netdev - Re: [PATCH v2.39 7/7] datapath: Add basic MPLS support to kernel

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Message-ID: <CAEP_g=_Eds_XvPLUOhAAMZbxkzmBJEtSR9vVG2kkuRLehrfbDg@mail.gmail.com>
Date:	Thu, 19 Sep 2013 12:21:33 -0500
From:	Jesse Gross <jesse@...ira.com>
To:	Simon Horman <horms@...ge.net.au>
Cc:	Ben Pfaff <blp@...ira.com>, Pravin Shelar <pshelar@...ira.com>,
	"dev@...nvswitch.org" <dev@...nvswitch.org>,
	netdev <netdev@...r.kernel.org>, Ravi K <rkerur@...il.com>,
	Isaku Yamahata <yamahata@...inux.co.jp>,
	Joe Stringer <joe@...d.net.nz>
Subject: Re: [PATCH v2.39 7/7] datapath: Add basic MPLS support to kernel

On Thu, Sep 19, 2013 at 10:57 AM, Simon Horman <horms@...ge.net.au> wrote:
> On Mon, Sep 16, 2013 at 03:38:21PM -0500, Jesse Gross wrote:
>> On Mon, Sep 9, 2013 at 12:20 AM, Simon Horman <horms@...ge.net.au> wrote:
>> > @@ -616,6 +736,13 @@ int ovs_execute_actions(struct datapath *dp, struct sk_buff *skb)
>> >                 goto out_loop;
>> >         }
>> >
>> > +       /* Needed to initialise inner protocol on kernels older
>> > +        * than v3.11 where skb->inner_protocol is not present
>> > +        * and compatibility code uses the OVS_CB(skb) to store
>> > +        * the inner protocol.
>> > +        */
>> > +       ovs_skb_set_inner_protocol(skb, skb->protocol);
>>
>> The comment makes it sound like this code should just be deleted when
>> upstreaming. However, I believe that we still need to initialize this
>> field, right? Is this the best place do it or should it be conditional
>> on adding a first MPLS header? (i.e. what happens if inner_protocol is
>> already set and the packet simply passes through OVS?)
>
> I believe there are several problems here.
>
> The first one, which my comment was written around is that I think that if
> inner_protocol is a field of struct sk_buff then we can rely on it already
> being initialised.  However, if we are using compatibility code, where
> inner_protcol is called in the callback field of struct sk_buff then I
> think that OVS needs to initialise it.

I'm not sure that it's true that inner_protocol is already initialized
- I grepped the tree and the only assignment that I found is in
skbuff.c in __copy_skb_header().

> A second problem is one that you raise which I had not considered
> which is how to handle things if inner_protocol is already set.
>
> I believe this should only occur in the case where inner_protocol
> is a field of struct sk_buff and I think it would be most convenient
> to set it conditionally in ovs_skb_reset_inner_protocol().
> I think that if it is not set it should be zero but it should be
> safe to check for values less than ETH_P_802_3_MIN.

It's probably OK to check for values less than ETH_P_802_3_MIN but I'm
not sure that it's the most correct thing to do since skb->protocol
could contain these values (such as ETH_P_802_2). It's unlikely that
they will be GSO packets but it seems better to use the more strict
check against zero.

One other consideration in the OVS case - with recirculation we may
hit this code multiple times and the difference in behavior could be
surprising. However, on the other hand, we need to be careful because
skb->cb is not guaranteed to be initialized to zero.
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