ethtool: rss: support skipping contexts during dump

Applications may want to deal with dynamic RSS contexts only.
So dumping context 0 will be counter-productive for them.
Support starting the dump from a given context ID.

Alternative would be to implement a dump flag to skip just
context 0, not sure which is better...
Reviewed-by: Edward Cree <ecree.xilinx@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/netlink/specs/ethtool.yaml

@@ -1028,6 +1028,9 @@ attribute-sets:
       -
         name: input_xfrm
         type: u32
+      -
+        name: start-context
+        type: u32
   -
     name: plca
     attributes:
@@ -1766,6 +1769,7 @@ operations:
         request:
           attributes:
             - header
+            - start-context
         reply: *rss-reply
     -
       name: plca-get-cfg

Documentation/networking/ethtool-netlink.rst

@@ -1866,10 +1866,18 @@ RSS context of an interface similar to ``ETHTOOL_GRSSH`` ioctl request.
 
 Request contents:
 
-=====================================  ======  ==========================
+=====================================  ======  ============================
   ``ETHTOOL_A_RSS_HEADER``             nested  request header
   ``ETHTOOL_A_RSS_CONTEXT``            u32     context number
-=====================================  ======  ==========================
+  ``ETHTOOL_A_RSS_START_CONTEXT``      u32     start context number (dumps)
+=====================================  ======  ============================
+
+``ETHTOOL_A_RSS_CONTEXT`` specifies which RSS context number to query,
+if not set context 0 (the main context) is queried. Dumps can be filtered
+by device (only listing contexts of a given netdev). Filtering single
+context number is not supported but ``ETHTOOL_A_RSS_START_CONTEXT``
+can be used to start dumping context from the given number (primarily
+used to ignore context 0s and only dump additional contexts).
 
 Kernel response contents:
 

include/uapi/linux/ethtool_netlink.h

@@ -965,6 +965,7 @@ enum {
 	ETHTOOL_A_RSS_INDIR,		/* binary */
 	ETHTOOL_A_RSS_HKEY,		/* binary */
 	ETHTOOL_A_RSS_INPUT_XFRM,	/* u32 */
+	ETHTOOL_A_RSS_START_CONTEXT,	/* u32 */
 
 	__ETHTOOL_A_RSS_CNT,
 	ETHTOOL_A_RSS_MAX = (__ETHTOOL_A_RSS_CNT - 1),

net/ethtool/netlink.h

@@ -449,7 +449,7 @@ extern const struct nla_policy ethnl_module_get_policy[ETHTOOL_A_MODULE_HEADER +
 extern const struct nla_policy ethnl_module_set_policy[ETHTOOL_A_MODULE_POWER_MODE_POLICY + 1];
 extern const struct nla_policy ethnl_pse_get_policy[ETHTOOL_A_PSE_HEADER + 1];
 extern const struct nla_policy ethnl_pse_set_policy[ETHTOOL_A_PSE_MAX + 1];
-extern const struct nla_policy ethnl_rss_get_policy[ETHTOOL_A_RSS_CONTEXT + 1];
+extern const struct nla_policy ethnl_rss_get_policy[ETHTOOL_A_RSS_START_CONTEXT + 1];
 extern const struct nla_policy ethnl_plca_get_cfg_policy[ETHTOOL_A_PLCA_HEADER + 1];
 extern const struct nla_policy ethnl_plca_set_cfg_policy[ETHTOOL_A_PLCA_MAX + 1];
 extern const struct nla_policy ethnl_plca_get_status_policy[ETHTOOL_A_PLCA_HEADER + 1];

net/ethtool/rss.c

@@ -28,6 +28,7 @@ struct rss_reply_data {
 const struct nla_policy ethnl_rss_get_policy[] = {
 	[ETHTOOL_A_RSS_HEADER] = NLA_POLICY_NESTED(ethnl_header_policy),
 	[ETHTOOL_A_RSS_CONTEXT] = { .type = NLA_U32 },
+	[ETHTOOL_A_RSS_START_CONTEXT] = { .type = NLA_U32 },
 };
 
 static int
@@ -38,6 +39,10 @@ rss_parse_request(struct ethnl_req_info *req_info, struct nlattr **tb,
 
 	if (tb[ETHTOOL_A_RSS_CONTEXT])
 		request->rss_context = nla_get_u32(tb[ETHTOOL_A_RSS_CONTEXT]);
+	if (tb[ETHTOOL_A_RSS_START_CONTEXT]) {
+		NL_SET_BAD_ATTR(extack, tb[ETHTOOL_A_RSS_START_CONTEXT]);
+		return -EINVAL;
+	}
 
 	return 0;
 }
@@ -214,6 +219,7 @@ struct rss_nl_dump_ctx {
 
 	/* User wants to only dump contexts from given ifindex */
 	unsigned int		match_ifindex;
+	unsigned int		start_ctx;
 };
 
 static struct rss_nl_dump_ctx *rss_dump_ctx(struct netlink_callback *cb)
@@ -236,6 +242,10 @@ int ethnl_rss_dump_start(struct netlink_callback *cb)
 		NL_SET_BAD_ATTR(info->extack, tb[ETHTOOL_A_RSS_CONTEXT]);
 		return -EINVAL;
 	}
+	if (tb[ETHTOOL_A_RSS_START_CONTEXT]) {
+		ctx->start_ctx = nla_get_u32(tb[ETHTOOL_A_RSS_START_CONTEXT]);
+		ctx->ctx_idx = ctx->start_ctx;
+	}
 
 	ret = ethnl_parse_header_dev_get(&req_info,
 					 tb[ETHTOOL_A_RSS_HEADER],
@@ -317,7 +327,7 @@ rss_dump_one_dev(struct sk_buff *skb, struct netlink_callback *cb,
 		if (ret)
 			return ret;
 	}
-	ctx->ctx_idx = 0;
+	ctx->ctx_idx = ctx->start_ctx;
 
 	return 0;
 }