Re: [PATCH] public/io/netif.h: change semantics of "request-multicast-control" flag

[Ian Campbell <ian.campbell@citrix.com>]

On Thu, 2016-01-21 at 11:48 +0000, Paul Durrant wrote:
> > -----Original Message-----
> > From: xen-devel-bounces@lists.xen.org [mailto:xen-devel-
> > bounces@lists.xen.org] On Behalf Of Paul Durrant
> > Sent: 20 January 2016 13:14
> > To: Ian Campbell; xen-devel@lists.xenproject.org
> > Cc: Ian Jackson; Keir (Xen.org); Jan Beulich; Tim (Xen.org)
> > Subject: Re: [Xen-devel] [PATCH] public/io/netif.h: change semantics of
> > "request-multicast-control" flag
> > 
> > > -----Original Message-----
> > > From: Ian Campbell [mailto:ian.campbell@citrix.com]
> > > Sent: 20 January 2016 13:06
> > > To: Paul Durrant; xen-devel@lists.xenproject.org
> > > Cc: Ian Jackson; Jan Beulich; Keir (Xen.org); Tim (Xen.org)
> > > Subject: Re: [PATCH] public/io/netif.h: change semantics of "request-
> > > multicast-control" flag
> > > 
> > > On Wed, 2016-01-20 at 12:50 +0000, Paul Durrant wrote:
> > > > My patch b2700877 "move and amend multicast control documentation"
> > > > clarified use of the multicast control protocol between frontend
> > > > and
> > > > backend. However, it transpires that the restrictions that
> > > > documentation
> > > > placed on the "request-multicast-control" flag make it hard for a
> > > > frontend to enable 'all multicast' promiscuous mode, in that to do
> > > > so
> > > > would require the frontend and backend to disconnect and re-
> > > > connect.
> > > 
> > > Do we therefore think that this document reflected reality, i.e.
> > > might this
> > > not be "just" a documentation bug?
> > > 
> > > (Or maybe we can't tell because the only previous implementation was
> > years
> > > ago in Solaris or something)
> > 
> > That's my concern. I hope it's just a documentation bug, but I don't
> > know.
> > Also I've already done an implementation in Linux netback according to
> > the
> > restricted semantics.
> > 
> > > 
> > > > This patch adds a new "feature-dynamic-multicast-control" flag to
> > > > allow
> > > > a backend to advertise that it will watch "request-multicast-
> > > > control"
> > hence
> > > > allowing it to be meaningfully modified by the frontend at any time
> > > > rather
> > > > than only when the frontend and backend are disconnected.
> > > 
> > > Would allowing XEN_NETIF_EXTRA_TYPE_MCAST_{ADD,DEL} to take a
> > bcast
> > > address
> > > be easier on the backend, in that it would just need to be a static feature
> > > rather than watching stuff on the fly?
> > 
> > The documented semantics of the list are 'exact match' so sending a bcast
> > address doesn't do much good with a backend that doesn't know to treat is
> > specially hence a frontend can't tell whether 'all multicast' mode is going to
> > work without the extra feature flag. As for watching "request-multcast-
> > control" vs. add/remove of bcast, the complexity of implementation is
> > cheaper for the latter but I think the former is 'nicer'.
> > 
> 
> Are you ok with the xenstore watch approach (and leaving the patch as is)
> or would you prefer to spec. the bcast address as a wildcard and submit a
> new patch?

I'm fine with the watch approach, was just suggesting the alternative in
case it turned out to be much easier.

Ian.

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