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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@nvidia.com> Link: https://lore.kernel.org/netdev/ZxUo0Dc0M5Y6l9qF@shredder.mtl.com/ Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com> Reviewed-by: Ido Schimmel <idosch@nvidia.com> Link: https://patch.msgid.link/20241023135251.1752488-2-vladimir.oltean@nxp.com Signed-off-by: Jakub Kicinski <kuba@kernel.org>