net: ethtool: only allow set_rxnfc with rss + ring_cookie if driver opts in

Ethtool ntuple filters with FLOW_RSS were originally defined as adding
the base queue ID (ring_cookie) to the value from the indirection table,
so that the same table could distribute over more than one set of queues
when used by different filters.
However, some drivers / hardware ignore the ring_cookie, and simply use
the indirection table entries as queue IDs directly. Thus, for drivers
which have not opted in by setting ethtool_ops.cap_rss_rxnfc_adds to
declare that they support the original (addition) semantics, reject in
ethtool_set_rxnfc any filter which combines FLOW_RSS and a nonzero ring.
(For a ring_cookie of zero, both behaviours are equivalent.)
Set the cap bit in sfc, as it is known to support this feature.

Signed-off-by: Edward Cree <[e-mail]>
Reviewed-by: Martin Habets <[e-mail]>
Link: https://patch.msgid.link/cc3d[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

drivers/net/ethernet/sfc/ef100_ethtool.c

···
 	.get_rxfh_indir_size	= efx_ethtool_get_rxfh_indir_size,
 	.get_rxfh_key_size	= efx_ethtool_get_rxfh_key_size,
 	.rxfh_per_ctx_key	= true,
+	.cap_rss_rxnfc_adds	= true,
 	.rxfh_priv_size		= sizeof(struct efx_rss_context_priv),
 	.get_rxfh		= efx_ethtool_get_rxfh,
 	.set_rxfh		= efx_ethtool_set_rxfh,

drivers/net/ethernet/sfc/ethtool.c

···
 	.get_rxfh_indir_size	= efx_ethtool_get_rxfh_indir_size,
 	.get_rxfh_key_size	= efx_ethtool_get_rxfh_key_size,
 	.rxfh_per_ctx_key	= true,
+	.cap_rss_rxnfc_adds	= true,
 	.rxfh_priv_size		= sizeof(struct efx_rss_context_priv),
 	.get_rxfh		= efx_ethtool_get_rxfh,
 	.set_rxfh		= efx_ethtool_set_rxfh,

include/linux/ethtool.h

···
  * @rxfh_per_ctx_key: device supports setting different RSS key for each
  *	additional context. Netlink API should report hfunc, key, and input_xfrm
  *	for every context, not just context 0.
+ * @cap_rss_rxnfc_adds: device supports nonzero ring_cookie in filters with
+ *	%FLOW_RSS flag; the queue ID from the filter is added to the value from
+ *	the indirection table to determine the delivery queue.
  * @rxfh_indir_space: max size of RSS indirection tables, if indirection table
  *	size as returned by @get_rxfh_indir_size may change during lifetime
  *	of the device. Leave as 0 if the table size is constant.
···
 	u32     cap_rss_ctx_supported:1;
 	u32	cap_rss_sym_xor_supported:1;
 	u32	rxfh_per_ctx_key:1;
+	u32	cap_rss_rxnfc_adds:1;
 	u32	rxfh_indir_space;
 	u16	rxfh_key_space;
 	u16	rxfh_priv_size;

net/ethtool/ioctl.c

···
 	if (rc)
 		return rc;
 
+	/* Nonzero ring with RSS only makes sense if NIC adds them together */
+	if (info.flow_type & FLOW_RSS && !ops->cap_rss_rxnfc_adds &&
+	    ethtool_get_flow_spec_ring(info.fs.ring_cookie))
+		return -EINVAL;
+
 	if (ops->get_rxfh) {
 		struct ethtool_rxfh_param rxfh = {};