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

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.

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.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Jan Beulich <jbeulich@suse.com>
Cc: Keir Fraser <keir@xen.org>
Cc: Tim Deegan <tim@xen.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>

xen/include/public/io/netif.h

@@ -136,18 +136,28 @@
  */
 
 /*
- * "feature-multicast-control" advertises the capability to filter ethernet
- * multicast packets in the backend. To enable use of this capability the
- * frontend must set "request-multicast-control" before moving into the
- * connected state.
+ * "feature-multicast-control" and "feature-dynamic-multicast-control"
+ * advertise the capability to filter ethernet multicast packets in the
+ * backend. If the frontend wishes to take advantage of this feature then
+ * it may set "request-multicast-control". If the backend only advertises
+ * "feature-multicast-control" then "request-multicast-control" must be set
+ * before the frontend moves into the connected state. The backend will
+ * sample the value on this state transition and any subsequent change in
+ * value will have no effect. However, if the backend also advertises
+ * "feature-dynamic-multicast-control" then "request-multicast-control"
+ * may be set by the frontend at any time. In this case, the backend will
+ * watch the value and re-sample on watch events.
  *
- * If "request-multicast-control" is set then the backend transmit side should
- * no longer flood multicast packets to the frontend, it should instead drop any
- * multicast packet that does not match in a filter list. The list is
- * amended by the frontend by sending dummy transmit requests containing
- * XEN_NETIF_EXTRA_TYPE_MCAST_{ADD,DEL} extra-info fragments as specified below.
- * Once enabled by the frontend, the feature cannot be disabled except by
- * closing and re-connecting to the backend.
+ * If the sampled value of "request-multicast-control" is set then the
+ * backend transmit side should no longer flood multicast packets to the
+ * frontend, it should instead drop any multicast packet that does not
+ * match in a filter list.
+ * The list is amended by the frontend by sending dummy transmit requests
+ * containing XEN_NETIF_EXTRA_TYPE_MCAST_{ADD,DEL} extra-info fragments as
+ * specified below.
+ * Note that the filter list may be amended even if the sampled value of
+ * "request-multicast-control" is not set, however the filter should only
+ * be applied if it is set.
  */
 
 /*