net: sched: use block index as a handle instead of qdisc when block is shared

As the tcm_ifindex with value TCM_IFINDEX_MAGIC_BLOCK is invalid ifindex,
use it to indicate that we work with block, instead of qdisc.
So if tcm_ifindex is set to TCM_IFINDEX_MAGIC_BLOCK, tcm_parent is used
to carry block_index.

If the block is set to be shared between at least 2 qdiscs, it is
forbidden to use the qdisc handle to add/delete filters. In that case,
userspace has to pass block_index.

Also, for dump of the filters, in case the block is shared in between at
least 2 qdiscs, the each filter is dumped with tcm_ifindex value
TCM_IFINDEX_MAGIC_BLOCK and tcm_parent set to block_index. That gives
the user clear indication, that the filter belongs to a shared block
and not only to one qdisc under which it is dumped.
Suggested-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Acked-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/rtnetlink.h

@@ -541,9 +541,19 @@ struct tcmsg {
 	int		tcm_ifindex;
 	__u32		tcm_handle;
 	__u32		tcm_parent;
+/* tcm_block_index is used instead of tcm_parent
+ * in case tcm_ifindex == TCM_IFINDEX_MAGIC_BLOCK
+ */
+#define tcm_block_index tcm_parent
 	__u32		tcm_info;
 };
 
+/* For manipulation of filters in shared block, tcm_ifindex is set to
+ * TCM_IFINDEX_MAGIC_BLOCK, and tcm_parent is aliased to tcm_block_index
+ * which is the block index.
+ */
+#define TCM_IFINDEX_MAGIC_BLOCK (0xFFFFFFFFU)
+
 enum {
 	TCA_UNSPEC,
 	TCA_KIND,