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

[Paul Durrant <Paul.Durrant@citrix.com>]

> -----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?

  Paul

>   Paul
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