Re: [IPoIB] Identify Multicast packets and fix IGMP breakage

[Jason Gunthorpe <jgunthorpe-ePGOBjL8dl3ta4EC/59zMFaTQe2KTcn/@public.gmane.org>]

On Thu, Aug 26, 2010 at 04:17:08PM -0500, Christoph Lameter wrote:
> On Thu, 26 Aug 2010, Roland Dreier wrote:
> 
> > also it's not clear to me why it's OK to do this test of the DGID if the
> > packet didn't have a GRH -- presumably we are just looking at random
> > uninitialized memory so we might incorrectly say some packets are
> > multicast if that byte happens to be 0xff.  (or does that not matter?
> > if so why can't we just always make everything PACKET_MULTICAST?)
>
> We will do an skb_pull to skip over the GRH next? 

The GRH bytes are always present, but the HW may not fill them in if 
IB_WC_GRH is not set.

Checking for the broadcast address should be done by comparing the
MGID against dev->broadcast + 4, like Yossi was describing. This can
be done as a sub-case of the multicast check since dev->broadcast+4
will always start with 0xff.

IPv6 does not have a broadcast address, so the additional work
to check the v6 MGID format Yossi was discussing is not required.

Jason
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