netdev - [PATCH v2] rds: Expose feature parameters via sysfs

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Message-ID: <20250611224020.318684-2-konrad.wilk@oracle.com>
Date: Wed, 11 Jun 2025 18:39:19 -0400
From: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To: allison.henderson@...cle.com, netdev@...r.kernel.org,
        linux-rdma@...r.kernel.org, rds-devel@....oracle.com, tj@...nel.org,
        andrew@...n.ch
Cc: hannes@...xchg.org, mkoutny@...e.com, cgroups@...r.kernel.org,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Subject: [PATCH v2] rds: Expose feature parameters via sysfs

We would like to have a programatic way for applications
to query which of the features defined in include/uapi/linux/rds.h
are actually implemented by the kernel.

The problem is that applications can be built against newer
kernel (or older) and they may have the feature implemented or not.

The lack of a certain feature would signify that the kernel
does not support it. The presence of it signifies the existence
of it.

This would provide the application to query the sysfs and figure
out what is supported.

This patch would expose this extra sysfs file:

/sys/kernel/rds/features

which would contain string values of what the RDS driver supports.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
---
 net/rds/af_rds.c | 37 +++++++++++++++++++++++++++++++++++++
 1 file changed, 37 insertions(+)

diff --git a/net/rds/af_rds.c b/net/rds/af_rds.c
index 8435a20968ef..46cb8655df20 100644
--- a/net/rds/af_rds.c
+++ b/net/rds/af_rds.c
@@ -33,6 +33,7 @@
 #include <linux/module.h>
 #include <linux/errno.h>
 #include <linux/kernel.h>
+#include <linux/kobject.h>
 #include <linux/gfp.h>
 #include <linux/in.h>
 #include <linux/ipv6.h>
@@ -871,6 +872,33 @@ static void rds6_sock_info(struct socket *sock, unsigned int len,
 }
 #endif
 
+#ifdef CONFIG_SYSFS
+static ssize_t features_show(struct kobject *kobj, struct kobj_attribute *attr,
+			     char *buf)
+{
+	return snprintf(buf, PAGE_SIZE, "get_tos\n"
+			"set_tos\n"
+			"socket_cancel_sent_to\n"
+			"socket_get_mr\n"
+			"socket_free_mr\n"
+			"socket_recverr\n"
+			"socket_cong_monitor\n"
+			"socket_get_mr_for_dest\n"
+			"socket_so_transport\n"
+			"socket_so_rxpath_latency\n");
+}
+static struct kobj_attribute rds_features_attr = __ATTR_RO(features);
+
+static struct attribute *rds_sysfs_attrs[] = {
+	&rds_features_attr.attr,
+	NULL,
+};
+static const struct attribute_group rds_sysfs_attr_group = {
+	.attrs = rds_sysfs_attrs,
+	.name = "rds",
+};
+#endif
+
 static void rds_exit(void)
 {
 	sock_unregister(rds_family_ops.family);
@@ -882,6 +910,9 @@ static void rds_exit(void)
 	rds_stats_exit();
 	rds_page_exit();
 	rds_bind_lock_destroy();
+#ifdef CONFIG_SYSFS
+	sysfs_remove_group(kernel_kobj, &rds_sysfs_attr_group);
+#endif
 	rds_info_deregister_func(RDS_INFO_SOCKETS, rds_sock_info);
 	rds_info_deregister_func(RDS_INFO_RECV_MESSAGES, rds_sock_inc_info);
 #if IS_ENABLED(CONFIG_IPV6)
@@ -923,6 +954,12 @@ static int __init rds_init(void)
 	if (ret)
 		goto out_proto;
 
+#ifdef CONFIG_SYSFS
+	ret = sysfs_create_group(kernel_kobj, &rds_sysfs_attr_group);
+	if (ret)
+		goto out_proto;
+#endif
+
 	rds_info_register_func(RDS_INFO_SOCKETS, rds_sock_info);
 	rds_info_register_func(RDS_INFO_RECV_MESSAGES, rds_sock_inc_info);
 #if IS_ENABLED(CONFIG_IPV6)
-- 
2.43.5