libnvme: add support for per-path diagnostic counters

Add support for retrieving per-path diagnostic counters such as
command_retry_count, command_error_count, and multipath_failover_count.
These counters improve visibility into NVMe native multipath behavior
and can be useful for tools such as nvme-top to display real-time
statistics.

Unlike other sysfs attributes, these counters can change dynamically.
Annotate them with "!accessors:none" and provide custom implementations
to always retrieve the latest (non-cached) values.

Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
Link: https://patch.msgid.link/20260421145038.3458987-7-nilay@linux.ibm.com
Signed-off-by: Daniel Wagner <wagi@kernel.org>

Author: shroffni

libnvme/src/libnvme.ld

@@ -163,6 +163,9 @@ LIBNVME_3 {
 		libnvme_path_get_write_ticks;
 		libnvme_path_reset_stat;
 		libnvme_path_update_stat;
+		libnvme_path_get_command_retry_count;
+		libnvme_path_get_command_error_count;
+		libnvme_path_get_multipath_failover_count;
 		libnvme_random_uuid;
 		libnvme_read_config;
 		libnvme_read_hostid;

libnvme/src/nvme/private.h

@@ -213,6 +213,9 @@ struct libnvme_path {			// !generate-accessors
 	char *numa_nodes;		// !accessors:none
 	int grpid;
 	int queue_depth;		// !accessors:none
+	long multipath_failover_count;	// !accessors:none
+	long command_retry_count;	// !accessors:none
+	long command_error_count;	// !accessors:none
 };
 
 struct libnvme_ns_head {

libnvme/src/nvme/tree.c

@@ -885,6 +885,39 @@ __public char *libnvme_path_get_numa_nodes(libnvme_path_t p)
 	return p->numa_nodes;
 }
 
+__public long libnvme_path_get_multipath_failover_count(libnvme_path_t p)
+{
+	__cleanup_free char *failover_count = NULL;
+
+	failover_count = libnvme_get_path_attr(p, "multipath_failover_count");
+	if (failover_count)
+		sscanf(failover_count, "%ld", &p->multipath_failover_count);
+
+	return p->multipath_failover_count;
+}
+
+__public long libnvme_path_get_command_retry_count(libnvme_path_t p)
+{
+	__cleanup_free char *retry_count = NULL;
+
+	retry_count = libnvme_get_path_attr(p, "command_retry_count");
+	if (retry_count)
+		sscanf(retry_count, "%ld", &p->command_retry_count);
+
+	return p->command_retry_count;
+}
+
+__public long libnvme_path_get_command_error_count(libnvme_path_t p)
+{
+	__cleanup_free char *error_count = NULL;
+
+	error_count = libnvme_get_path_attr(p, "command_error_count");
+	if (error_count)
+		sscanf(error_count, "%ld", &p->command_error_count);
+
+	return p->command_error_count;
+}
+
 static libnvme_stat_t libnvme_path_get_stat(libnvme_path_t p, unsigned int idx)
 {
 	if (idx > 1)

libnvme/src/nvme/tree.h

@@ -685,6 +685,33 @@ char *libnvme_path_get_ana_state(libnvme_path_t p);
  */
 char *libnvme_path_get_numa_nodes(libnvme_path_t p);
 
+/**
+ * libnvme_path_get_multipath_failover_count() - Get multipath failover count
+ * @p: &libnvme_path_t object
+ *
+ * Return: Number of times I/Os have to be failed over to another active path
+ * from path @p maybe due to any transient error observed on path @p
+ */
+long libnvme_path_get_multipath_failover_count(libnvme_path_t p);
+
+/**
+ * libnvme_path_get_command_retry_count() - Get command retry count
+ * @p: &libnvme_path_t object
+ *
+ * Return: Number of times any command issued to the namespace represented by
+ * path @p has to be retried
+ */
+long libnvme_path_get_command_retry_count(libnvme_path_t p);
+
+/**
+ * libnvme_path_get_command_error_count() - Get command error count
+ * @p: &libnvme_path_t object
+ *
+ * Return: Number of times command issued to the namespace represented by path
+ * @p returns non-zero status or error
+ */
+long libnvme_path_get_command_error_count(libnvme_path_t p);
+
 /**
  * libnvme_path_get_ctrl() - Parent controller of an libnvme_path_t object
  * @p:	&libnvme_path_t object