linux-kernel - Re: [PATCH v3 net-next 1/6] net: sched: propagate "skip_sw" flag to struct flow_cls_common

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Message-ID: <Zx3r6Al78WpARBe4@shredder.mtl.com>
Date: Sun, 27 Oct 2024 09:29:44 +0200
From: Ido Schimmel <idosch@...dia.com>
To: Vladimir Oltean <vladimir.oltean@....com>
Cc: netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Andrew Lunn <andrew@...n.ch>,
	Florian Fainelli <f.fainelli@...il.com>,
	Petr Machata <petrm@...dia.com>,
	Claudiu Manoil <claudiu.manoil@....com>,
	Alexandre Belloni <alexandre.belloni@...tlin.com>,
	UNGLinuxDriver@...rochip.com, Jamal Hadi Salim <jhs@...atatu.com>,
	Cong Wang <xiyou.wangcong@...il.com>, Jiri Pirko <jiri@...nulli.us>,
	Vlad Buslov <vladbu@...dia.com>, Simon Horman <horms@...nel.org>,
	Christian Marangi <ansuelsmth@...il.com>,
	Arun Ramadoss <arun.ramadoss@...rochip.com>,
	Arınç ÜNAL <arinc.unal@...nc9.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 net-next 1/6] net: sched: propagate "skip_sw" flag to
 struct flow_cls_common_offload

On Wed, Oct 23, 2024 at 04:52:46PM +0300, Vladimir Oltean wrote:
> Background: switchdev ports offload the Linux bridge, and most of the
> packets they handle will never see the CPU. The ports between which
> there exists no hardware data path are considered 'foreign' to switchdev.
> These can either be normal physical NICs without switchdev offload, or
> incompatible switchdev ports, or virtual interfaces like veth/dummy/etc.
> 
> In some cases, an offloaded filter can only do half the work, and the
> rest must be handled by software. Redirecting/mirroring from the ingress
> of a switchdev port towards a foreign interface is one example of
> combined hardware/software data path. The most that the switchdev port
> can do is to extract the matching packets from its offloaded data path
> and send them to the CPU. From there on, the software filter runs
> (a second time, after the first run in hardware) on the packet and
> performs the mirred action.
> 
> It makes sense for switchdev drivers which allow this kind of "half
> offloading" to sense the "skip_sw" flag of the filter/action pair, and
> deny attempts from the user to install a filter that does not run in
> software, because that simply won't work.
> 
> In fact, a mirred action on a switchdev port towards a dummy interface
> appears to be a valid way of (selectively) monitoring offloaded traffic
> that flows through it. IFF_PROMISC was also discussed years ago, but
> (despite initial disagreement) there seems to be consensus that this
> flag should not affect the destination taken by packets, but merely
> whether or not the NIC discards packets with unknown MAC DA for local
> processing.
> 
> [1] https://lore.kernel.org/netdev/20190830092637.7f83d162@ceranb/
> [2] https://lore.kernel.org/netdev/20191002233750.13566-1-olteanv@gmail.com/
> Suggested-by: Ido Schimmel <idosch@...dia.com>
> Link: https://lore.kernel.org/netdev/ZxUo0Dc0M5Y6l9qF@shredder.mtl.com/
> Signed-off-by: Vladimir Oltean <vladimir.oltean@....com>

Reviewed-by: Ido Schimmel <idosch@...dia.com>