[ovs-dev] [PATCH] [RFC] flow: Do not clear L3+ fields of flow in flow_push_mpls()

Jarno Rajahalme jrajahalme at nicira.com
Thu May 1 15:21:36 UTC 2014


> On May 1, 2014, at 7:53 AM, Ben Pfaff <blp at nicira.com> wrote:
> 
>> On Mon, Apr 28, 2014 at 03:57:58PM -0700, Jarno Rajahalme wrote:
>> 
>>> On Mar 20, 2014, at 10:05 AM, Ben Pfaff <blp at nicira.com> wrote:
>>> 
>>>> On Fri, Mar 14, 2014 at 04:19:52PM +0900, Simon Horman wrote:
>>>> When creating a flow in the datapath as the result of an upcall
>>>> the match itself is the match supplied in the upcall while
>>>> the mask of the match, if supplied, is generated based on the
>>>> flow and mask composed during action translation.
>>>> 
>>>> In the case of, for example a UDP packet, the match will include
>>>> of L2, L3 and L4 fields. However, if the flow is cleared in
>>>> flow_push_mpls() then the mask that is synthesised from it will
>>>> not include L3 and L4 fields. This seems incorrect and the kernel
>>>> datapath complains about this mismatch.
>>>> 
>>>> Signed-off-by: Simon Horman <horms at verge.net.au>
>>> 
>>> The goal of clearing the fields is to ensure that later flow tables
>>> can't match on fields that aren't visible anymore.  That's important for
>>> accurate OpenFlow implementation, so I'd rather not change it.  On the
>>> other hand, I see the point you're making, but I don't immediately
>>> understand why it happens that way.  After all, I can change fields with
>>> OpenFlow actions and the datapath flows work out OK, why doesn't this
>>> work out OK too?  Do you understand the reason?
>> 
>> As the flow?s dl_type is changed to an MPLS type, later non-MPLS rules
>> will not match on the modified flow. AFAIK, you can match on L3/L4
>> fields only by also matching on the corresponding dl_type as a
>> prerequisite, no?
> 
> Yes, that's true.
> 
>> If this holds, I?d rather not clear the fields so we can properly do a
>> set IPv4 action followed by an MPLS push action. Currently the the
>> MPLS action clears the flow values at the translation time set in the
>> preceding action, so that at the commit time the values intended for
>> set IPv4 action are lost.
> 
> Are you sure?  compose_mpls_push_action() call commit_odp_actions() to
> avoid this very problem.
> 

I did not notice this, sorry about that.

> Assuming I'm right about that, at this point what I really want is an
> example of a situation that's broken in the current code and not broken
> with this patch applied, so that I can understand exactly what we're
> getting at here.
> 

It seems that a check on the nw_proto before committing set ports actions is enough to avoid introducing new problems as part of my masked set actions patch series.

  Jarno

> Thanks,
> 
> Ben.



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