[PATCH 2/2] x86, microcode: Add option to allow downgrading of microcode

[Andi Kleen <andi@firstfloor.org>]

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

For testing purposes it can be useful to downgrade microcode.
Normally the driver only allows upgrading.

Add a module_param (default off) that allows downgrading.

Note the module_param can currently not be set for early
ucode update, only for late.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
---
 arch/x86/kernel/microcode_intel_lib.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/x86/kernel/microcode_intel_lib.c b/arch/x86/kernel/microcode_intel_lib.c
index ce69320..18d5325 100644
--- a/arch/x86/kernel/microcode_intel_lib.c
+++ b/arch/x86/kernel/microcode_intel_lib.c
@@ -26,11 +26,16 @@
 #include <linux/uaccess.h>
 #include <linux/kernel.h>
 #include <linux/module.h>
+#include <linux/moduleparam.h>
 
 #include <asm/microcode_intel.h>
 #include <asm/processor.h>
 #include <asm/msr.h>
 
+static bool allow_downgrade;
+module_param(allow_downgrade, bool, 0644);
+MODULE_PARM_DESC(allow_downgrade, "Allow downgrading microcode");
+
 static inline int
 update_match_cpu(unsigned int csig, unsigned int cpf,
 		 unsigned int sig, unsigned int pf)
@@ -41,6 +46,8 @@ update_match_cpu(unsigned int csig, unsigned int cpf,
 int
 update_match_revision(struct microcode_header_intel *mc_header, int rev)
 {
+	if (allow_downgrade)
+		return 1;
 	return (mc_header->rev <= rev) ? 0 : 1;
 }
 
-- 
1.8.3.1