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During flow processing, the flow wildcards are checked as a series of stages, and these stages are intended to carry dependencies in a single direction. But when the neighbor discovery processing, for example, was executed there is an incorrect dependency chain - we need fields from stage 4 to determine whether we need fields from stage 3. We can build a set of flow rules to demonstrate this: table=0,priority=100,ipv6,ipv6_src=1000::/10 actions=resubmit(,1) table=0,priority=0 actions=NORMAL table=1,priority=110,ipv6,ipv6_dst=1000::3 actions=resubmit(,2) table=1,priority=100,ipv6,ipv6_dst=1000::4 actions=resubmit(,2) table=1,priority=0 actions=NORMAL table=2,priority=120,icmp6,nw_ttl=255,icmp_type=135,icmp_code=0,nd_sll=10🇩🇪ad:be:ef:10 actions=NORMAL table=2,priority=100,tcp actions=NORMAL table=2,priority=100,icmp6 actions=NORMAL table=2,priority=0 actions=NORMAL With this set of flows, any IPv6 packet that executes through this pipeline will have the corresponding nd_sll field flagged as required match for classification even if that field doesn't make sense in such a context (for example, TCP packets). When the corresponding flow is installed into the kernel datapath, this field is not reflected when the revalidator executes the dump stage (see net/openvswitch/flow_netlink.c for more details). During the sweep stage, revalidator will compare the dumped WC with a generated WC - these will mismatch because the generated WC will match on the Neighbor Discovery fields, while the datapath WC will not match on these fields. We will then invalidate the flow and as a side effect force an upcall. By redefining the boundary, we shift these fields to the l4 subtable, and cause masks to be generated matching just the requisite fields. The list of fields being shifted: struct in6_addr nd_target; struct eth_addr arp_sha; struct eth_addr arp_tha; ovs_be16 tcp_flags; ovs_be16 pad2; struct ovs_key_nsh nsh; A standout field would be tcp_flags moving from l3 subtable matches to the l4 subtable matches. This reverts a partial performance optimization in the case of stateless firewalling. The tcp_flags field might have been a good candidate to retain in the l3 segment, but it got overloaded with ICMPv6 ND matching, and therefore we can't preserve this kind of optimization. Two other approaches were considered - moving the nd_target field alone and collapsing the l3/l4 segments into a single subtable for matching. Moving any field individually introduces ABI mismatch, and doesn't completely address the problems with other neighbor discovery related fields (such as nd_sll/nd_tll). Collapsing the two subtables creates an issue with datapath flow explosion, since the l3 and l4 fields will be unwildcarded together (this can be seen with some of the existing classifier tests). A simple test is added to showcase the behavior. Fixes: 476f36e83bc5 ("Classifier: Staged subtable matching.") Reported-at: https://bugzilla.redhat.com/show_bug.cgi?id=2081773 Reported-by: Numan Siddique <nusiddiq@redhat.com> Suggested-by: Ilya Maximets <i.maximets@ovn.org> Signed-off-by: Aaron Conole <aconole@redhat.com> Acked-by: Eelco Chaudron <echaudro@redhat.com> Acked-by: Cian Ferriter <cian.ferriter@intel.com> Tested-by: Numan Siddique <numans@ovn.org> Signed-off-by: Ilya Maximets <i.maximets@ovn.org>