[ovs-dev] Unwildcarding of fields modified by set_field action

[Jan Scheurich]

Sorry for resending, but an earlier version of the message was blocked 
by list filters:

Hi,


When testing the OVS datapath in the context of work on L3 tunneling and 
packet-type aware pipeline, we have observed that ofproto-dpif-xlate 
unwildcards match fields that are being modified by set_field actions, 
even when those fields are never matched on in the pipeline.

As a result the megaflow cache is bloated with unnecessarily many 
entries that only differ in such a non-matched field and the datapath 
classifier lookup cost increases due to larger miniflow keys/masks.

Example:

OpenFlow flow entry:

ovs-ofctl -Oopenflow13 add-flow br0 
“in_port=2,actions=set_field:fe:4a:7a:ae:7c:fb->dl_dst,1”

Datapath flows:

recirc_id(0),in_port(3),eth(dst=fe:4a:7a:ae:7c:f1),eth_type(0x0800),ipv4(frag=no), 
packets:26, bytes:2548, used:0.571s, 
actions:set(eth(dst=fe:4a:7a:ae:7c:fb)),2

recirc_id(0),in_port(3),eth(dst=fe:4a:7a:ae:7c:f2),eth_type(0x0800),ipv4(frag=no), 
packets:26, bytes:2548, used:0.571s, 
actions:set(eth(dst=fe:4a:7a:ae:7c:fb)),2

recirc_id(0),in_port(3),eth(dst=fe:4a:7a:ae:7c:f3),eth_type(0x0800),ipv4(frag=no), 
packets:26, bytes:2548, used:0.571s, 
actions:set(eth(dst=fe:4a:7a:ae:7c:fb)),2

…

We do not understand why this is done and, if there are no good reasons 
to keep it, would suggest to remove the unwildcarding of fields modified 
by set_field actions.

What we do understand is that fields are unwildcarded when they are 
copied into packet registers or other packet fields. The reason is that 
these do/may affect the resulting packet. But that is not the case here.

In the context of L3 tunneling and PTAP the current behavior produces 
incorrect results because setting the dl_dst of a packet received from 
an L3 port adds a match on dl_dst==00:00:00:00:00:00 to the Megaflow 
which can never match because the packet received from the L3 tunnel 
does not have a dl_dst field.

Thanks, Jan