x86, microcode: Add a disable chicken bit

Add a cmdline param which disables the microcode loader. This is useful
mostly in debugging situations where we want to turn off microcode
loading, both early from the initrd and late, as a means to be able to
rule out its influence on the machine.

Signed-off-by: Borislav Petkov <bp@suse.de>
Link: http://lkml.kernel.org/r/1400525957-11525-3-git-send-email-bp@alien8.de
Signed-off-by: H. Peter Anvin <hpa@zytor.com>

arch/x86/include/asm/microcode.h

@@ -25,6 +25,7 @@ struct cpu_signature {
 struct device;
 
 enum ucode_state { UCODE_ERROR, UCODE_OK, UCODE_NFOUND };
+extern bool dis_ucode_ldr;
 
 struct microcode_ops {
 	enum ucode_state (*request_microcode_user) (int cpu,

arch/x86/kernel/cpu/microcode/core.c

@@ -97,6 +97,9 @@ MODULE_LICENSE("GPL");
 
 static struct microcode_ops	*microcode_ops;
 
+bool dis_ucode_ldr;
+module_param(dis_ucode_ldr, bool, 0);
+
 /*
  * Synchronization.
  *
@@ -546,6 +549,9 @@ static int __init microcode_init(void)
 	struct cpuinfo_x86 *c = &cpu_data(0);
 	int error;
 
+	if (dis_ucode_ldr)
+		return 0;
+
 	if (c->x86_vendor == X86_VENDOR_INTEL)
 		microcode_ops = init_intel_microcode();
 	else if (c->x86_vendor == X86_VENDOR_AMD)

arch/x86/kernel/cpu/microcode/core_early.c

@@ -17,9 +17,11 @@
  *	2 of the License, or (at your option) any later version.
  */
 #include <linux/module.h>
+#include <asm/microcode.h>
 #include <asm/microcode_intel.h>
 #include <asm/microcode_amd.h>
 #include <asm/processor.h>
+#include <asm/cmdline.h>
 
 #define QCHAR(a, b, c, d) ((a) + ((b) << 8) + ((c) << 16) + ((d) << 24))
 #define CPUID_INTEL1 QCHAR('G', 'e', 'n', 'u')
@@ -72,10 +74,33 @@ static int x86_family(void)
 	return x86;
 }
 
+static bool __init check_loader_disabled_bsp(void)
+{
+#ifdef CONFIG_X86_32
+	const char *cmdline = (const char *)__pa_nodebug(boot_command_line);
+	const char *opt	    = "dis_ucode_ldr";
+	const char *option  = (const char *)__pa_nodebug(opt);
+	bool *res = (bool *)__pa_nodebug(&dis_ucode_ldr);
+
+#else /* CONFIG_X86_64 */
+	const char *cmdline = boot_command_line;
+	const char *option  = "dis_ucode_ldr";
+	bool *res = &dis_ucode_ldr;
+#endif
+
+	if (cmdline_find_option_bool(cmdline, option))
+		*res = true;
+
+	return *res;
+}
+
 void __init load_ucode_bsp(void)
 {
 	int vendor, x86;
 
+	if (check_loader_disabled_bsp())
+		return;
+
 	if (!have_cpuid_p())
 		return;
 
@@ -96,10 +121,22 @@ void __init load_ucode_bsp(void)
 	}
 }
 
+static bool check_loader_disabled_ap(void)
+{
+#ifdef CONFIG_X86_32
+	return __pa_nodebug(dis_ucode_ldr);
+#else
+	return dis_ucode_ldr;
+#endif
+}
+
 void load_ucode_ap(void)
 {
 	int vendor, x86;
 
+	if (check_loader_disabled_ap())
+		return;
+
 	if (!have_cpuid_p())
 		return;