[ovs-dev] [PATCH 2/2] [RFC] classifier: Add support for conjunctive matches.

[Ben Pfaff]

On Thu, Oct 30, 2014 at 08:26:15AM -0700, Ben Pfaff wrote:
> On Thu, Oct 30, 2014 at 12:06:30PM +0100, Thomas Graf wrote:
> > On 10/29/14 at 11:46pm, Ben Pfaff wrote:
> > > A "conjunctive match" allows higher-level matches in the flow table, such
> > > as set membership matches, without causing a cross-product explosion for
> > > multidimensional matches.  Please refer to the documentation that this
> > > commit adds to ovs-ofctl(8) for a better explanation, including an example.
> > 
> > High level remark before I dig into the code:
> > 
> >         conjunction(id, k/n)
> > 
> > I like n being the number of dimensions. I wonder whether we should
> > have k start at 1 instead of 0 though. It would read as 1/2 - "1st
> > dimension out of 2", 2/2 - "2nd dimension out of 2".
> 
> I'd be happy with that as the formatted syntax, but due to the use of
> bitmaps for the dimensions it would either cause extra +1/-1 in
> different places, or change the limit from 64 to 63 dimensions (which is
> probably still about 60 more than actually useful).
> 
> I'll think about it for the next version.

I decided to change the formatted representation but not the OpenFlow
binary format; that is, the formatting and parsing code for the
human-readable representation adds or subtracts one from the dimension
numbers.

This change will be in the next version I post.

> > > - Fixing reg0 as the field to use for conjunctive match lookup doesn't seem
> > >   too great.  We could make it configurable per-flow, or we could add a new
> > >   field specifically for this purpose.  I am not sure that it should be
> > >   fixed to 32 bits, either.
> > 
> > I guess we can't go wrong with a separate 64 bits field.
> 
> We're starting to have a lot of fields.  When struct flow exceeds 252
> (63 * 4) bytes, we'll have to change how struct miniflow works because
> we can't make the 'map' member wider than 63 bits.  Still, we do have
> some room.

Jarno also suggested using a dedicated field, so I went with that.  At
the moment it is 32 bits.  That would not be difficult to change, but
it would extend NXAST_CONJUNCTION from 16 to 24 bytes.