diff --git a/docs/arm-sip-service.md b/docs/arm-sip-service.md
new file mode 100644
index 0000000..7ebb724
--- /dev/null
+++ b/docs/arm-sip-service.md
@@ -0,0 +1,91 @@
+
+ARM SiP Service
+===============
+
+This document enumerates and describes the ARM SiP (Silicon Provider) services.
+
+SiP services are non-standard, platform-specific services offered by the silicon
+implementer or platform provider. They are accessed via. `SMC` ("SMC calls")
+instruction executed from Exception Levels below EL3. SMC calls for SiP
+services:
+
+* Follow [SMC Calling Convention][SMCCC];
+* Use SMC function IDs that fall in the SiP range, which are `0xc2000000` -
+  `0xc200ffff` for 64-bit calls, and `0x82000000` - `0x8200ffff` for 32-bit
+  calls.
+
+The ARM SiP implementation offers the following services:
+
+* Performance Measurement Framework (PMF)
+* Execution State Switching service
+
+Source definitions for ARM SiP service are located in the `arm_sip_svc.h` header
+file.
+
+Performance Measurement Framework (PMF)
+---------------------------------------
+
+The [Performance Measurement Framework](./firmware-design.md#13--performance-measurement-framework)
+allows callers to retrieve timestamps captured at various paths in ARM Trusted
+Firmware execution. It's described in detail in [Firmware Design document][Firmware Design].
+
+Execution State Switching service
+---------------------------------
+
+Execution State Switching service provides a mechanism for a non-secure lower
+Exception Level (either EL2, or NS EL1 if EL2 isn't implemented) to request to
+switch its execution state (a.k.a. Register Width), either from AArch64 to
+AArch32, or from AArch32 to AArch64, for the calling CPU. This service is only
+available when ARM Trusted Firmware is built for AArch64 (i.e. when build option
+`ARCH` is set to `aarch64`).
+
+### `ARM_SIP_SVC_EXE_STATE_SWITCH`
+
+    Arguments:
+        uint32_t Function ID
+        uint32_t PC hi
+        uint32_t PC lo
+        uint32_t Cookie hi
+        uint32_t Cookie lo
+
+    Return:
+        uint32_t
+
+The function ID parameter must be `0x82000020`. It uniquely identifies the
+Execution State Switching service being requested.
+
+The parameters _PC hi_ and _PC lo_ defines upper and lower words, respectively,
+of the entry point (physical address) at which execution should start, after
+Execution State has been switched. When calling from AArch64, _PC hi_ must be 0.
+
+When execution starts at the supplied entry point after Execution State has been
+switched, the parameters _Cookie hi_ and _Cookie lo_ are passed in CPU registers
+0 and 1, respectively. When calling from AArch64, _Cookie hi_ must be 0.
+
+This call can only be made on the primary CPU, before any secondaries were
+brought up with `CPU_ON` PSCI call. Otherwise, the call will always fail.
+
+The effect of switching execution state is as if the Exception Level were
+entered for the first time, following power on. This means CPU registers that
+have a defined reset value by the Architecture will assume that value. Other
+registers should not be expected to hold their values before the call was made.
+CPU endianness, however, is preserved from the previous execution state. Note
+that this switches the execution state of the calling CPU only. This is not a
+substitute for PSCI `SYSTEM_RESET`.
+
+The service may return the following error codes:
+
+  - `STATE_SW_E_PARAM`: If any of the parameters were deemed invalid for
+    a specific request.
+  - `STATE_SW_E_DENIED`: If the call is not successful, or when ARM Trusted
+    Firmware is built for AArch32.
+
+If the call is successful, the caller wouldn't observe the SMC returning.
+Instead, execution starts at the supplied entry point, with the CPU registers 0
+and 1 populated with the supplied _Cookie hi_ and _Cookie lo_ values,
+respectively.
+
+- - - - - - - - - - - - - - - - - - - - - - - - - -
+
+[Firmware Design]: ./firmware-design.md
+[SMCCC]: http://infocenter.arm.com/help/topic/com.arm.doc.den0028a/index.html "SMC Calling Convention PDD (ARM DEN 0028A)"
diff --git a/include/lib/aarch64/arch.h b/include/lib/aarch64/arch.h
index 834434e..efb6b91 100644
--- a/include/lib/aarch64/arch.h
+++ b/include/lib/aarch64/arch.h
@@ -211,7 +211,8 @@
 #define MDCR_DEF_VAL		(MDCR_SDD_BIT | MDCR_SPD32(MDCR_SPD32_DISABLE))
 
 /* HCR definitions */
-#define HCR_RW_BIT		(1ull << 31)
+#define HCR_RW_SHIFT		31
+#define HCR_RW_BIT		(1ull << HCR_RW_SHIFT)
 #define HCR_AMO_BIT		(1 << 5)
 #define HCR_IMO_BIT		(1 << 4)
 #define HCR_FMO_BIT		(1 << 3)
diff --git a/include/lib/el3_runtime/context_mgmt.h b/include/lib/el3_runtime/context_mgmt.h
index 31bf681..6c43c66 100644
--- a/include/lib/el3_runtime/context_mgmt.h
+++ b/include/lib/el3_runtime/context_mgmt.h
@@ -33,6 +33,8 @@
 
 #ifndef AARCH32
 #include <arch.h>
+#include <assert.h>
+#include <stdint.h>
 #endif
 
 /*******************************************************************************
diff --git a/include/lib/psci/psci_lib.h b/include/lib/psci/psci_lib.h
index 2169d6d..b02e7ab 100644
--- a/include/lib/psci/psci_lib.h
+++ b/include/lib/psci/psci_lib.h
@@ -106,6 +106,7 @@
 			  void *handle,
 			  u_register_t flags);
 int psci_setup(const psci_lib_args_t *lib_args);
+int psci_secondaries_brought_up(void);
 void psci_warmboot_entrypoint(void);
 void psci_register_spd_pm_hook(const spd_pm_ops_t *pm);
 void psci_prepare_next_non_secure_ctx(
diff --git a/include/plat/arm/common/arm_sip_svc.h b/include/plat/arm/common/arm_sip_svc.h
index 640bbaf..f3eac31 100644
--- a/include/plat/arm/common/arm_sip_svc.h
+++ b/include/plat/arm/common/arm_sip_svc.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2016, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2016-2017, ARM Limited and Contributors. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are met:
@@ -38,8 +38,11 @@
 /*					0x8200ff02 is reserved */
 #define ARM_SIP_SVC_VERSION		0x8200ff03
 
+/* Function ID for requesting state switch of lower EL */
+#define ARM_SIP_SVC_EXE_STATE_SWITCH	0x82000020
+
 /* ARM SiP Service Calls version numbers */
 #define ARM_SIP_SVC_VERSION_MAJOR		0x0
-#define ARM_SIP_SVC_VERSION_MINOR		0x1
+#define ARM_SIP_SVC_VERSION_MINOR		0x2
 
 #endif /* __ARM_SIP_SVC_H__ */
diff --git a/include/plat/arm/common/plat_arm.h b/include/plat/arm/common/plat_arm.h
index 8ea32b9..2b7e16c 100644
--- a/include/plat/arm/common/plat_arm.h
+++ b/include/plat/arm/common/plat_arm.h
@@ -124,6 +124,9 @@
 
 #endif /* __ARM_RECOM_STATE_ID_ENC__ */
 
+/* ARM State switch error codes */
+#define STATE_SW_E_PARAM		(-2)
+#define STATE_SW_E_DENIED		(-3)
 
 /* IO storage utility functions */
 void arm_io_setup(void);
@@ -230,4 +233,12 @@
 /* Allow platform to override psci_pm_ops during runtime */
 const plat_psci_ops_t *plat_arm_psci_override_pm_ops(plat_psci_ops_t *ops);
 
+/* Execution state switch in ARM platforms */
+int arm_execution_state_switch(unsigned int smc_fid,
+		uint32_t pc_hi,
+		uint32_t pc_lo,
+		uint32_t cookie_hi,
+		uint32_t cookie_lo,
+		void *handle);
+
 #endif /* __PLAT_ARM_H__ */
diff --git a/lib/psci/psci_common.c b/lib/psci/psci_common.c
index 1be37c0..cde8645 100644
--- a/lib/psci/psci_common.c
+++ b/lib/psci/psci_common.c
@@ -916,6 +916,27 @@
 #endif
 }
 
+/******************************************************************************
+ * Return whether any secondaries were powered up with CPU_ON call. A CPU that
+ * have ever been powered up would have set its MPDIR value to something other
+ * than PSCI_INVALID_MPIDR. Note that MPDIR isn't reset back to
+ * PSCI_INVALID_MPIDR when a CPU is powered down later, so the return value is
+ * meaningful only when called on the primary CPU during early boot.
+ *****************************************************************************/
+int psci_secondaries_brought_up(void)
+{
+	int idx, n_valid = 0;
+
+	for (idx = 0; idx < ARRAY_SIZE(psci_cpu_pd_nodes); idx++) {
+		if (psci_cpu_pd_nodes[idx].mpidr != PSCI_INVALID_MPIDR)
+			n_valid++;
+	}
+
+	assert(n_valid);
+
+	return (n_valid > 1);
+}
+
 #if ENABLE_PLAT_COMPAT
 /*******************************************************************************
  * PSCI Compatibility helper function to return the 'power_state' parameter of
diff --git a/plat/arm/common/arm_common.mk b/plat/arm/common/arm_common.mk
index 9cf2b7e..c7db9d8 100644
--- a/plat/arm/common/arm_common.mk
+++ b/plat/arm/common/arm_common.mk
@@ -164,6 +164,7 @@
 BL31_SOURCES		+=	plat/arm/common/arm_bl31_setup.c		\
 				plat/arm/common/arm_pm.c			\
 				plat/arm/common/arm_topology.c			\
+				plat/arm/common/execution_state_switch.c	\
 				plat/common/plat_psci_common.c
 
 ifeq (${ENABLE_PMF}, 1)
diff --git a/plat/arm/common/arm_sip_svc.c b/plat/arm/common/arm_sip_svc.c
index eb8ec9e..62a2ef5 100644
--- a/plat/arm/common/arm_sip_svc.c
+++ b/plat/arm/common/arm_sip_svc.c
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2016, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2016-2017, ARM Limited and Contributors. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are met:
@@ -30,6 +30,7 @@
 
 #include <arm_sip_svc.h>
 #include <debug.h>
+#include <plat_arm.h>
 #include <pmf.h>
 #include <runtime_svc.h>
 #include <stdint.h>
@@ -60,6 +61,8 @@
 			void *handle,
 			u_register_t flags)
 {
+	int call_count = 0;
+
 	/*
 	 * Dispatch PMF calls to PMF SMC handler and return its return
 	 * value
@@ -70,12 +73,34 @@
 	}
 
 	switch (smc_fid) {
-	case ARM_SIP_SVC_CALL_COUNT:
+	case ARM_SIP_SVC_EXE_STATE_SWITCH: {
+		u_register_t pc;
+
+		/* Allow calls from non-secure only */
+		if (!is_caller_non_secure(flags))
+			SMC_RET1(handle, STATE_SW_E_DENIED);
+
+		/* Validate supplied entry point */
+		pc = (u_register_t) ((x1 << 32) | (uint32_t) x2);
+		if (arm_validate_ns_entrypoint(pc))
+			SMC_RET1(handle, STATE_SW_E_PARAM);
+
 		/*
-		 * Return the number of SiP Service Calls. PMF is the only
-		 * SiP service implemented; so return number of PMF calls
+		 * Pointers used in execution state switch are all 32 bits wide
 		 */
-		SMC_RET1(handle, PMF_NUM_SMC_CALLS);
+		return arm_execution_state_switch(smc_fid, (uint32_t) x1,
+				(uint32_t) x2, (uint32_t) x3, (uint32_t) x4,
+				handle);
+		}
+
+	case ARM_SIP_SVC_CALL_COUNT:
+		/* PMF calls */
+		call_count += PMF_NUM_SMC_CALLS;
+
+		/* State switch call */
+		call_count += 1;
+
+		SMC_RET1(handle, call_count);
 
 	case ARM_SIP_SVC_UID:
 		/* Return UID to the caller */
diff --git a/plat/arm/common/execution_state_switch.c b/plat/arm/common/execution_state_switch.c
new file mode 100644
index 0000000..5068cdf
--- /dev/null
+++ b/plat/arm/common/execution_state_switch.c
@@ -0,0 +1,197 @@
+/*
+ * Copyright (c) 2017, ARM Limited and Contributors. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * Redistributions of source code must retain the above copyright notice, this
+ * list of conditions and the following disclaimer.
+ *
+ * Redistributions in binary form must reproduce the above copyright notice,
+ * this list of conditions and the following disclaimer in the documentation
+ * and/or other materials provided with the distribution.
+ *
+ * Neither the name of ARM nor the names of its contributors may be used
+ * to endorse or promote products derived from this software without specific
+ * prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
+ * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <arch_helpers.h>
+#include <arm_sip_svc.h>
+#include <context.h>
+#include <context_mgmt.h>
+#include <plat_arm.h>
+#include <psci.h>
+#include <smcc_helpers.h>
+#include <string.h>
+#include <utils.h>
+
+/*
+ * Handle SMC from a lower exception level to switch its execution state
+ * (either from AArch64 to AArch32, or vice versa).
+ *
+ * smc_fid:
+ *	SMC function ID - either ARM_SIP_SVC_STATE_SWITCH_64 or
+ *	ARM_SIP_SVC_STATE_SWITCH_32.
+ * pc_hi, pc_lo:
+ *	PC upon re-entry to the calling exception level; width dependent on the
+ *	calling exception level.
+ * cookie_hi, cookie_lo:
+ *	Opaque pointer pairs received from the caller to pass it back, upon
+ *	re-entry.
+ * handle:
+ *	Handle to saved context.
+ */
+int arm_execution_state_switch(unsigned int smc_fid,
+		uint32_t pc_hi,
+		uint32_t pc_lo,
+		uint32_t cookie_hi,
+		uint32_t cookie_lo,
+		void *handle)
+{
+	/* Execution state can be switched only if EL3 is AArch64 */
+#ifdef AARCH64
+	int caller_64, from_el2, el, endianness, thumb = 0;
+	u_register_t spsr, pc, scr, sctlr;
+	entry_point_info_t ep;
+	cpu_context_t *ctx = (cpu_context_t *) handle;
+	el3_state_t *el3_ctx = get_el3state_ctx(ctx);
+
+	/* That the SMC originated from NS is already validated by the caller */
+
+	/*
+	 * Disallow state switch if any of the secondaries have been brought up.
+	 */
+	if (psci_secondaries_brought_up())
+		goto exec_denied;
+
+	spsr = read_ctx_reg(el3_ctx, CTX_SPSR_EL3);
+	caller_64 = (GET_RW(spsr) == MODE_RW_64);
+
+	if (caller_64) {
+		/*
+		 * If the call originated from AArch64, expect 32-bit pointers when
+		 * switching to AArch32.
+		 */
+		if ((pc_hi != 0) || (cookie_hi != 0))
+			goto invalid_param;
+
+		pc = pc_lo;
+
+		/* Instruction state when entering AArch32 */
+		thumb = pc & 1;
+	} else {
+		/* Construct AArch64 PC */
+		pc = (((u_register_t) pc_hi) << 32) | pc_lo;
+	}
+
+	/* Make sure PC is 4-byte aligned, except for Thumb */
+	if ((pc & 0x3) && !thumb)
+		goto invalid_param;
+
+	/*
+	 * EL3 controls register width of the immediate lower EL only. Expect
+	 * this request from EL2/Hyp unless:
+	 *
+	 * - EL2 is not implemented;
+	 * - EL2 is implemented, but was disabled. This can be inferred from
+	 *   SCR_EL3.HCE.
+	 */
+	from_el2 = caller_64 ? (GET_EL(spsr) == MODE_EL2) :
+		(GET_M32(spsr) == MODE32_hyp);
+	scr = read_ctx_reg(el3_ctx, CTX_SCR_EL3);
+	if (!from_el2) {
+		/* The call is from NS privilege level other than HYP */
+
+		/*
+		 * Disallow switching state if there's a Hypervisor in place;
+		 * this request must be taken up with the Hypervisor instead.
+		 */
+		if (scr & SCR_HCE_BIT)
+			goto exec_denied;
+	}
+
+	/*
+	 * Return to the caller using the same endianness. Extract
+	 * endianness bit from the respective system control register
+	 * directly.
+	 */
+	sctlr = from_el2 ? read_sctlr_el2() : read_sctlr_el1();
+	endianness = !!(sctlr & SCTLR_EE_BIT);
+
+	/* Construct SPSR for the exception state we're about to switch to */
+	if (caller_64) {
+		int impl;
+
+		/*
+		 * Switching from AArch64 to AArch32. Ensure this CPU implements
+		 * the target EL in AArch32.
+		 */
+		impl = from_el2 ? EL_IMPLEMENTED(2) : EL_IMPLEMENTED(1);
+		if (impl != EL_IMPL_A64_A32)
+			goto exec_denied;
+
+		/* Return to the equivalent AArch32 privilege level */
+		el = from_el2 ? MODE32_hyp : MODE32_svc;
+		spsr = SPSR_MODE32(el, thumb ? SPSR_T_THUMB : SPSR_T_ARM,
+				endianness, DISABLE_ALL_EXCEPTIONS);
+	} else {
+		/*
+		 * Switching from AArch32 to AArch64. Since it's not possible to
+		 * implement an EL as AArch32-only (from which this call was
+		 * raised), it's safe to assume AArch64 is also implemented.
+		 */
+		el = from_el2 ? MODE_EL2 : MODE_EL1;
+		spsr = SPSR_64(el, MODE_SP_ELX, DISABLE_ALL_EXCEPTIONS);
+	}
+
+	/*
+	 * Use the context management library to re-initialize the existing
+	 * context with the execution state flipped. Since the library takes
+	 * entry_point_info_t pointer as the argument, construct a dummy one
+	 * with PC, state width, endianness, security etc. appropriately set.
+	 * Other entries in the entry point structure are irrelevant for
+	 * purpose.
+	 */
+	zeromem(&ep, sizeof(ep));
+	ep.pc = pc;
+	ep.spsr = spsr;
+	SET_PARAM_HEAD(&ep, PARAM_EP, VERSION_1,
+			((endianness ? EP_EE_BIG : EP_EE_LITTLE) | NON_SECURE |
+			 EP_ST_DISABLE));
+
+	/*
+	 * Re-initialize the system register context, and exit EL3 as if for the
+	 * first time. State switch is effectively a soft reset of the
+	 * calling EL.
+	 */
+	cm_init_my_context(&ep);
+	cm_prepare_el3_exit(NON_SECURE);
+
+	/*
+	 * State switch success. The caller of SMC wouldn't see the SMC
+	 * returning. Instead, execution starts at the supplied entry point,
+	 * with context pointers populated in registers 0 and 1.
+	 */
+	SMC_RET2(handle, cookie_hi, cookie_lo);
+
+invalid_param:
+	SMC_RET1(handle, STATE_SW_E_PARAM);
+
+exec_denied:
+#endif
+	/* State switch denied */
+	SMC_RET1(handle, STATE_SW_E_DENIED);
+}