net: ethtool: fix off-by-one error in max RSS context IDs

Both ethtool_ops.rxfh_max_context_id and the default value used when
 it's not specified are supposed to be exclusive maxima (the former
 is documented as such; the latter, U32_MAX, cannot be used as an ID
 since it equals ETH_RXFH_CONTEXT_ALLOC), but xa_alloc() expects an
 inclusive maximum.
Subtract one from 'limit' to produce an inclusive maximum, and pass
 that to xa_alloc().
Increase bnxt's max by one to prevent a (very minor) regression, as
 BNXT_MAX_ETH_RSS_CTX is an inclusive max.  This is safe since bnxt
 is not actually hard-limited; BNXT_MAX_ETH_RSS_CTX is just a
 leftover from old driver code that managed context IDs itself.
Rename rxfh_max_context_id to rxfh_max_num_contexts to make its
 semantics (hopefully) more obvious.

Fixes: 847a8ab ("net: ethtool: let the core choose RSS context IDs")
Signed-off-by: Edward Cree <ecree.xilinx@gmail.com>
Link: https://patch.msgid.link/5a2d11a599aa5b0cc6141072c01accfb7758650c.1723045898.git.ecree.xilinx@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: Edward Cree

drivers/net/ethernet/broadcom/bnxt/bnxt_ethtool.c

@@ -5290,7 +5290,7 @@ void bnxt_ethtool_free(struct bnxt *bp)
 const struct ethtool_ops bnxt_ethtool_ops = {
 	.cap_link_lanes_supported	= 1,
 	.cap_rss_ctx_supported		= 1,
-	.rxfh_max_context_id		= BNXT_MAX_ETH_RSS_CTX,
+	.rxfh_max_num_contexts		= BNXT_MAX_ETH_RSS_CTX + 1,
 	.rxfh_indir_space		= BNXT_MAX_RSS_TABLE_ENTRIES_P5,
 	.rxfh_priv_size			= sizeof(struct bnxt_rss_ctx),
 	.supported_coalesce_params = ETHTOOL_COALESCE_USECS |

include/linux/ethtool.h

@@ -736,10 +736,10 @@ struct kernel_ethtool_ts_info {
  * @rxfh_key_space: same as @rxfh_indir_space, but for the key.
  * @rxfh_priv_size: size of the driver private data area the core should
  *	allocate for an RSS context (in &struct ethtool_rxfh_context).
- * @rxfh_max_context_id: maximum (exclusive) supported RSS context ID.  If this
- *	is zero then the core may choose any (nonzero) ID, otherwise the core
- *	will only use IDs strictly less than this value, as the @rss_context
- *	argument to @create_rxfh_context and friends.
+ * @rxfh_max_num_contexts: maximum (exclusive) supported RSS context ID.
+ *	If this is zero then the core may choose any (nonzero) ID, otherwise
+ *	the core will only use IDs strictly less than this value, as the
+ *	@rss_context argument to @create_rxfh_context and friends.
  * @supported_coalesce_params: supported types of interrupt coalescing.
  * @supported_ring_params: supported ring params.
  * @get_drvinfo: Report driver/device information. Modern drivers no
@@ -954,7 +954,7 @@ struct ethtool_ops {
 	u32	rxfh_indir_space;
 	u16	rxfh_key_space;
 	u16	rxfh_priv_size;
-	u32	rxfh_max_context_id;
+	u32	rxfh_max_num_contexts;
 	u32	supported_coalesce_params;
 	u32	supported_ring_params;
 	void	(*get_drvinfo)(struct net_device *, struct ethtool_drvinfo *);

net/ethtool/ioctl.c

@@ -1449,12 +1449,13 @@ static noinline_for_stack int ethtool_set_rxfh(struct net_device *dev,
 		}
 
 		if (ops->create_rxfh_context) {
-			u32 limit = ops->rxfh_max_context_id ?: U32_MAX;
+			u32 limit = ops->rxfh_max_num_contexts ?: U32_MAX;
 			u32 ctx_id;
 
 			/* driver uses new API, core allocates ID */
 			ret = xa_alloc(&dev->ethtool->rss_ctx, &ctx_id, ctx,
-				       XA_LIMIT(1, limit), GFP_KERNEL_ACCOUNT);
+				       XA_LIMIT(1, limit - 1),
+				       GFP_KERNEL_ACCOUNT);
 			if (ret < 0) {
 				kfree(ctx);
 				goto out;