[Bug 296591] Bridge(4) flaps when the first vmnet(4) interface is added

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=296591

--- Comment #4 from Poul-Henning Kamp <[email protected]> ---
Apologies for the delay.

I set up a machine to experiment with this, and this patch does not solve the
problem.

Adding a bunch of printfs throughout the path, it looks like it is
bridge_mutecaps() calling bridge_set_ifcap() that causes the flap.

Bridge(4) says:

     The TOE, TSO, TXCSUM and TXCSUM6 capabilities on all interfaces added to
     the bridge are disabled if any of the interfaces do not support/enable
     them.  The LRO capability is always disabled.  All the capabilities are
     restored when the interface is removed from the bridge.  Changing
     capabilities at run-time may cause NIC reinit and a link flap.

That text clearly did not do it for me.

I think I can see why it was done this way, and I'll leave to others to decide
if that really is the proper behavior, but it sure is neither intuitive nor
communicated clearly.

I think bridge_mutecaps()/bridge_set_ifcap() should printf() why they cause a
flap, something like:

   "bridge60: Flapping to enable/disable member interface option TXCSUM, see
bridge(4)"

so that the admin is made aware of the reason for the flap.

And bridge(4) manpage should be expanded to explain how to either avoid (remove
the options before addm) or work around (add a dummy interface) this issue.

As it is now, it just looks like FreeBSD is flakey.

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