[ovs-dev] [PATCH] ovn-controller: Loopback prevention flows for local ports only.

[Ben Pfaff]

On Wed, Mar 30, 2016 at 09:47:27PM -0700, Han Zhou wrote:
> On Wed, Mar 30, 2016 at 4:48 PM, Ben Pfaff <blp at ovn.org> wrote:
> >
> > On Tue, Mar 29, 2016 at 04:55:11PM -0700, Han Zhou wrote:
> > > Currently in physical_run() we added per-port loopback prevention
> > > flows for all lports. The flows are actually required only for
> > > local ports on the chassis. This change greatly reduces number of
> > > flows in table 34.
> > >
> > > Signed-off-by: Han Zhou <zhouhan at gmail.com>
> >
> > We could do this with a single logical flow of the form:
> >
> > match="(inport[0] && !outport[0]) || (!inport[0]) && outport[0]) ||
> >        (inport[1] && !outport[1]) || (!inport[1]) && outport[1]) ||
> >                                   ...
> >        (inport[15] && !outport[15]) || (!inport[15]) && outport[15])",
> > actions="next;"
> >
> > That's a fairly inefficient solution due to the construction of the OVS
> > classifier but we still might consider it at some point.
> >
> > I applied this to master, thanks!
> 
> I was thinking about:
> 
> match="inport == outport", actions="drop"
> 
> And then I realized that this is not possible without big change to OVS
> classifier. But I didn't thought of this smart bit-trick to implement the
> comparison. Thanks for enlightening :)

Tricks like this can implement any comparison, but usually in a really
inefficient manner.

If we add OpenFlow extension actions for arithmetic, this particular
operation could be efficient.  For example, one table could do something
equivalent to "reg0 = inport ^ outport;" or "reg0 = inport - outport;"
and then the next one could have entries like this:

    priority=100, match="reg0 == 0", actions="drop;"
    priority=0,   match="1;",        actions="next;"

However, this kind of thing doesn't generalize very nicely, so I've
refrained from working on it.