[PATCH V2 00/12] POWER DSCR fixes, improvements, docs and tests

@@ -1112,8 +1112,6 @@ static void setup_ksp_vsid(struct task_struct *p, unsigned long sp)
 /*
  * Copy a thread..
  */
-extern unsigned long dscr_default; /* defined in arch/powerpc/kernel/sysfs.c */
-
 int copy_thread(unsigned long clone_flags, unsigned long usp,
 		unsigned long arg, struct task_struct *p)
 {

@@ -295,6 +295,15 @@ struct thread_struct {
 #endif
 #ifdef CONFIG_PPC64
 	unsigned long	dscr;
+	/*
+	 * This member element dscr_inherit indicates that the process
+	 * has explicitly attempted and changed the DSCR register value
+	 * for itself. Hence kernel wont use the default CPU DSCR value
+	 * contained in the PACA structure anymore during process context
+	 * switch. Once this variable is set, this behaviour will also be
+	 * inherited to all the children of this process from that point
+	 * onwards.
+	 */
 	int		dscr_inherit;
 	unsigned long	ppr;	/* used to save/restore SMT priority */
 #endif

@@ -496,13 +496,34 @@ static DEVICE_ATTR(spurr, 0400, show_spurr, NULL);
 static DEVICE_ATTR(purr, 0400, show_purr, store_purr);
 static DEVICE_ATTR(pir, 0400, show_pir, NULL);
 
+/*
+ * This is the system wide DSCR register default value. Any
+ * change to this default value through the sysfs interface
+ * will update all per cpu DSCR default values across the
+ * system stored in their respective PACA structures.
+ */
 static unsigned long dscr_default;
 
+/**
+ * read_dscr() - Fetch the cpu specific DSCR default
+ * @val:	Returned cpu specific DSCR default value
+ *
+ * This function returns the per cpu DSCR default value
+ * for any cpu which is contained in it's PACA structure.
+ */
 static void read_dscr(void *val)
 {
 	*(unsigned long *)val = get_paca()->dscr_default;
 }
 
+
+/**
+ * write_dscr() - Update the cpu specific DSCR default
+ * @val:	New cpu specific DSCR default value to update
+ *
+ * This function updates the per cpu DSCR default value
+ * for any cpu which is contained in it's PACA structure.
+ */
 static void write_dscr(void *val)
 {
 	get_paca()->dscr_default = *(unsigned long *)val;

@@ -520,12 +541,29 @@ static void add_write_permission_dev_attr(struct device_attribute *attr)
 	attr->attr.mode |= 0200;
 }
 
+/**
+ * show_dscr_default() - Fetch the system wide DSCR default
+ * @dev:	Device structure
+ * @attr:	Device attribute structure
+ * @buf:	Interface buffer
+ *
+ * This function returns the system wide DSCR default value.
+ */
 static ssize_t show_dscr_default(struct device *dev,
 		struct device_attribute *attr, char *buf)
 {
 	return sprintf(buf, "%lx\n", dscr_default);
 }
 
+/**
+ * store_dscr_default() - Update the system wide DSCR default
+ * @dev:	Device structure
+ * @attr:	Device attribute structure
+ * @buf:	Interface buffer
+ * @count:	Size of the update
+ *
+ * This function updates the system wide DSCR default value.
+ */
 static ssize_t __used store_dscr_default(struct device *dev,
 		struct device_attribute *attr, const char *buf,
 		size_t count)

@@ -13,7 +13,7 @@ CFLAGS := -Wall -O2 -flto -Wall -Werror -DGIT_VERSION='"$(GIT_VERSION)"' -I$(CUR
 
 export CC CFLAGS
 
-TARGETS = pmu copyloops mm tm primitives
+TARGETS = pmu copyloops mm tm primitives dscr
 
 endif
 

@@ -0,0 +1,17 @@
+PROGS := dscr_default_test
+
+CFLAGS := $(CFLAGS) -lpthread
+
+all: $(PROGS)
+
+$(PROGS): ../harness.c
+
+run_tests: all
+	@-for PROG in $(PROGS); do \
+		./$$PROG; \
+	done;
+
+clean:
+	rm -f $(PROGS) *.o
+
+.PHONY: all run_tests clean

@@ -0,0 +1,120 @@
+/*
+ * POWER Data Stream Control Register (DSCR)
+ *
+ * This header file contains helper functions and macros
+ * required for all the DSCR related test cases.
+ *
+ * Copyright (C) 2012 Anton Blanchard <anton@au.ibm.com>, IBM
+ * Copyright (C) 2015 Anshuman Khandual <khandual@linux.vnet.ibm.com>, IBM
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version
+ * 2 of the License, or (at your option) any later version.
+ */
+#ifndef _SELFTESTS_POWERPC_DSCR_DSCR_H
+#define _SELFTESTS_POWERPC_DSCR_DSCR_H
+
+#include <unistd.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <fcntl.h>
+#include <dirent.h>
+#include <pthread.h>
+#include <sched.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <sys/wait.h>
+
+#include "utils.h"
+
+#define SPRN_DSCR	0x11	/* Privilege state SPR */
+#define SPRN_DSCR_USR	0x03	/* Problem state SPR */
+#define THREADS		100	/* Max threads */
+#define COUNT		100	/* Max iterations */
+#define DSCR_MAX	16	/* Max DSCR value */
+#define LEN_MAX		100	/* Max name length */
+
+#define DSCR_DEFAULT	"/sys/devices/system/cpu/dscr_default"
+#define CPU_PATH	"/sys/devices/system/cpu/"
+
+#define rmb()  asm volatile("lwsync":::"memory")
+#define wmb()  asm volatile("lwsync":::"memory")
+
+#define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))
+
+/* Prilvilege state DSCR access */
+inline unsigned long get_dscr(void)
+{
+	unsigned long ret;
+
+	asm volatile("mfspr %0,%1" : "=r" (ret): "i" (SPRN_DSCR));
+
+	return ret;
+}
+
+inline void set_dscr(unsigned long val)
+{
+	asm volatile("mtspr %1,%0" : : "r" (val), "i" (SPRN_DSCR));
+}
+
+/* Problem state DSCR access */
+inline unsigned long get_dscr_usr(void)
+{
+	unsigned long ret;
+
+	asm volatile("mfspr %0,%1" : "=r" (ret): "i" (SPRN_DSCR_USR));
+
+	return ret;
+}
+
+inline void set_dscr_usr(unsigned long val)
+{
+	asm volatile("mtspr %1,%0" : : "r" (val), "i" (SPRN_DSCR_USR));
+}
+
+/* Default DSCR access */
+unsigned long get_default_dscr(void)
+{
+	int fd = -1;
+	char buf[16];
+	unsigned long val;
+
+	if (fd == -1) {
+		fd = open(DSCR_DEFAULT, O_RDONLY);
+		if (fd == -1) {
+			perror("open() failed\n");
+			exit(1);
+		}
+	}
+	memset(buf, 0, sizeof(buf));
+	lseek(fd, 0, SEEK_SET);
+	read(fd, buf, sizeof(buf));
+	sscanf(buf, "%lx", &val);
+	close(fd);
+	return val;
+}
+
+void set_default_dscr(unsigned long val)
+{
+	int fd = -1;
+	char buf[16];
+
+	if (fd == -1) {
+		fd = open(DSCR_DEFAULT, O_RDWR);
+		if (fd == -1) {
+			perror("open() failed\n");
+			exit(1);
+		}
+	}
+	sprintf(buf, "%lx\n", val);
+	write(fd, buf, strlen(buf));
+	close(fd);
+}
+
+double uniform_deviate(int seed)
+{
+	return seed * (1.0 / (RAND_MAX + 1.0));
+}
+#endif	/* _SELFTESTS_POWERPC_DSCR_DSCR_H */

@@ -0,0 +1,121 @@
+/*
+ * POWER Data Stream Control Register (DSCR) default test
+ *
+ * This test modifies the system wide default DSCR through
+ * it's sysfs interface and then verifies that all threads
+ * see the correct changed DSCR value immediately.
+ *
+ * Copyright (C) 2012 Anton Blanchard <anton@au.ibm.com>, IBM
+ * Copyright (C) 2015 Anshuman Khandual <khandual@linux.vnet.ibm.com>, IBM
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version
+ * 2 of the License, or (at your option) any later version.
+ */
+#include "dscr.h"
+
+static unsigned long dscr;		/* System DSCR default */
+static unsigned long sequence;
+static unsigned long result[THREADS];
+
+static void *do_test(void *in)
+{
+	unsigned long thread = (unsigned long)in;
+	unsigned long i;
+
+	for (i = 0; i < COUNT; i++) {
+		unsigned long d, cur_dscr, cur_dscr_usr;
+		unsigned long s1, s2;
+
+		s1 = ACCESS_ONCE(sequence);
+		if (s1 & 1)
+			continue;
+		rmb();
+
+		d = dscr;
+		cur_dscr = get_dscr();
+		cur_dscr_usr = get_dscr_usr();
+
+		rmb();
+		s2 = sequence;
+
+		if (s1 != s2)
+			continue;
+
+		if (cur_dscr != d) {
+			fprintf(stderr, "thread %ld kernel DSCR should be %ld "
+				"but is %ld\n", thread, d, cur_dscr);
+			result[thread] = 1;
+			pthread_exit(&result[thread]);
+		}
+
+		if (cur_dscr_usr != d) {
+			fprintf(stderr, "thread %ld user DSCR should be %ld "
+				"but is %ld\n", thread, d, cur_dscr_usr);
+			result[thread] = 1;
+			pthread_exit(&result[thread]);
+		}
+	}
+	result[thread] = 0;
+	pthread_exit(&result[thread]);
+}
+
+int test_body(void)
+{
+	pthread_t threads[THREADS];
+	unsigned long i, *status[THREADS];
+
+	/* Initial DSCR default */
+	dscr = 1;
+	set_default_dscr(dscr);
+
+	/* Spawn all testing threads */
+	for (i = 0; i < THREADS; i++) {
+		if (pthread_create(&threads[i], NULL, do_test, (void *)i)) {
+			perror("pthread_create() failed\n");
+			exit(1);
+		}
+	}
+
+	srand(getpid());
+
+	/* Keep changing the DSCR default */
+	for (i = 0; i < COUNT; i++) {
+		double ret = uniform_deviate(rand());
+
+		if (ret < 0.0001) {
+			sequence++;
+			wmb();
+
+			dscr++;
+			if (dscr > DSCR_MAX)
+				dscr = 0;
+
+			set_default_dscr(dscr);
+
+			wmb();
+			sequence++;
+		}
+	}
+
+	/* Individual testing thread exit status */
+	for (i = 0; i < THREADS; i++) {
+		if (pthread_join(threads[i], (void **)&(status[i]))) {
+			perror("pthread_join() failed\n");
+			exit(1);
+		}
+
+		if (*status[i]) {
+			printf("%ldth thread failed to join with %ld status\n",
+								i, *status[i]);
+			return 1;
+		}
+	}
+	return 0;
+}
+
+int main(int argc, char *argv[])
+{
+	return test_harness(test_body, "dscr_default_test");
+}

@@ -1,4 +1,4 @@
-PROGS := dscr_default_test dscr_explicit_test
+PROGS := dscr_default_test dscr_explicit_test dscr_user_test
 
 CFLAGS := $(CFLAGS) -lpthread
 

@@ -0,0 +1,62 @@
+/*
+ * POWER Data Stream Control Register (DSCR) SPR test
+ *
+ * This test modifies the DSCR value through both the SPR number
+ * based mtspr instruction and then makes sure that the same is
+ * reflected through mfspr instruction using either of the SPR
+ * numbers.
+ *
+ * When using the privilege state SPR, the instructions such as
+ * mfspr or mtspr are priviledged and the kernel emulates them
+ * for us. Instructions using problem state SPR can be exuecuted
+ * directly without any emulation if the HW supports them. Else
+ * they also get emulated by the kernel.
+ *
+ * Copyright (C) 2013 Anton Blanchard <anton@au.ibm.com>, IBM
+ * Copyright (C) 2015 Anshuman Khandual <khandual@linux.vnet.ibm.com>, IBM
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version
+ * 2 of the License, or (at your option) any later version.
+ */
+#include "dscr.h"
+
+static int check_dscr(char *str)
+{
+	unsigned long cur_dscr, cur_dscr_usr;
+
+	cur_dscr = get_dscr();
+	cur_dscr_usr = get_dscr_usr();
+	if (cur_dscr != cur_dscr_usr) {
+		printf("%s set, kernel get %lx != user get %lx\n",
+					str, cur_dscr, cur_dscr_usr);
+		return 1;
+	}
+	return 0;
+}
+
+int test_body(void)
+{
+	int i;
+
+	check_dscr("");
+
+	for (i = 0; i < COUNT; i++) {
+		set_dscr(i);
+		if (check_dscr("kernel"))
+			return 1;
+	}
+
+	for (i = 0; i < COUNT; i++) {
+		set_dscr_usr(i);
+		if (check_dscr("user"))
+			return 1;
+	}
+	return 0;
+}
+
+int main(int argc, char *argv[])
+{
+	return test_harness(test_body, "dscr_user_test");
+}

@@ -1,4 +1,5 @@
-PROGS := dscr_default_test dscr_explicit_test dscr_user_test
+PROGS := dscr_default_test dscr_explicit_test dscr_user_test	\
+	 dscr_inherit_test
 
 CFLAGS := $(CFLAGS) -lpthread
 

@@ -0,0 +1,96 @@
+/*
+ * POWER Data Stream Control Register (DSCR) fork test
+ *
+ * This testcase modifies the DSCR using mtspr, forks and then
+ * verifies that the child process has the correct changed DSCR
+ * value using mfspr.
+ *
+ * When using the privilege state SPR, the instructions such as
+ * mfspr or mtspr are priviledged and the kernel emulates them
+ * for us. Instructions using problem state SPR can be exuecuted
+ * directly without any emulation if the HW supports them. Else
+ * they also get emulated by the kernel.
+ *
+ * Copyright (C) 2012 Anton Blanchard <anton@au.ibm.com>, IBM
+ * Copyright (C) 2015 Anshuman Khandual <khandual@linux.vnet.ibm.com>, IBM
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version
+ * 2 of the License, or (at your option) any later version.
+ */
+#include "dscr.h"
+
+int test_body(void)
+{
+	unsigned long i, dscr = 0;
+	pid_t pid;
+
+	srand(getpid());
+	set_dscr(dscr);
+
+	for (i = 0; i < COUNT; i++) {
+		unsigned long cur_dscr, cur_dscr_usr;
+
+		dscr++;
+		if (dscr > DSCR_MAX)
+			dscr = 0;
+
+		if (i % 2 == 0)
+			set_dscr_usr(dscr);
+		else
+			set_dscr(dscr);
+
+		/*
+		 * XXX: Force a context switch out so that DSCR
+		 * current value is copied into the thread struct
+		 * which is required for the child to inherit the
+		 * changed value.
+		 */
+		sleep(1);
+
+		pid = fork();
+		if (pid == -1) {
+			perror("fork() failed\n");
+			exit(1);
+		} else if (pid) {
+			int status;
+
+			if (waitpid(pid, &status, 0) == -1) {
+				perror("waitpid() failed\n");
+				exit(1);
+			}
+
+			if (!WIFEXITED(status)) {
+				fprintf(stderr, "Child didn't exit cleanly\n");
+				exit(1);
+			}
+
+			if (WEXITSTATUS(status) != 0) {
+				fprintf(stderr, "Child didn't exit cleanly\n");
+				return 1;
+			}
+		} else {
+			cur_dscr = get_dscr();
+			if (cur_dscr != dscr) {
+				fprintf(stderr, "Kernel DSCR should be %ld "
+					"but is %ld\n", dscr, cur_dscr);
+				exit(1);
+			}
+
+			cur_dscr_usr = get_dscr_usr();
+			if (cur_dscr_usr != dscr) {
+				fprintf(stderr, "User DSCR should be %ld "
+					"but is %ld\n", dscr, cur_dscr_usr);
+				exit(1);
+			}
+			exit(0);
+		}
+	}
+	return 0;
+}
+
+int main(int argc, char *argv[])
+{
+	return test_harness(test_body, "dscr_inherit_test");
+}

@@ -1,5 +1,6 @@
 PROGS := dscr_default_test dscr_explicit_test dscr_user_test	\
-	 dscr_inherit_test dscr_inherit_exec_test
+	 dscr_inherit_test dscr_inherit_exec_test		\
+	 dscr_sysfs_test
 
 CFLAGS := $(CFLAGS) -lpthread
 

@@ -0,0 +1,89 @@
+/*
+ * POWER Data Stream Control Register (DSCR) sysfs interface test
+ *
+ * This test updates to system wide DSCR default through the sysfs interface
+ * and then verifies that all the CPU specific DSCR defaults are updated as
+ * well verified from their sysfs interfaces.
+ *
+ * Copyright (C) 2015 Anshuman Khandual <khandual@linux.vnet.ibm.com>, IBM
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version
+ * 2 of the License, or (at your option) any later version.
+ */
+#include "dscr.h"
+
+static int check_cpu_dscr_default(char *file, unsigned long val)
+{
+	char buf[10];
+	int fd, rc;
+
+	fd = open(file, O_RDWR);
+	if (fd == -1) {
+		perror("open() failed\n");
+		return 1;
+	}
+
+	rc = read(fd, buf, sizeof(buf));
+	if (rc == -1) {
+		perror("read() failed\n");
+		return 1;
+	}
+	close(fd);
+
+	buf[rc] = '\0';
+	if (strtol(buf, NULL, 16) != val) {
+		printf("DSCR match failed: %ld (system) %ld (cpu)\n",
+					val, strtol(buf, NULL, 16));
+		return 1;
+	}
+	return 0;
+}
+
+static int check_all_cpu_dscr_defaults(unsigned long val)
+{
+	DIR *sysfs;
+	struct dirent *dp;
+	char file[LEN_MAX];
+
+	sysfs = opendir(CPU_PATH);
+	if (!sysfs) {
+		perror("opendir() failed\n");
+		return 1;
+	}
+
+	while ((dp = readdir(sysfs))) {
+		if (!(dp->d_type & DT_DIR))
+			continue;
+		if (!strcmp(dp->d_name, "cpuidle"))
+			continue;
+		if (!strstr(dp->d_name, "cpu"))
+			continue;
+
+		sprintf(file, "%s%s/dscr", CPU_PATH, dp->d_name);
+		if (check_cpu_dscr_default(file, val))
+			return 1;
+	}
+	closedir(sysfs);
+	return 0;
+}
+
+int test_body(void)
+{
+	int i, j;
+
+	for (i = 0; i < COUNT; i++) {
+		for (j = 0; j < DSCR_MAX; j++) {
+			set_default_dscr(j);
+			if (check_all_cpu_dscr_defaults(j))
+				return 1;
+		}
+	}
+	return 0;
+}
+
+int main(int argc, char *argv[])
+{
+	return test_harness(test_body, "dscr_sysfs_test");
+}

@@ -1,6 +1,6 @@
 PROGS := dscr_default_test dscr_explicit_test dscr_user_test	\
 	 dscr_inherit_test dscr_inherit_exec_test		\
-	 dscr_sysfs_test
+	 dscr_sysfs_test dscr_sysfs_thread_test
 
 CFLAGS := $(CFLAGS) -lpthread
 

@@ -0,0 +1,114 @@
+/*
+ * POWER Data Stream Control Register (DSCR) sysfs thread test
+ *
+ * This test updates the system wide DSCR default value through
+ * sysfs interface which should then update all the CPU specific
+ * DSCR default values which must also be then visible to threads
+ * executing on individual CPUs on the system.
+ *
+ * Copyright (C) 2015 Anshuman Khandual <khandual@linux.vnet.ibm.com>, IBM
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version
+ * 2 of the License, or (at your option) any later version.
+ */
+#define _GNU_SOURCE
+#include "dscr.h"
+
+static pthread_mutex_t lock;	/* Pthread lock */
+static cpu_set_t cpuset;	/* Thread cpu set */
+static int target;		/* Thread target cpu */
+static unsigned long *result;	/* Thread exit status array */
+
+static void *test_thread_dscr(void *in)
+{
+	unsigned long cur_dscr, cur_dscr_usr;
+	unsigned long val = (unsigned long)in;
+
+	pthread_mutex_lock(&lock);
+	target++;
+	if (target == sysconf(_SC_NPROCESSORS_ONLN))
+		target =  0;
+	CPU_ZERO(&cpuset);
+	CPU_SET(target, &cpuset);
+
+	if (sched_setaffinity(0, sizeof(cpuset), &cpuset)) {
+		perror("sched_settarget_cpu() failed\n");
+		pthread_mutex_unlock(&lock);
+		result[target] = 1;
+		pthread_exit(&result[target]);
+	}
+	pthread_mutex_unlock(&lock);
+
+	cur_dscr = get_dscr();
+	cur_dscr_usr = get_dscr_usr();
+
+	if (val != cur_dscr) {
+		printf("[cpu %d] Kernel DSCR should be %ld but is %ld\n",
+					sched_getcpu(), val, cur_dscr);
+		result[target] = 1;
+		pthread_exit(&result[target]);
+	}
+
+	if (val != cur_dscr_usr) {
+		printf("[cpu %d] User DSCR should be %ld but is %ld\n",
+					sched_getcpu(), val, cur_dscr_usr);
+		result[target] = 1;
+		pthread_exit(&result[target]);
+	}
+	result[target] = 0;
+	pthread_exit(&result[target]);
+}
+
+static int check_cpu_dscr_thread(unsigned long val)
+{
+	pthread_t threads[sysconf(_SC_NPROCESSORS_ONLN)];
+	unsigned long *status[sysconf(_SC_NPROCESSORS_ONLN)];
+	unsigned long i;
+
+	for (i = 0; i < sysconf(_SC_NPROCESSORS_ONLN); i++) {
+		if (pthread_create(&threads[i], NULL,
+				test_thread_dscr, (void *)val)) {
+			perror("pthread_create() failed\n");
+			return 1;
+		}
+	}
+
+	for (i = 0; i < sysconf(_SC_NPROCESSORS_ONLN); i++) {
+		if (pthread_join(threads[i], (void **)&(status[i]))) {
+			perror("pthread_join() failed\n");
+			return 1;
+		}
+
+		if (*status[i]) {
+			printf(" %ldth thread join failed with %ld\n",
+							i, *status[i]);
+			return 1;
+		}
+	}
+	return 0;
+}
+
+int test_body(void)
+{
+	int i, j;
+
+	result = malloc(sizeof(unsigned long) * sysconf(_SC_NPROCESSORS_ONLN));
+	pthread_mutex_init(&lock, NULL);
+	target = 0;
+	for (i = 0; i < COUNT; i++) {
+		for (j = 0; j < DSCR_MAX; j++) {
+			set_default_dscr(j);
+			if (check_cpu_dscr_thread(j))
+				return 1;
+		}
+	}
+	free(result);
+	return 0;
+}
+
+int main(int argc, char *argv[])
+{
+	return test_harness(test_body, "dscr_sysfs_thread_test");
+}

@@ -1,5 +1,5 @@
 PROGS := dscr_default_test dscr_explicit_test dscr_user_test	\
-	 dscr_inherit_test
+	 dscr_inherit_test dscr_inherit_exec_test
 
 CFLAGS := $(CFLAGS) -lpthread
 

@@ -0,0 +1,118 @@
+/*
+ * POWER Data Stream Control Register (DSCR) fork exec test
+ *
+ * This testcase modifies the DSCR using mtspr, forks & execs and
+ * verifies that the child is using the changed DSCR using mfspr.
+ *
+ * When using the privilege state SPR, the instructions such as
+ * mfspr or mtspr are priviledged and the kernel emulates them
+ * for us. Instructions using problem state SPR can be exuecuted
+ * directly without any emulation if the HW supports them. Else
+ * they also get emulated by the kernel.
+ *
+ * Copyright (C) 2012 Anton Blanchard <anton@au.ibm.com>, IBM
+ * Copyright (C) 2015 Anshuman Khandual <khandual@linux.vnet.ibm.com>, IBM
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version
+ * 2 of the License, or (at your option) any later version.
+ */
+#include "dscr.h"
+
+static char prog[LEN_MAX];
+
+static void do_exec(unsigned long parent_dscr)
+{
+	unsigned long cur_dscr, cur_dscr_usr;
+
+	cur_dscr = get_dscr();
+	cur_dscr_usr = get_dscr_usr();
+
+	if (cur_dscr != parent_dscr) {
+		fprintf(stderr, "Parent DSCR %ld was not inherited "
+				"over exec (kernel value)\n", parent_dscr);
+		exit(1);
+	}
+
+	if (cur_dscr_usr != parent_dscr) {
+		fprintf(stderr, "Parent DSCR %ld was not inherited "
+				"over exec (user value)\n", parent_dscr);
+		exit(1);
+	}
+	exit(0);
+}
+
+int test_body(void)
+{
+	unsigned long i, dscr = 0;
+	pid_t pid;
+
+	for (i = 0; i < COUNT; i++) {
+		dscr++;
+		if (dscr > DSCR_MAX)
+			dscr = 0;
+
+		if (dscr == get_default_dscr())
+			continue;
+
+		if (i % 2 == 0)
+			set_dscr_usr(dscr);
+		else
+			set_dscr(dscr);
+
+		/*
+		 * XXX: Force a context switch out so that DSCR
+		 * current value is copied into the thread struct
+		 * which is required for the child to inherit the
+		 * changed value.
+		 */
+		sleep(1);
+
+		pid = fork();
+		if (pid == -1) {
+			perror("fork() failed\n");
+			exit(1);
+		} else if (pid) {
+			int status;
+
+			if (waitpid(pid, &status, 0) == -1) {
+				perror("waitpid() failed\n");
+				exit(1);
+			}
+
+			if (!WIFEXITED(status)) {
+				fprintf(stderr, "Child didn't exit cleanly\n");
+				exit(1);
+			}
+
+			if (WEXITSTATUS(status) != 0) {
+				fprintf(stderr, "Child didn't exit cleanly\n");
+				return 1;
+			}
+		} else {
+			char dscr_str[16];
+
+			sprintf(dscr_str, "%ld", dscr);
+			execlp(prog, prog, "exec", dscr_str, NULL);
+			exit(1);
+		}
+	}
+	return 0;
+}
+
+int main(int argc, char *argv[])
+{
+	if (argc == 3 && !strcmp(argv[1], "exec")) {
+		unsigned long parent_dscr;
+
+		parent_dscr = atoi(argv[2]);
+		do_exec(parent_dscr);
+	} else if (argc != 1) {
+		fprintf(stderr, "Usage: %s\n", argv[0]);
+		exit(1);
+	}
+
+	strncpy(prog, argv[0], strlen(argv[0]));
+	return test_harness(test_body, "dscr_inherit_exec_test");
+}