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

Jiri Benc jbenc at redhat.com
Fri Jun 1 16:38:53 UTC 2018

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.

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.

Please go read what Eric described about reading the value back.

As for the patch,

Nacked-by: Jiri Benc <jbenc at redhat.com>


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