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

From: Petr Machata <petrm@nvidia.com>
To: Jamal Hadi Salim <jhs@mojatatu.com>
Cc: Petr Machata <petrm@nvidia.com>, <netdev@vger.kernel.org>,
	Jiri Pirko <jiri@nvidia.com>,
	"David S. Miller" <davem@davemloft.net>,
	Jakub Kicinski <kuba@kernel.org>,
	Ido Schimmel <idosch@nvidia.com>,
	Cong Wang <xiyou.wangcong@gmail.com>
Subject: Re: [PATCH net-next 1/7] net: sched: Add a trap-and-forward action
Date: Fri, 9 Apr 2021 13:03:19 +0200	[thread overview]
Message-ID: <877dlb67pk.fsf@nvidia.com> (raw)
In-Reply-To: <b60df78a-1aba-ba27-6508-4c67b0496020@mojatatu.com>

Jamal Hadi Salim <jhs@mojatatu.com> writes:

> 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

Trap has been in since 4.13, so 2017ish. It's done and dusted at this
point.

> 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

Trap drops implicitly. We need a "trap, but don't drop". Expressed in
terms of existing actions it would be "mirred egress redirect dev
$cpu_port". But how to express $cpu_port except again by a HW-specific
magic token I don't know.