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Re: [Xen-devel] netfront and promiscuous/multicast packets

To: James Harper <james.harper@xxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] netfront and promiscuous/multicast packets
From: David Edmondson <dme@xxxxxxx>
Date: Mon, 18 Aug 2008 08:48:34 +0100
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On Thu, Aug 14, 2008 at 09:49:08AM +1000, James Harper wrote:
> Okay google tells me that Keir submitted a patch late last year to
> enable netfront to send multicast filter lists to netback via extra_info
> structures on the ring.
> What is the best way to tell if the particular netback I am connected to
> actually supports this? Send a dummy EXTRA_TYPE_MCAST_ADD request and
> see if it succeeds?

Look for the 'feature-multicast-control' flag on the backend node in
xenstore. If it's present and set to '1', you can enable the behaviour
(see below) by setting 'request-multicast-control' to '1' on the
frontend node.

> I'm pretty certain that my Dom0's (Debian based) won't support this so
> the point is moot for now anyway as I can't test it.

This was added to support Solaris, where we have a different model for
dom0 networking.

The behaviour without any flag set is that the guest should get all of
the packets it might rightfully be interested in. That would be:
    - all packets to its' unicast address,
    - all broadcast packets,
    - all multicast packets.
It may of course receive more packets than this set.

If the flag is set, the frontend won't receive all multicast packets,
only those for which is has issued an MCAST_ADD request. It still gets
the unitcast and broadcast packets.

This feature means that we avoid pushing the dom0 NIC into promiscuous
mode on NICs that support multiple unicast addresses (and our drivers
know how to use that support, obviously).

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