[PATCH v4 12/13] x86/retpoline: Add boot time option to disable retpoline

[David Woodhouse]

From: Andi Kleen <ak@xxxxxxxxxxxxxxx>

Add a noretpoline option boot to disable retpoline and patch
out the extra sequences. It cannot patch out the jumps
to the thunk functions generated by the compiler, but they
turn into a single indirect branch now.

Signed-off-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
---
 Documentation/admin-guide/kernel-parameters.txt |  3 +++
 arch/x86/kernel/cpu/intel.c                     | 10 ++++++++++
 2 files changed, 13 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index af7104a..c48db34 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -2623,6 +2623,9 @@
 
 	nohugeiomap	[KNL,x86] Disable kernel huge I/O mappings.
 
+	noretpoline	[X86] Disable the retpoline kernel indirect branch speculation
+			workarounds. System may allow data leaks with this option.
+
 	nosmt		[KNL,S390] Disable symmetric multithreading (SMT).
 			Equivalent to smt=1.
 
diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
index b1af220..4ab46f5 100644
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -31,6 +31,16 @@
 #include <asm/apic.h>
 #endif
 
+#ifdef RETPOLINE
+static int __init noretpoline_setup(char *__unused)
+{
+	pr_info("Retpoline runtime disabled\n");
+	setup_force_cpu_cap(X86_BUG_NO_RETPOLINE);
+	return 1;
+}
+__setup("noretpoline", noretpoline_setup);
+#endif
+
 /*
  * Just in case our CPU detection goes bad, or you have a weird system,
  * allow a way to override the automatic disabling of MPX.
-- 
2.7.4