x86/speculation/mds: Add 'mitigations=' support for MDS

Add MDS to the new 'mitigations=' cmdline option.
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

Documentation/admin-guide/kernel-parameters.txt

@@ -2556,6 +2556,7 @@
 					       spectre_v2_user=off [X86]
 					       spec_store_bypass_disable=off [X86,PPC]
 					       l1tf=off [X86]
+					       mds=off [X86]
 
 			auto (default)
 				Mitigate all CPU vulnerabilities, but leave SMT
@@ -2570,6 +2571,7 @@
 				if needed.  This is for users who always want to
 				be fully mitigated, even if it means losing SMT.
 				Equivalent to: l1tf=flush,nosmt [X86]
+					       mds=full,nosmt [X86]
 
 	mminit_loglevel=
 			[KNL] When CONFIG_DEBUG_MEMORY_INIT is set, this

arch/x86/kernel/cpu/bugs.c

@@ -233,7 +233,7 @@ static const char * const mds_strings[] = {
 
 static void __init mds_select_mitigation(void)
 {
-	if (!boot_cpu_has_bug(X86_BUG_MDS)) {
+	if (!boot_cpu_has_bug(X86_BUG_MDS) || cpu_mitigations_off()) {
 		mds_mitigation = MDS_MITIGATION_OFF;
 		return;
 	}
@@ -244,7 +244,8 @@ static void __init mds_select_mitigation(void)
 
 		static_branch_enable(&mds_user_clear);
 
-		if (mds_nosmt && !boot_cpu_has(X86_BUG_MSBDS_ONLY))
+		if (!boot_cpu_has(X86_BUG_MSBDS_ONLY) &&
+		    (mds_nosmt || cpu_mitigations_auto_nosmt()))
 			cpu_smt_disable(false);
 	}