[PATCH 3.16 81/86] x86/speculation/mds: Add 'mitigations=' support for MDS

[Ben Hutchings]

3.16.68-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>

commit 5c14068f87d04adc73ba3f41c2a303d3c3d1fa12 upstream.

Add MDS to the new 'mitigations=' cmdline option.

Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
[bwh: Backported to 3.16:
 - Drop the auto,nosmt option, which we can't support
 - Adjust filenames, context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -1920,6 +1920,7 @@ bytes respectively. Such letter suffixes
 					       nospectre_v2 [X86]
 					       spectre_v2_user=off [X86]
 					       spec_store_bypass_disable=off [X86]
+					       mds=off [X86]
 
 			auto (default)
 				Mitigate all CPU vulnerabilities, but leave SMT
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -287,7 +287,7 @@ 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;
 	}