netdev - RE: [PATCH net-next v2] openvswitch: enable NSH support

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Date:   Sun, 13 Aug 2017 21:13:57 +0000
From:   Jan Scheurich <jan.scheurich@...csson.com>
To:     Jiri Benc <jbenc@...hat.com>
CC:     "Yang, Yi Y" <yi.y.yang@...el.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "dev@...nvswitch.org" <dev@...nvswitch.org>
Subject: RE: [PATCH net-next v2] openvswitch: enable NSH support

> From: Jiri Benc [mailto:jbenc@...hat.com]
> Sent: Friday, 11 August, 2017 12:23
> 
> On Fri, 11 Aug 2017 10:09:36 +0000, Jan Scheurich wrote:
> > Unless someone can explain to me why the datapath should understand the
> > internal structure/format of metadata in push_nsh, I would strongly
> > vote to keep the metadata as variable length octet sequence in the
> > non-structured OVS_ACTION_ATTR_PUSH_NSH
> 
> Could be but it still needs to be in a different attribute and not in
> the ovs_action_push_nsh structure.
> 
> Separate attributes for MD1/MD2 has the advantage of easier validation:
> with a separate MD1 type attribute, the size check is easier. With an
> unstructured MD attribute, we'd need to look into the
> OVS_ACTION_ATTR_NSH_BASE_HEADER attribute for mdtype and then validate
> the unstructured MD attribute size manually. Not a big deal, though.
> I don't have strong opinion here.
> 
> But I do have strong opinion that MD needs to go into a separate
> attribute, whether there are separate attributes for MD1/2 or not.

Jiri, I am not too familiar with conventions on the OVS netlink interface regarding the handling of variable length fields. What is the benefit of structuring the push_nsh action into

OVS_ACTION_ATTR_PUSH_NSH
+-- OVS_ACTION_ATTR_NSH_BASE_HEADER
+-- OVS_ACTION_ATTR_NSH_MD

instead of grouping the base header fields and the variable length MD into a single monolithic attribute OVS_ACTION_ATTR_PUSH_NSH? Is the main concern a potential future need to add additional parameters to the push_nsh datapath action? Are there examples for such structured actions other than OVS_ACTION_ATTR_CT where the need is obvious because it is very polymorphic?

BR, Jan

BTW:  The name OVS_ACTION_ATTR_NSH_BASE_HEADER is misleading because in the NSH draft the term base header is used for the first 32-bit word, whereas here it includes also the 32-bit Service Path header.