s390: add sysfs attributes for spectre

Set CONFIG_GENERIC_CPU_VULNERABILITIES and provide the two functions
cpu_show_spectre_v1 and cpu_show_spectre_v2 to report the spectre
mitigations.
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/Kconfig

@@ -120,6 +120,7 @@ config S390
 	select GENERIC_CLOCKEVENTS
 	select GENERIC_CPU_AUTOPROBE
 	select GENERIC_CPU_DEVICES if !SMP
+	select GENERIC_CPU_VULNERABILITIES
 	select GENERIC_FIND_FIRST_BIT
 	select GENERIC_SMP_IDLE_THREAD
 	select GENERIC_TIME_VSYSCALL

arch/s390/kernel/nospec-branch.c

 // SPDX-License-Identifier: GPL-2.0
 #include <linux/module.h>
+#include <linux/device.h>
 #include <asm/nospec-branch.h>
 
 static int __init nobp_setup_early(char *str)
@@ -42,6 +43,24 @@ static int __init nospec_report(void)
 }
 arch_initcall(nospec_report);
 
+#ifdef CONFIG_SYSFS
+ssize_t cpu_show_spectre_v1(struct device *dev,
+			    struct device_attribute *attr, char *buf)
+{
+	return sprintf(buf, "Mitigation: __user pointer sanitization\n");
+}
+
+ssize_t cpu_show_spectre_v2(struct device *dev,
+			    struct device_attribute *attr, char *buf)
+{
+	if (IS_ENABLED(CC_USING_EXPOLINE) && !nospec_disable)
+		return sprintf(buf, "Mitigation: execute trampolines\n");
+	if (__test_facility(82, S390_lowcore.alt_stfle_fac_list))
+		return sprintf(buf, "Mitigation: limited branch prediction.\n");
+	return sprintf(buf, "Vulnerable\n");
+}
+#endif
+
 #ifdef CONFIG_EXPOLINE
 
 int nospec_disable = IS_ENABLED(CONFIG_EXPOLINE_OFF);