[PATCH v5 10/12] x86/retpoline: Add boot time option to disable retpoline

[David Woodhouse <dwmw@amazon.co.uk>]

From: Andi Kleen <ak@linux.intel.com>

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

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
---
 Documentation/admin-guide/kernel-parameters.txt |  3 +++
 arch/x86/kernel/cpu/intel.c                     | 11 +++++++++++
 2 files changed, 14 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 520fdec..f30f9b4 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -2596,6 +2596,9 @@
 
 	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 b720dac..35e123e 100644
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -31,6 +31,17 @@
 #include <asm/apic.h>
 #endif
 
+#ifdef RETPOLINE
+static int __init noretpoline_setup(char *__unused)
+{
+	pr_info("Retpoline runtime disabled\n");
+	setup_clear_cpu_cap(X86_FEATURE_RETPOLINE);
+	setup_clear_cpu_cap(X86_FEATURE_RETPOLINE_AMD);
+	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