Re: [PATCH v2] rds: Expose feature parameters via sysfs

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Wed, Jun 11, 2025 at 06:39:19PM -0400, Konrad Rzeszutek Wilk wrote:
> 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@oracle.com>

Argh, forgot to include:

Suggested-by: Andrew Lunn <andrew@lunn.ch>
> ---
>  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
>