arch/x86: add SMM caller

Useful for debugging or for cases where we need to enter SMM.
Probably should be moved to commonlib or libpayload.

Change-Id: I7a9cc626dae9a7751034615ef409eebc6035f5c3
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/25193
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
diff --git a/src/arch/x86/include/smm.h b/src/arch/x86/include/smm.h
new file mode 100644
index 0000000..d66e8906
--- /dev/null
+++ b/src/arch/x86/include/smm.h
@@ -0,0 +1,34 @@
+/*
+ * This file is part of the coreboot project.
+ *
+ * Copyright 2018 Google LLC
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; version 2 of the License.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ */
+
+#include <stddef.h>
+#include <stdint.h>
+#include <cpu/x86/smm.h>
+
+/*
+ * calls into SMM with the given cmd and subcmd in eax, and arg in ebx
+ *
+ * static inline because the resulting assembly is often smaller than
+ * the call sequence due to constant folding.
+ */
+static inline u32 call_smm(u8 cmd, u8 subcmd, void *arg)
+{
+	u32 res = 0;
+	__asm__ __volatile__ (
+		"outb %b0, %3"
+		: "=a" (res)
+		: "a" ((subcmd << 8) | cmd), "b" (arg), "i" (APM_CNT));
+	return res;
+}