[ovs-dev] [PATCH] dpif: Ensure ERSPAN GRE support

Gregory Rose gvrose8192 at gmail.com
Fri Jun 1 16:56:54 UTC 2018

On 6/1/2018 9:38 AM, Jiri Benc wrote:
> On Fri, 1 Jun 2018 09:15:33 -0700, Gregory Rose wrote:
>> Since ERSPAN over gre/ip_gre was added to the Linux 4.16 kernel the
>> compat interface is needed
>> for kernels up to 4.15 so that we can support ERSPAN.  If the built-in
>> gre/ip_gre kernel modules
>> don't have the ERSPAN support in them then we have to use the compat
>> interface.
> That's very wrong. The compat interface should not be used with
> upstream kernel (except perhaps for very very very old kernels). We
> converted the API to the standard rtnetlink for good reasons. New
> features are not supported using the compat API. You are potentially
> breaking future distribution kernels by reverting to an obsolete and
> deprecated API.


I think there must be some confusion here.  The compat interface should 
be used to support
features that are not in the kernel. ERSPAN in this case.  ERSPAN is 
introduced in 4.16 (let's
just agree to ignore 4.15 since it's not LTS).

So in kernels up to and including 4.14 we must use the compat interface 
to get ERSPAN support

If there is another way to do it I'm all ears.

> You'll have to find a different way to do what you need. Eric described
> pretty nicely a way to achieve that and how the fallbacks work, please
> re-read his emails and modify the code accordingly.
>> The target for USE_UPSTREAM_TUNNEL is moved to 4.16 now.  That's when
>> ERSPAN becomes
>> fully supported.  Going forward the ERSPAN feature is the determinant
>> for whether gr/ip_gre
>> compat mode is used or not.
> And with the next added feature to the kernel, that next feature will be
> what determines whether the compat mode will be used? And then next and
> so on? This doesn't work. ERSPAN must not be the decision factor.
> Instead, rtnetlink must be tried first and if and only if it fails,
> compat mode can be used.

So how do we support ERSPAN in kernels that don't have it?  I thought 
that's what the
compat layer code is there for.


- Greg

> Please go read what Eric described about reading the value back.
> As for the patch,
> Nacked-by: Jiri Benc <jbenc at redhat.com>
>   Jiri

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