diff --git a/bl31/aarch64/runtime_exceptions.S b/bl31/aarch64/runtime_exceptions.S
index 500e87b..2fa9f06 100644
--- a/bl31/aarch64/runtime_exceptions.S
+++ b/bl31/aarch64/runtime_exceptions.S
@@ -55,7 +55,7 @@
 	 * delegated to platform EA handler.
 	 *
 	 * Without RAS_EXTENSION, this macro synchronizes pending errors using
-         * a DSB, unmasks Asynchronous External Aborts and saves X30 before
+	 * a DSB, unmasks Asynchronous External Aborts and saves X30 before
 	 * setting the flag CTX_IS_IN_EL3.
 	 */
 	.macro check_and_unmask_ea
@@ -506,6 +506,17 @@
 	/* Copy SCR_EL3.NS bit to the flag to indicate caller's security */
 	bfi	x7, x18, #0, #1
 
+	mov	sp, x12
+
+	/*
+	 * Per SMCCC documentation, bits [23:17] must be zero for Fast
+	 * SMCs. Other values are reserved for future use. Ensure that
+	 * these bits are zeroes, if not report as unknown SMC.
+	 */
+	tbz	x0, #FUNCID_TYPE_SHIFT, 2f  /* Skip check if its a Yield Call*/
+	tst	x0, #(FUNCID_FC_RESERVED_MASK << FUNCID_FC_RESERVED_SHIFT)
+	b.ne	smc_unknown
+
 	/*
 	 * Per SMCCCv1.3 a caller can set the SVE hint bit in the SMC FID
 	 * passed through x0. Copy the SVE hint bit to flags and mask the
@@ -513,11 +524,10 @@
 	 * A service/dispatcher can retrieve the SVE hint bit state from
 	 * flags using the appropriate helper.
 	 */
+2:
 	bfi	x7, x0, #FUNCID_SVE_HINT_SHIFT, #FUNCID_SVE_HINT_MASK
 	bic	x0, x0, #(FUNCID_SVE_HINT_MASK << FUNCID_SVE_HINT_SHIFT)
 
-	mov	sp, x12
-
 	/* Get the unique owning entity number */
 	ubfx	x16, x0, #FUNCID_OEN_SHIFT, #FUNCID_OEN_WIDTH
 	ubfx	x15, x0, #FUNCID_TYPE_SHIFT, #FUNCID_TYPE_WIDTH
@@ -584,7 +594,7 @@
 	 * Unknown SMC call. Populate return value with SMC_UNK and call
 	 * el3_exit() which will restore the remaining architectural state
 	 * i.e., SYS, GP and PAuth registers(if any) prior to issuing the ERET
-         * to the desired lower EL.
+	 * to the desired lower EL.
 	 */
 	mov	x0, #SMC_UNK
 	str	x0, [x6, #CTX_GPREGS_OFFSET + CTX_GPREG_X0]
diff --git a/include/lib/smccc.h b/include/lib/smccc.h
index cce91af..63637d5 100644
--- a/include/lib/smccc.h
+++ b/include/lib/smccc.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2016-2022, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2016-2023, ARM Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -37,9 +37,13 @@
 #define FUNCID_OEN_MASK			U(0x3f)
 #define FUNCID_OEN_WIDTH		U(6)
 
-#define FUNCID_SVE_HINT_SHIFT          U(16)
-#define FUNCID_SVE_HINT_MASK           U(1)
-#define FUNCID_SVE_HINT_WIDTH          U(1)
+#define FUNCID_FC_RESERVED_SHIFT	U(17)
+#define FUNCID_FC_RESERVED_MASK		U(0x7f)
+#define FUNCID_FC_RESERVED_WIDTH	U(7)
+
+#define FUNCID_SVE_HINT_SHIFT		U(16)
+#define FUNCID_SVE_HINT_MASK		U(1)
+#define FUNCID_SVE_HINT_WIDTH		U(1)
 
 #define FUNCID_NUM_SHIFT		U(0)
 #define FUNCID_NUM_MASK			U(0xffff)