s390: report spectre mitigation via syslog

BugLink: http://bugs.launchpad.net/bugs/1768474

[ Upstream commit bc035599 ]

Add a boot message if either of the spectre defenses is active.
The message is
    "Spectre V2 mitigation: execute trampolines."
or  "Spectre V2 mitigation: limited branch prediction."
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

arch/s390/kernel/nospec-branch.c

@@ -33,6 +33,16 @@ static int __init nospec_setup_early(char *str)
 }
 early_param("nospec", nospec_setup_early);
 
+static int __init nospec_report(void)
+{
+	if (IS_ENABLED(CC_USING_EXPOLINE) && !nospec_disable)
+		pr_info("Spectre V2 mitigation: execute trampolines.\n");
+	if (__test_facility(82, S390_lowcore.alt_stfle_fac_list))
+		pr_info("Spectre V2 mitigation: limited branch prediction.\n");
+	return 0;
+}
+arch_initcall(nospec_report);
+
 #ifdef CONFIG_EXPOLINE
 
 int nospec_disable = IS_ENABLED(CONFIG_EXPOLINE_OFF);