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

commit 5c14068f upstream.

Add MDS to the new 'mitigations=' cmdline option.
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/kernel-parameters.txt

@@ -2488,6 +2488,7 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 					       spectre_v2_user=off [X86]
 					       spec_store_bypass_disable=off [X86]
 					       l1tf=off [X86]
+					       mds=off [X86]
 
 			auto (default)
 				Mitigate all CPU vulnerabilities, but leave SMT
@@ -2502,6 +2503,7 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 				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

@@ -230,7 +230,7 @@ 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;
 	}
@@ -241,7 +241,8 @@ 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);
 	}