Jamal Hadi Salim <jhs@...atatu.com>
Cc:     Petr Machata <petrm@...dia.com>, netdev@...r.kernel.org,
        Jiri Pirko <jiri@...dia.com>,
        "David S. Miller" <davem@...emloft.net>,
        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

On Thu, 8 Apr 2021 10:05:07 -0400 Jamal Hadi Salim wrote:
> 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

To make sure I understand - are you saying that trap should become 
more general and support both "and then drop" as well as "and then 
pass" semantics?

Seems like that ship has sailed, but also - how does it make it any
better WRT not having HW only opcodes? Or are you saying one is better
than two?