UBUNTU: SAUCE: powerpc/speculation: Support 'mitigations=' cmdline option

BugLink: https://bugs.launchpad.net/bugs/1830176

The update to stable 4.4.180 added powerpc Spectre v1 and v2 mitigations,
so configure them with the 'mitigations=' cmdline option.

From upstream commit 782e69ef ("powerpc/speculation: Support
'mitigations=' cmdline option").
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

Documentation/kernel-parameters.txt

@@ -2271,7 +2271,8 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 				expose users to several CPU vulnerabilities.
 				Equivalent to: nopti [X86,PPC]
 					       nobp=0 [S390]
-					       nospectre_v2 [X86,S390]
+					       nospectre_v1 [PPC]
+					       nospectre_v2 [X86,PPC,S390]
 					       spec_store_bypass_disable=off [X86,PPC]
 					       l1tf=off [X86]
 					       mds=off [X86]

arch/powerpc/kernel/security.c

@@ -58,7 +58,7 @@ void setup_barrier_nospec(void)
 	enable = security_ftr_enabled(SEC_FTR_FAVOUR_SECURITY) &&
 		 security_ftr_enabled(SEC_FTR_BNDS_CHK_SPEC_BAR);
 
-	if (!no_nospec)
+	if (!no_nospec && !cpu_mitigations_off())
 		enable_barrier_nospec(enable);
 }
 
@@ -117,7 +117,7 @@ static int __init handle_nospectre_v2(char *p)
 early_param("nospectre_v2", handle_nospectre_v2);
 void setup_spectre_v2(void)
 {
-	if (no_spectrev2)
+	if (no_spectrev2 || cpu_mitigations_off())
 		do_btb_flush_fixups();
 	else
 		btb_flush_enabled = true;