On Tue, Jun 02, 2015 at 08:25:14AM -0700, Jesse Gross wrote: > On Tue, Jun 2, 2015 at 8:03 AM, Ben Pfaff <b...@nicira.com> wrote: > > On Tue, Jun 02, 2015 at 07:58:48AM -0700, Ben Pfaff wrote: > >> On Mon, Jun 01, 2015 at 03:35:23PM -0700, Jesse Gross wrote: > >> > On Mon, Jun 1, 2015 at 2:55 PM, Ben Pfaff <b...@nicira.com> wrote: > >> > > On Mon, Jun 01, 2015 at 01:49:30PM -0700, Jesse Gross wrote: > >> > >> We have a special flow_metadata structure to represent the parts > >> > >> of a packet that aren't carried in the payload itself. This is > >> > >> used in the case where we need to send the packet as a Packet In > >> > >> to an OpenFlow controller. This is a subset of the more general > >> > >> struct flow. > >> > >> > >> > >> In practice, almost all operations we do on this structure involve > >> > >> converting it to or from a match or have code that is the same as > >> > >> a match. Serialization to NXM and back is done as a match. There > >> > >> is special flow_metadata formatting code that is almost identical > >> > >> to match formatting. > >> > >> > >> > >> The uses for struct flow_metadata aren't performance critical > >> > >> when it comes to memory, so we can save quite a bit of code by > >> > >> just using a match structure directly instead. In addition, as > >> > >> metadata increases and becomes more complex (Geneve options require > >> > >> some special handling beyond just additional fields), using the > >> > >> match structure means we only have to do this work in one place. > >> > >> > >> > >> Signed-off-by: Jesse Gross <je...@nicira.com> > >> > > > >> > > Most of this rationale makes sense to me, except that I'm surprised > >> > > that struct flow_metadata (which doesn't have masks) gets converted to > >> > > struct match (which does). Maybe I should just dig into the code, but > >> > > is there some big reason why this is the case? > >> > > >> > The main reason is just that struct match is the more general superset > >> > and the encode/decoding/formatting code operates on that to be able to > >> > handle everything so it's cleaner to just use a match even if it is > >> > slightly less memory efficient. > >> > >> It just seems conceptually wrong to me to have masks around in cases > >> where there's no concept of one, where the masks just take up space and > >> get ignored. I'm trying to figure out how much that bothers me. > > > > So, I guess, the summary of my thoughts about this right now is that for > > me it's a no-brainer to switch from a specialized structure to struct > > flow, but struct match is a little harder for me conceptually. > > I guess I should have been more clear: we actually are using the > wildcards to indicate which fields are significant. Effectively the > purpose of the conversion code between struct flow_metadata and struct > match is to synthesize these wildcards. There would be little benefit > to switching to struct flow instead because we would still need to do > the same operation.
OK, then I need to read the patch more closely, thanks for the clarification. _______________________________________________ dev mailing list dev@openvswitch.org http://openvswitch.org/mailman/listinfo/dev
