Multicast join now works.

src/drivers/net/mlx_ipoib/arbel.h

 #define ARBEL_HCR_INIT2RTR_QPEE		0x001a
 #define ARBEL_HCR_RTR2RTS_QPEE		0x001b
 #define ARBEL_HCR_2RST_QPEE		0x0021
+#define ARBEL_HCR_READ_MGM		0x0025
+#define ARBEL_HCR_WRITE_MGM		0x0026
+#define ARBEL_HCR_MGID_HASH		0x0027
 
 /* Service types */
 #define ARBEL_ST_UD			0x03
 
 #define ARBEL_INVALID_LKEY		0x00000100UL
 
+/*
+ * Datatypes that seem to be missing from the autogenerated documentation
+ *
+ */
+struct arbelprm_mgm_hash_st {
+	pseudo_bit_t reserved0[0x00020];
+/* -------------- */
+	pseudo_bit_t hash[0x00010];
+	pseudo_bit_t reserved1[0x00010];
+};
+
 /*
  * Wrapper structures for hardware datatypes
  *
 struct MLX_DECLARE_STRUCT ( arbelprm_cq_arm_db_record );
 struct MLX_DECLARE_STRUCT ( arbelprm_cq_ci_db_record );
 struct MLX_DECLARE_STRUCT ( arbelprm_hca_command_register );
+struct MLX_DECLARE_STRUCT ( arbelprm_mgm_entry );
+struct MLX_DECLARE_STRUCT ( arbelprm_mgm_hash );
 struct MLX_DECLARE_STRUCT ( arbelprm_qp_db_record );
 struct MLX_DECLARE_STRUCT ( arbelprm_qp_ee_state_transitions );
 struct MLX_DECLARE_STRUCT ( arbelprm_query_dev_lim );
 #define ARBEL_HCR_OUT_LEN( _command )	( ( (_command) >> 21 ) & 0x7fc )
 
 /** Build HCR command from component parts */
-#define ARBEL_HCR_CMD( _opcode, _in_mbox, _in_len, _out_mbox, _out_len )     \
+#define ARBEL_HCR_INOUT_CMD( _opcode, _in_mbox, _in_len,		     \
+			     _out_mbox, _out_len )			     \
 	( (_opcode) |							     \
 	  ( (_in_mbox) ? ARBEL_HCR_IN_MBOX : 0 ) |			     \
 	  ( ( (_in_len) / 4 ) << 14 ) |					     \
 	  ( ( (_out_len) / 4 ) << 23 ) )
 
 #define ARBEL_HCR_IN_CMD( _opcode, _in_mbox, _in_len )			     \
-	ARBEL_HCR_CMD ( _opcode, _in_mbox, _in_len, 0, 0 )
+	ARBEL_HCR_INOUT_CMD ( _opcode, _in_mbox, _in_len, 0, 0 )
 
 #define ARBEL_HCR_OUT_CMD( _opcode, _out_mbox, _out_len )		     \
-	ARBEL_HCR_CMD ( _opcode, 0, 0, _out_mbox, _out_len )
+	ARBEL_HCR_INOUT_CMD ( _opcode, 0, 0, _out_mbox, _out_len )
 
 #define ARBEL_HCR_VOID_CMD( _opcode )					     \
-	ARBEL_HCR_CMD ( _opcode, 0, 0, 0, 0 )
+	ARBEL_HCR_INOUT_CMD ( _opcode, 0, 0, 0, 0 )
 
 /*
  * Doorbell record allocation

src/drivers/net/mlx_ipoib/mt25218.c

 			   0x03, NULL, qpn, NULL );
 }
 
+static inline int
+arbel_cmd_read_mgm ( struct arbel *arbel, unsigned int index,
+		     struct arbelprm_mgm_entry *mgm ) {
+	return arbel_cmd ( arbel,
+			   ARBEL_HCR_OUT_CMD ( ARBEL_HCR_READ_MGM,
+					       1, sizeof ( *mgm ) ),
+			   0, NULL, index, mgm );
+}
+
+static inline int
+arbel_cmd_write_mgm ( struct arbel *arbel, unsigned int index,
+		      const struct arbelprm_mgm_entry *mgm ) {
+	return arbel_cmd ( arbel,
+			   ARBEL_HCR_IN_CMD ( ARBEL_HCR_WRITE_MGM,
+					      1, sizeof ( *mgm ) ),
+			   0, mgm, index, NULL );
+}
+
+static inline int
+arbel_cmd_mgid_hash ( struct arbel *arbel, const struct ib_gid *gid,
+		      struct arbelprm_mgm_hash *hash ) {
+	return arbel_cmd ( arbel,
+			   ARBEL_HCR_INOUT_CMD ( ARBEL_HCR_MGID_HASH,
+						 1, sizeof ( *gid ),
+						 0, sizeof ( *hash ) ),
+			   0, gid, 0, hash );
+}
+
 /***************************************************************************
  *
  * Completion queue operations
 	}
 }
 
+/***************************************************************************
+ *
+ * Multicast group operations
+ *
+ ***************************************************************************
+ */
+
+/**
+ * Attach to multicast group
+ *
+ * @v ibdev		Infiniband device
+ * @v qp		Queue pair
+ * @v gid		Multicast GID
+ * @ret rc		Return status code
+ */
+static int arbel_mcast_attach ( struct ib_device *ibdev,
+				struct ib_queue_pair *qp,
+				struct ib_gid *gid ) {
+	struct arbel *arbel = ibdev->dev_priv;
+	struct arbelprm_mgm_hash hash;
+	struct arbelprm_mgm_entry mgm;
+	unsigned int index;
+	int rc;
+
+	/* Generate hash table index */
+	if ( ( rc = arbel_cmd_mgid_hash ( arbel, gid, &hash ) ) != 0 ) {
+		DBGC ( arbel, "Arbel %p could not hash GID: %s\n",
+		       arbel, strerror ( rc ) );
+		return rc;
+	}
+	index = MLX_GET ( &hash, hash );
+
+	/* Check for existing hash table entry */
+	if ( ( rc = arbel_cmd_read_mgm ( arbel, index, &mgm ) ) != 0 ) {
+		DBGC ( arbel, "Arbel %p could not read MGM %#x: %s\n",
+		       arbel, index, strerror ( rc ) );
+		return rc;
+	}
+	if ( MLX_GET ( &mgm, mgmqp_0.qi ) != 0 ) {
+		/* FIXME: this implementation allows only a single QP
+		 * per multicast group, and doesn't handle hash
+		 * collisions.  Sufficient for IPoIB but may need to
+		 * be extended in future.
+		 */
+		DBGC ( arbel, "Arbel %p MGID index %#x already in use\n",
+		       arbel, index );
+		return -EBUSY;
+	}
+
+	/* Update hash table entry */
+	MLX_FILL_2 ( &mgm, 8,
+		     mgmqp_0.qpn_i, qp->qpn,
+		     mgmqp_0.qi, 1 );
+	memcpy ( &mgm.u.dwords[4], gid, sizeof ( *gid ) );
+	if ( ( rc = arbel_cmd_write_mgm ( arbel, index, &mgm ) ) != 0 ) {
+		DBGC ( arbel, "Arbel %p could not write MGM %#x: %s\n",
+		       arbel, index, strerror ( rc ) );
+		return rc;
+	}
+
+	return 0;
+}
+
+/**
+ * Detach from multicast group
+ *
+ * @v ibdev		Infiniband device
+ * @v qp		Queue pair
+ * @v gid		Multicast GID
+ */
+static void arbel_mcast_detach ( struct ib_device *ibdev,
+				 struct ib_queue_pair *qp __unused,
+				 struct ib_gid *gid ) {
+	struct arbel *arbel = ibdev->dev_priv;
+	struct arbelprm_mgm_hash hash;
+	struct arbelprm_mgm_entry mgm;
+	unsigned int index;
+	int rc;
+
+	/* Generate hash table index */
+	if ( ( rc = arbel_cmd_mgid_hash ( arbel, gid, &hash ) ) != 0 ) {
+		DBGC ( arbel, "Arbel %p could not hash GID: %s\n",
+		       arbel, strerror ( rc ) );
+		return;
+	}
+	index = MLX_GET ( &hash, hash );
+
+	/* Clear hash table entry */
+	memset ( &mgm, 0, sizeof ( mgm ) );
+	if ( ( rc = arbel_cmd_write_mgm ( arbel, index, &mgm ) ) != 0 ) {
+		DBGC ( arbel, "Arbel %p could not write MGM %#x: %s\n",
+		       arbel, index, strerror ( rc ) );
+		return;
+	}
+}
+
+
+
 /** Arbel Infiniband operations */
 static struct ib_device_operations arbel_ib_operations = {
 	.create_cq	= arbel_create_cq,
 	.post_send	= arbel_post_send,
 	.post_recv	= arbel_post_recv,
 	.poll_cq	= arbel_poll_cq,
+	.mcast_attach	= arbel_mcast_attach,
+	.mcast_detach	= arbel_mcast_detach,
 };
 
 /**
 		return -EIO;
 	}
 	mlx->own_qp->owner_priv = netdev;
+	struct ib_gid *bcast_gid = ( struct ib_gid * ) &ib_data.bcast_gid;
+	if ( ( rc = ib_mcast_attach ( ibdev, mlx->own_qp,
+				      bcast_gid ) ) != 0 ) {
+		DBG ( "Could not attach to broadcast GID: %s\n",
+		      strerror ( rc ) );
+		return rc;
+	}
+				      
 
 	mac = ( ( struct ib_mac * ) netdev->ll_addr );
 	mac->qpn = htonl ( mlx->own_qp->qpn );

src/include/gpxe/infiniband.h

 			     struct ib_completion_queue *cq,
 			     ib_completer_t complete_send,
 			     ib_completer_t complete_recv );
+	/**
+	 * Attach to multicast group
+	 *
+	 * @v ibdev		Infiniband device
+	 * @v qp		Queue pair
+	 * @v gid		Multicast GID
+	 * @ret rc		Return status code
+	 */
+	int ( * mcast_attach ) ( struct ib_device *ibdev,
+				 struct ib_queue_pair *qp,
+				 struct ib_gid *gid );
+	/**
+	 * Detach from multicast group
+	 *
+	 * @v ibdev		Infiniband device
+	 * @v qp		Queue pair
+	 * @v gid		Multicast GID
+	 */
+	void ( * mcast_detach ) ( struct ib_device *ibdev,
+				  struct ib_queue_pair *qp,
+				  struct ib_gid *gid );
 };
 
 /** An Infiniband device */
 extern struct ib_work_queue * ib_find_wq ( struct ib_completion_queue *cq,
 					   unsigned long qpn, int is_send );
 
+/**
+ * Attach to multicast group
+ *
+ * @v ibdev		Infiniband device
+ * @v qp		Queue pair
+ * @v gid		Multicast GID
+ * @ret rc		Return status code
+ */
+static inline __attribute__ (( always_inline )) int
+ib_mcast_attach ( struct ib_device *ibdev, struct ib_queue_pair *qp,
+		  struct ib_gid *gid ) {
+	return ibdev->op->mcast_attach ( ibdev, qp, gid );
+}
+
+/**
+ * Detach from multicast group
+ *
+ * @v ibdev		Infiniband device
+ * @v qp		Queue pair
+ * @v gid		Multicast GID
+ */
+static inline __attribute__ (( always_inline )) void
+ib_mcast_detach ( struct ib_device *ibdev, struct ib_queue_pair *qp,
+		  struct ib_gid *gid ) {
+	ibdev->op->mcast_detach ( ibdev, qp, gid );
+}
 
 
 extern struct ll_protocol infiniband_protocol;