diff --git a/cmake/install.cmake b/cmake/install.cmake
index ec22027..ab6292a 100644
--- a/cmake/install.cmake
+++ b/cmake/install.cmake
@@ -118,6 +118,8 @@
                         ${INTERFACE_SRC_DIR}/multi_core/tfm_multi_core_ns_api.c
                         ${INTERFACE_SRC_DIR}/multi_core/tfm_multi_core_psa_ns_api.c
                         ${INTERFACE_SRC_DIR}/multi_core/tfm_ns_mailbox_rtos_api.c
+                        ${INTERFACE_SRC_DIR}/multi_core/tfm_ns_mailbox_thread.c
+                        ${INTERFACE_SRC_DIR}/multi_core/tfm_ns_mailbox_test.c
             DESTINATION ${INSTALL_INTERFACE_SRC_DIR})
 else()
     install(FILES       ${INTERFACE_SRC_DIR}/tfm_ns_interface.c
diff --git a/interface/include/multi_core/tfm_mailbox.h b/interface/include/multi_core/tfm_mailbox.h
index 5824543..74527a2 100644
--- a/interface/include/multi_core/tfm_mailbox.h
+++ b/interface/include/multi_core/tfm_mailbox.h
@@ -93,20 +93,28 @@
  * to hold the PSA client call return result from SPE
  */
 struct mailbox_reply_t {
-    int32_t return_val;
+    int32_t    return_val;
+    const void *owner;                      /* Handle of owner task. */
+    int32_t    *reply;                      /* Address of reply value belonging
+                                             * to owner task.
+                                             */
+#ifdef TFM_MULTI_CORE_NS_OS_MAILBOX_THREAD
+    uint8_t    *woken_flag;                 /* Indicate that owner task has been
+                                             * or should be woken up, after the
+                                             * reply is received.
+                                             */
+#else
+    bool        is_woken;                   /* Indicate that owner task has been
+                                             * or should be woken up, after the
+                                             * reply is received.
+                                             */
+#endif
 };
 
 /* A single slot structure in NSPE mailbox queue */
 struct ns_mailbox_slot_t {
     struct mailbox_msg_t   msg;
     struct mailbox_reply_t reply;
-    const void             *owner;          /* Handle of the owner task of this
-                                             * slot
-                                             */
-    bool                   is_woken;        /* Indicate that owner task has been
-                                             * or should be woken up, after the
-                                             * replied is received.
-                                             */
 };
 
 typedef uint32_t   mailbox_queue_status_t;
@@ -135,6 +143,8 @@
                                                  * NS thread requests a mailbox
                                                  * queue slot.
                                                  */
+
+    bool                     is_full;           /* Queue if full */
 #endif
 };
 
diff --git a/interface/include/multi_core/tfm_ns_mailbox.h b/interface/include/multi_core/tfm_ns_mailbox.h
index 69d0b29..7ba15bf 100644
--- a/interface/include/multi_core/tfm_ns_mailbox.h
+++ b/interface/include/multi_core/tfm_ns_mailbox.h
@@ -12,6 +12,8 @@
 
 #include <stdbool.h>
 #include <stdint.h>
+
+#include "cmsis_compiler.h"
 #include "tfm_mailbox.h"
 
 #ifdef __cplusplus
@@ -88,6 +90,20 @@
 }
 #endif
 
+#ifdef TFM_MULTI_CORE_NS_OS_MAILBOX_THREAD
+/**
+ * \brief Handling PSA client calls in a dedicated NS mailbox thread.
+ *        This function constructs NS mailbox messages, transmits them to SPE
+ *        mailbox and returns the results to NS PSA client.
+ *
+ * \param[in] args              The pointer to the structure of PSA client call
+ *                              parameters.
+ */
+void tfm_ns_mailbox_thread_runner(void *args);
+#else /* TFM_MULTI_CORE_NS_OS_MAILBOX_THREAD */
+#define tfm_ns_mailbox_thread_runner(args)        do {} while (0)
+#endif /* TFM_MULTI_CORE_NS_OS_MAILBOX_THREAD */
+
 /**
  * \brief Platform specific NSPE mailbox initialization.
  *        Invoked by \ref tfm_ns_mailbox_init().
@@ -202,6 +218,47 @@
  * \param[in] task_handle       The handle to the task to be woken up.
  */
 void tfm_ns_mailbox_os_wake_task_isr(const void *task_handle);
+
+/**
+ * \brief Create and initialize a message queue
+ *
+ * \param[in] msg_size        The maximum message size in bytes
+ * \param[in] msg_count       The maximum number of messages in queue
+ *
+ * \return Returns handle of the message queue created, or NULL in case of error
+ */
+void *tfm_ns_mailbox_os_mq_create(size_t msg_size, uint8_t msg_count);
+
+/**
+ * \brief Send a request via message queue
+ *
+ * \param[in] mq_handle       The handle of message queue
+ * \param[in] msg_ptr         The pointer to the message to be sent
+ *
+ * \note The message size must be the same as the value set in
+ *       \ref tfm_ns_mailbox_os_mq_create.
+ *
+ * \return \ref MAILBOX_SUCCESS if the message is successfully sent, or
+ *         other return code in case of error
+ */
+int32_t tfm_ns_mailbox_os_mq_send(void *mq_handle, const void *msg_ptr);
+
+/**
+ * \brief Receive a request from message queue
+ *
+ * \param[in] mq_handle       The handle of message queue
+ * \param[in] msg_ptr         The pointer to buffer for message to be received
+ *
+ * \return \ref MAILBOX_SUCCESS if the message is successfully received, or
+ *         other return code in case of error
+ *
+ * \note The message size is the same as the value set in
+ *       \ref tfm_ns_mailbox_os_mq_create.
+ *
+ * \note The function should be blocked until a message is received from message
+ *       queue, unless a fatal error occurs.
+ */
+int32_t tfm_ns_mailbox_os_mq_receive(void *mq_handle, void *msg_ptr);
 #else /* TFM_MULTI_CORE_NS_OS */
 #define tfm_ns_mailbox_os_wait_reply()         do {} while (0)
 
@@ -233,8 +290,28 @@
  * \brief Initialize the statistics module in TF-M NSPE mailbox.
  *
  * \note This function is only available when multi-core tests are enabled.
+ *
+ * \param[in] ns_queue          The NSPE mailbox queue to be tracked.
  */
-void tfm_ns_mailbox_tx_stats_init(void);
+void tfm_ns_mailbox_tx_stats_init(struct ns_mailbox_queue_t *ns_queue);
+
+/**
+ * \brief Re-initialize the statistics module in TF-M NSPE mailbox.
+ *        Clean up statistics data.
+ *
+ * \note This function is only available when multi-core tests are enabled.
+ *
+ * \return \ref MAILBOX_SUCCESS if the operation succeeded, or other return code
+           in case of error
+ */
+int32_t tfm_ns_mailbox_tx_stats_reinit(void);
+
+/**
+ * \brief Update the statistics result of NSPE mailbox message transmission.
+ *
+ * \note This function is only available when multi-core tests are enabled.
+ */
+void tfm_ns_mailbox_tx_stats_update(void);
 
 /**
  * \brief Calculate the average number of used NS mailbox queue slots each time
@@ -251,6 +328,58 @@
 void tfm_ns_mailbox_stats_avg_slot(struct ns_mailbox_stats_res_t *stats_res);
 #endif
 
+#ifdef TFM_MULTI_CORE_NS_OS
+/*
+ * When NSPE mailbox only covers a single non-secure core, spinlock only
+ * requires to disable IRQ.
+ */
+static inline void ns_mailbox_spin_lock(void)
+{
+    __disable_irq();
+}
+
+/*
+ * It is assumed that IRQ is always enabled when spinlock is acquired.
+ * Otherwise, the waiting thread won't be woken up.
+ */
+static inline void ns_mailbox_spin_unlock(void)
+{
+    __enable_irq();
+}
+#else /* TFM_MULTI_CORE_NS_OS */
+/*
+ * Local spinlock is implemented as a dummy one when integrating with NS bare
+ * metal environment since interrupt is not required in NS mailbox.
+ */
+#define ns_mailbox_spin_lock()   do {} while (0)
+
+#define ns_mailbox_spin_unlock() do {} while (0)
+#endif /* TFM_MULTI_CORE_NS_OS */
+
+/* The following inline functions configure non-secure mailbox queue status */
+static inline void clear_queue_slot_empty(struct ns_mailbox_queue_t *queue_ptr,
+                                          uint8_t idx)
+{
+    if (idx < NUM_MAILBOX_QUEUE_SLOT) {
+        queue_ptr->empty_slots &= ~(1UL << idx);
+    }
+}
+
+static inline void set_queue_slot_pend(struct ns_mailbox_queue_t *queue_ptr,
+                                       uint8_t idx)
+{
+    if (idx < NUM_MAILBOX_QUEUE_SLOT) {
+        queue_ptr->pend_slots |= (1UL << idx);
+    }
+}
+
+static inline void clear_queue_slot_all_replied(
+                                           struct ns_mailbox_queue_t *queue_ptr,
+                                           mailbox_queue_status_t status)
+{
+    queue_ptr->replied_slots &= ~status;
+}
+
 #ifdef __cplusplus
 }
 #endif
diff --git a/interface/src/multi_core/tfm_ns_mailbox.c b/interface/src/multi_core/tfm_ns_mailbox.c
index 90836ff..922e5fe 100644
--- a/interface/src/multi_core/tfm_ns_mailbox.c
+++ b/interface/src/multi_core/tfm_ns_mailbox.c
@@ -7,7 +7,6 @@
 
 #include <string.h>
 
-#include "cmsis_compiler.h"
 #include "tfm_ns_mailbox.h"
 
 /* The pointer to NSPE mailbox queue */
@@ -15,39 +14,43 @@
 
 static int32_t mailbox_wait_reply(uint8_t idx);
 
-static inline void clear_queue_slot_empty(uint8_t idx)
-{
-    if (idx < NUM_MAILBOX_QUEUE_SLOT) {
-        mailbox_queue_ptr->empty_slots &= ~(1 << idx);
-    }
-}
-
 static inline void set_queue_slot_empty(uint8_t idx)
 {
     if (idx < NUM_MAILBOX_QUEUE_SLOT) {
-        mailbox_queue_ptr->empty_slots |= (1 << idx);
+        mailbox_queue_ptr->empty_slots |= (1UL << idx);
     }
 }
 
-static inline void set_queue_slot_pend(uint8_t idx)
+static inline void set_queue_slot_woken(uint8_t idx)
 {
     if (idx < NUM_MAILBOX_QUEUE_SLOT) {
-        mailbox_queue_ptr->pend_slots |= (1 << idx);
+        mailbox_queue_ptr->queue[idx].reply.is_woken = true;
+    }
+}
+
+static inline bool is_queue_slot_woken(uint8_t idx)
+{
+    if (idx < NUM_MAILBOX_QUEUE_SLOT) {
+        return mailbox_queue_ptr->queue[idx].reply.is_woken;
+    }
+
+    return false;
+}
+
+static inline void clear_queue_slot_woken(uint8_t idx)
+{
+    if (idx < NUM_MAILBOX_QUEUE_SLOT) {
+        mailbox_queue_ptr->queue[idx].reply.is_woken = false;
     }
 }
 
 static inline void clear_queue_slot_replied(uint8_t idx)
 {
     if (idx < NUM_MAILBOX_QUEUE_SLOT) {
-        mailbox_queue_ptr->replied_slots &= ~(1 << idx);
+        mailbox_queue_ptr->replied_slots &= ~(1UL << idx);
     }
 }
 
-static inline void clear_queue_slot_all_replied(mailbox_queue_status_t status)
-{
-    mailbox_queue_ptr->replied_slots &= ~status;
-}
-
 static inline bool is_queue_slot_replied(uint8_t idx)
 {
     if (idx < NUM_MAILBOX_QUEUE_SLOT) {
@@ -57,57 +60,6 @@
     return false;
 }
 
-static inline void set_queue_slot_woken(uint8_t idx)
-{
-    if (idx < NUM_MAILBOX_QUEUE_SLOT) {
-        mailbox_queue_ptr->queue[idx].is_woken = true;
-    }
-}
-
-static inline bool is_queue_slot_woken(uint8_t idx)
-{
-    if (idx < NUM_MAILBOX_QUEUE_SLOT) {
-        return mailbox_queue_ptr->queue[idx].is_woken;
-    }
-
-    return false;
-}
-
-static inline void clear_queue_slot_woken(uint8_t idx)
-{
-    if (idx < NUM_MAILBOX_QUEUE_SLOT) {
-        mailbox_queue_ptr->queue[idx].is_woken = false;
-    }
-}
-
-#ifdef TFM_MULTI_CORE_NS_OS
-/*
- * When NSPE mailbox only covers a single non-secure core, spinlock only
- * requires to disable IRQ.
- */
-static inline void ns_mailbox_spin_lock(void)
-{
-    __disable_irq();
-}
-
-/*
- * It is assumed that IRQ is always enabled when spinlock is acquired.
- * Otherwise, the waiting thread won't be woken up.
- */
-static inline void ns_mailbox_spin_unlock(void)
-{
-    __enable_irq();
-}
-#else /* TFM_MULTI_CORE_NS_OS */
-/*
- * Local spinlock is implemented as a dummy one when integrating with NS bare
- * metal environment since interrupt is not required in NS mailbox.
- */
-#define ns_mailbox_spin_lock()   do {} while (0)
-
-#define ns_mailbox_spin_unlock() do {} while (0)
-#endif /* TFM_MULTI_CORE_NS_OS */
-
 static uint8_t acquire_empty_slot(struct ns_mailbox_queue_t *queue)
 {
     uint8_t idx;
@@ -129,7 +81,7 @@
     }
 
     ns_mailbox_spin_lock();
-    clear_queue_slot_empty(idx);
+    clear_queue_slot_empty(queue, idx);
     ns_mailbox_spin_unlock();
 
     return idx;
@@ -138,66 +90,10 @@
 static void set_msg_owner(uint8_t idx, const void *owner)
 {
     if (idx < NUM_MAILBOX_QUEUE_SLOT) {
-        mailbox_queue_ptr->queue[idx].owner = owner;
+        mailbox_queue_ptr->queue[idx].reply.owner = owner;
     }
 }
 
-#ifdef TFM_MULTI_CORE_TEST
-void tfm_ns_mailbox_tx_stats_init(void)
-{
-    if (!mailbox_queue_ptr) {
-        return;
-    }
-
-    mailbox_queue_ptr->nr_tx = 0;
-    mailbox_queue_ptr->nr_used_slots = 0;
-}
-
-static void mailbox_tx_stats_update(struct ns_mailbox_queue_t *ns_queue)
-{
-    mailbox_queue_status_t empty_status;
-    uint8_t idx, nr_empty = 0;
-
-    if (!ns_queue) {
-        return;
-    }
-
-    ns_mailbox_spin_lock();
-    /* Count the number of used slots when this tx arrives */
-    empty_status = ns_queue->empty_slots;
-    ns_mailbox_spin_unlock();
-
-    if (empty_status) {
-        for (idx = 0; idx < NUM_MAILBOX_QUEUE_SLOT; idx++) {
-            if (empty_status & (0x1UL << idx)) {
-                nr_empty++;
-            }
-        }
-    }
-
-    ns_mailbox_spin_lock();
-    ns_queue->nr_used_slots += (NUM_MAILBOX_QUEUE_SLOT - nr_empty);
-    ns_queue->nr_tx++;
-    ns_mailbox_spin_unlock();
-}
-
-void tfm_ns_mailbox_stats_avg_slot(struct ns_mailbox_stats_res_t *stats_res)
-{
-    uint32_t nr_used_slots, nr_tx;
-
-    if (!mailbox_queue_ptr || !stats_res) {
-        return;
-    }
-
-    nr_used_slots = mailbox_queue_ptr->nr_used_slots;
-    nr_tx = mailbox_queue_ptr->nr_tx;
-
-    stats_res->avg_nr_slots = nr_used_slots / nr_tx;
-    nr_used_slots %= nr_tx;
-    stats_res->avg_nr_slots_tenths = nr_used_slots * 10 / nr_tx;
-}
-#endif
-
 static int32_t mailbox_tx_client_req(uint32_t call_type,
                                      const struct psa_client_params_t *params,
                                      int32_t client_id,
@@ -213,7 +109,7 @@
     }
 
 #ifdef TFM_MULTI_CORE_TEST
-    mailbox_tx_stats_update(mailbox_queue_ptr);
+    tfm_ns_mailbox_tx_stats_update();
 #endif
 
     /* Fill the mailbox message */
@@ -231,7 +127,7 @@
     set_msg_owner(idx, task_handle);
 
     tfm_ns_mailbox_hal_enter_critical();
-    set_queue_slot_pend(idx);
+    set_queue_slot_pend(mailbox_queue_ptr, idx);
     tfm_ns_mailbox_hal_exit_critical();
 
     tfm_ns_mailbox_hal_notify_peer();
@@ -315,7 +211,7 @@
 
     tfm_ns_mailbox_hal_enter_critical_isr();
     replied_status = mailbox_queue_ptr->replied_slots;
-    clear_queue_slot_all_replied(replied_status);
+    clear_queue_slot_all_replied(mailbox_queue_ptr, replied_status);
     tfm_ns_mailbox_hal_exit_critical_isr();
 
     if (!replied_status) {
@@ -336,7 +232,8 @@
         set_queue_slot_woken(idx);
         tfm_ns_mailbox_hal_exit_critical_isr();
 
-        tfm_ns_mailbox_os_wake_task_isr(mailbox_queue_ptr->queue[idx].owner);
+        tfm_ns_mailbox_os_wake_task_isr(
+                                     mailbox_queue_ptr->queue[idx].reply.owner);
 
         replied_status &= ~(0x1UL << idx);
         if (!replied_status) {
@@ -439,7 +336,7 @@
     ret = tfm_ns_mailbox_os_lock_init();
 
 #ifdef TFM_MULTI_CORE_TEST
-    tfm_ns_mailbox_tx_stats_init();
+    tfm_ns_mailbox_tx_stats_init(queue);
 #endif
 
     return ret;
diff --git a/interface/src/multi_core/tfm_ns_mailbox_rtos_api.c b/interface/src/multi_core/tfm_ns_mailbox_rtos_api.c
index 1ac1b4f..629e108 100644
--- a/interface/src/multi_core/tfm_ns_mailbox_rtos_api.c
+++ b/interface/src/multi_core/tfm_ns_mailbox_rtos_api.c
@@ -11,13 +11,15 @@
  * It can be replaced by RTOS specific implementation.
  */
 
+#ifdef TFM_MULTI_CORE_NS_OS_MAILBOX_THREAD
+#include "os_wrapper/msg_queue.h"
+#else
 #include "os_wrapper/semaphore.h"
+#endif
 #include "os_wrapper/thread.h"
 
 #include "tfm_ns_mailbox.h"
 
-#define MAX_SEMAPHORE_COUNT            NUM_MAILBOX_QUEUE_SLOT
-
 /*
  * Thread flag to manage wait/wake mechanism in mailbox.、
  * Thread flag can be RTOS specific.
@@ -26,7 +28,11 @@
  */
 #define MAILBOX_THREAD_FLAG            0x5FCA0000
 
+#ifndef TFM_MULTI_CORE_NS_OS_MAILBOX_THREAD
+#define MAX_SEMAPHORE_COUNT            NUM_MAILBOX_QUEUE_SLOT
+
 static void *ns_lock_handle = NULL;
+#endif
 
 const void *tfm_ns_mailbox_os_get_task_handle(void)
 {
@@ -43,6 +49,48 @@
     os_wrapper_thread_set_flag_isr((void *)task_handle, MAILBOX_THREAD_FLAG);
 }
 
+#ifdef TFM_MULTI_CORE_NS_OS_MAILBOX_THREAD
+void *tfm_ns_mailbox_os_mq_create(size_t msg_size, uint8_t msg_count)
+{
+    return os_wrapper_msg_queue_create(msg_size, msg_count);
+}
+
+int32_t tfm_ns_mailbox_os_mq_send(void *mq_handle, const void *msg_ptr)
+{
+    int32_t ret;
+
+    if (!mq_handle || !msg_ptr) {
+        return MAILBOX_INVAL_PARAMS;
+    }
+
+    while (1) {
+        ret = os_wrapper_msg_queue_send(mq_handle, msg_ptr);
+        if (ret == OS_WRAPPER_SUCCESS) {
+            return MAILBOX_SUCCESS;
+        }
+    }
+
+    return MAILBOX_GENERIC_ERROR;
+}
+
+int32_t tfm_ns_mailbox_os_mq_receive(void *mq_handle, void *msg_ptr)
+{
+    int32_t ret;
+
+    if (!mq_handle || !msg_ptr) {
+        return MAILBOX_INVAL_PARAMS;
+    }
+
+    while (1) {
+        ret = os_wrapper_msg_queue_receive(mq_handle, msg_ptr);
+        if (ret == OS_WRAPPER_SUCCESS) {
+            return MAILBOX_SUCCESS;
+        }
+    }
+
+    return MAILBOX_GENERIC_ERROR;
+}
+#else /* TFM_MULTI_CORE_NS_OS_MAILBOX_THREAD */
 int32_t tfm_ns_mailbox_os_lock_init(void)
 {
     ns_lock_handle = os_wrapper_semaphore_create(MAX_SEMAPHORE_COUNT,
@@ -65,3 +113,4 @@
 {
     return os_wrapper_semaphore_release(ns_lock_handle);
 }
+#endif /* TFM_MULTI_CORE_NS_OS_MAILBOX_THREAD */
diff --git a/interface/src/multi_core/tfm_ns_mailbox_test.c b/interface/src/multi_core/tfm_ns_mailbox_test.c
new file mode 100644
index 0000000..e12cdc9
--- /dev/null
+++ b/interface/src/multi_core/tfm_ns_mailbox_test.c
@@ -0,0 +1,78 @@
+/*
+ * Copyright (c) 2020-2021, Arm Limited. All rights reserved.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ *
+ */
+
+#include "tfm_ns_mailbox.h"
+
+static struct ns_mailbox_queue_t *stats_queue_ptr = NULL;
+
+void tfm_ns_mailbox_tx_stats_init(struct ns_mailbox_queue_t *ns_queue)
+{
+    if (!ns_queue) {
+        return;
+    }
+
+    ns_queue->nr_tx = 0;
+    ns_queue->nr_used_slots = 0;
+
+    stats_queue_ptr = ns_queue;
+}
+
+int32_t tfm_ns_mailbox_tx_stats_reinit(void)
+{
+    if (!stats_queue_ptr) {
+        return MAILBOX_INVAL_PARAMS;
+    }
+
+    stats_queue_ptr->nr_tx = 0;
+    stats_queue_ptr->nr_used_slots = 0;
+
+    return MAILBOX_SUCCESS;
+}
+
+void tfm_ns_mailbox_tx_stats_update(void)
+{
+    mailbox_queue_status_t empty_status;
+    uint8_t idx, nr_empty = 0;
+
+    if (!stats_queue_ptr) {
+        return;
+    }
+
+    ns_mailbox_spin_lock();
+    /* Count the number of used slots when this tx arrives */
+    empty_status = stats_queue_ptr->empty_slots;
+    ns_mailbox_spin_unlock();
+
+    if (empty_status) {
+        for (idx = 0; idx < NUM_MAILBOX_QUEUE_SLOT; idx++) {
+            if (empty_status & (0x1UL << idx)) {
+                nr_empty++;
+            }
+        }
+    }
+
+    ns_mailbox_spin_lock();
+    stats_queue_ptr->nr_used_slots += (NUM_MAILBOX_QUEUE_SLOT - nr_empty);
+    stats_queue_ptr->nr_tx++;
+    ns_mailbox_spin_unlock();
+}
+
+void tfm_ns_mailbox_stats_avg_slot(struct ns_mailbox_stats_res_t *stats_res)
+{
+    uint32_t nr_used_slots, nr_tx;
+
+    if (!stats_queue_ptr || !stats_res) {
+        return;
+    }
+
+    nr_used_slots = stats_queue_ptr->nr_used_slots;
+    nr_tx = stats_queue_ptr->nr_tx;
+
+    stats_res->avg_nr_slots = nr_used_slots / nr_tx;
+    nr_used_slots %= nr_tx;
+    stats_res->avg_nr_slots_tenths = nr_used_slots * 10 / nr_tx;
+}
diff --git a/interface/src/multi_core/tfm_ns_mailbox_thread.c b/interface/src/multi_core/tfm_ns_mailbox_thread.c
new file mode 100644
index 0000000..6a77b1b
--- /dev/null
+++ b/interface/src/multi_core/tfm_ns_mailbox_thread.c
@@ -0,0 +1,353 @@
+/*
+ * Copyright (c) 2020-2021, Arm Limited. All rights reserved.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ *
+ */
+
+#include <string.h>
+
+#include "tfm_ns_mailbox.h"
+
+/* Thread woken up flag */
+#define NOT_WOKEN        0x0
+#define WOKEN_UP        0x5C
+
+/*
+ * The request contains the parameters which application threads share with
+ * NS mailbox thread.
+ */
+struct ns_mailbox_req_t {
+    uint32_t                         call_type;   /* PSA client call type */
+    const struct psa_client_params_t *params_ptr; /* Pointer to PSA client call
+                                                   * parameters.
+                                                   */
+    int32_t                          client_id;   /* Optional client ID of the
+                                                   * non-secure caller.
+                                                   * It is required to identify
+                                                   * the non-secure task when
+                                                   * NSPE OS enforces non-secure
+                                                   * task isolation
+                                                   */
+    const void                       *owner;      /* Handle of owner task. */
+    int32_t                          *reply;      /* Address of reply value
+                                                   * belonging to owner task.
+                                                   */
+
+    uint8_t                          *woken_flag; /* Indicate that owner task
+                                                   * has been or should be woken
+                                                   * up, after the reply is
+                                                   * received.
+                                                   */
+};
+
+/* Message queue handle */
+static void *msgq_handle = NULL;
+
+/* The handle of the dedicated NS mailbox thread. */
+static const void *ns_mailbox_thread_handle = NULL;
+
+/* The pointer to NSPE mailbox queue */
+static struct ns_mailbox_queue_t *mailbox_queue_ptr = NULL;
+
+static inline void set_queue_slot_all_empty(mailbox_queue_status_t completed)
+{
+    mailbox_queue_ptr->empty_slots |= completed;
+}
+
+static inline void set_queue_slot_woken(uint8_t idx)
+{
+    if (idx < NUM_MAILBOX_QUEUE_SLOT) {
+        *mailbox_queue_ptr->queue[idx].reply.woken_flag = WOKEN_UP;
+    }
+}
+
+static uint8_t acquire_empty_slot(struct ns_mailbox_queue_t *queue)
+{
+    uint8_t idx;
+    mailbox_queue_status_t status;
+
+    while (1) {
+        ns_mailbox_spin_lock();
+        status = queue->empty_slots;
+        ns_mailbox_spin_unlock();
+
+        if (status) {
+            break;
+        }
+
+        /* No empty slot */
+        queue->is_full = true;
+        /* DSB to make sure the thread sleeps after the flag is set */
+        __DSB();
+
+        /* Wait for an empty slot released by a completed mailbox message */
+        tfm_ns_mailbox_os_wait_reply();
+        queue->is_full = false;
+    }
+
+    for (idx = 0; idx < NUM_MAILBOX_QUEUE_SLOT; idx++) {
+        if (status & (1 << idx)) {
+            break;
+        }
+    }
+
+    ns_mailbox_spin_lock();
+    clear_queue_slot_empty(queue, idx);
+    ns_mailbox_spin_unlock();
+
+    return idx;
+}
+
+static int32_t mailbox_tx_client_call_msg(const struct ns_mailbox_req_t *req,
+                                          uint8_t *slot_idx)
+{
+    struct mailbox_msg_t *msg_ptr;
+    struct mailbox_reply_t *reply_ptr;
+    uint8_t idx = NUM_MAILBOX_QUEUE_SLOT;
+
+    idx = acquire_empty_slot(mailbox_queue_ptr);
+    if (idx == NUM_MAILBOX_QUEUE_SLOT) {
+        return MAILBOX_QUEUE_FULL;
+    }
+
+#ifdef TFM_MULTI_CORE_TEST
+    tfm_ns_mailbox_tx_stats_update();
+#endif
+
+    /* Fill the mailbox message */
+    msg_ptr = &mailbox_queue_ptr->queue[idx].msg;
+    msg_ptr->call_type = req->call_type;
+    memcpy(&msg_ptr->params, req->params_ptr, sizeof(msg_ptr->params));
+    msg_ptr->client_id = req->client_id;
+
+    /* Prepare the reply structure */
+    reply_ptr = &mailbox_queue_ptr->queue[idx].reply;
+    reply_ptr->owner = req->owner;
+    reply_ptr->reply = req->reply;
+    reply_ptr->woken_flag = req->woken_flag;
+
+    /*
+     * Memory check can be added here to prevent a malicious application
+     * from providing addresses of other applications or privileged area.
+     */
+
+    tfm_ns_mailbox_hal_enter_critical();
+    set_queue_slot_pend(mailbox_queue_ptr, idx);
+    tfm_ns_mailbox_hal_exit_critical();
+
+    tfm_ns_mailbox_hal_notify_peer();
+
+    if (slot_idx) {
+        *slot_idx = idx;
+    }
+
+    return MAILBOX_SUCCESS;
+}
+
+static inline void ns_mailbox_set_reply_isr(uint8_t idx)
+{
+    int32_t *reply_ptr = mailbox_queue_ptr->queue[idx].reply.reply;
+
+    if (reply_ptr) {
+        *reply_ptr = mailbox_queue_ptr->queue[idx].reply.return_val;
+    }
+}
+
+static int32_t mailbox_wait_reply(const struct ns_mailbox_req_t *req)
+{
+    while (1) {
+        /*
+         * Check the completed flag to make sure that the current thread is
+         * woken up by reply event, rather than other events.
+         */
+        if (*req->woken_flag == WOKEN_UP) {
+            break;
+        }
+
+        /* Woken up from sleep */
+        tfm_ns_mailbox_os_wait_reply();
+    }
+
+    return MAILBOX_SUCCESS;
+}
+
+int32_t tfm_ns_mailbox_client_call(uint32_t call_type,
+                                   const struct psa_client_params_t *params,
+                                   int32_t client_id,
+                                   int32_t *reply)
+{
+    struct ns_mailbox_req_t req;
+    uint8_t woken_flag = NOT_WOKEN;
+    int32_t ret;
+
+    if (!mailbox_queue_ptr) {
+        return MAILBOX_INIT_ERROR;
+    }
+
+    if (!params || !reply) {
+        return MAILBOX_INVAL_PARAMS;
+    }
+
+    req.call_type = call_type;
+    req.params_ptr = params;
+    req.reply = reply;
+    req.woken_flag = &woken_flag;
+    req.owner = tfm_ns_mailbox_os_get_task_handle();
+    req.client_id = client_id;
+
+    ret = tfm_ns_mailbox_os_mq_send(msgq_handle, &req);
+    if (ret != MAILBOX_SUCCESS) {
+        return ret;
+    }
+
+    ret = mailbox_wait_reply(&req);
+
+    return ret;
+}
+
+void tfm_ns_mailbox_thread_runner(void *args)
+{
+    struct ns_mailbox_req_t req;
+    int32_t ret;
+
+    (void)args;
+
+    ns_mailbox_thread_handle = tfm_ns_mailbox_os_get_task_handle();
+
+    while (1) {
+        ret = tfm_ns_mailbox_os_mq_receive(msgq_handle, &req);
+        if (ret != MAILBOX_SUCCESS) {
+            continue;
+        }
+
+        /*
+         * Invalid client address. However, the pointer was already
+         * checked previously and therefore just simply ignore this
+         * client call request.
+         */
+        if (!req.params_ptr || !req.reply || !req.woken_flag) {
+            continue;
+        }
+
+        mailbox_tx_client_call_msg(&req, NULL);
+    }
+}
+
+int32_t tfm_ns_mailbox_wake_reply_owner_isr(void)
+{
+    uint8_t idx;
+    const void *task_handle;
+    mailbox_queue_status_t replied_status, complete_slots = 0x0;
+
+    if (!mailbox_queue_ptr) {
+        return MAILBOX_INIT_ERROR;
+    }
+
+    tfm_ns_mailbox_hal_enter_critical_isr();
+    replied_status = mailbox_queue_ptr->replied_slots;
+    clear_queue_slot_all_replied(mailbox_queue_ptr, replied_status);
+    tfm_ns_mailbox_hal_exit_critical_isr();
+
+    if (!replied_status) {
+        return MAILBOX_NO_PEND_EVENT;
+    }
+
+    for (idx = 0; idx < NUM_MAILBOX_QUEUE_SLOT; idx++) {
+        /*
+         * The reply has already received from SPE mailbox but
+         * the wake-up signal is not sent yet.
+         */
+        if (!(replied_status & (0x1UL << idx))) {
+            continue;
+        }
+
+        /*
+         * Write back the return result.
+         * When TFM_MULTI_CORE_NS_OS_MAILBOX_THREAD is enabled, a reply is
+         * returned inside ns_mailbox_set_reply_isr().
+         * When TFM_MULTI_CORE_NS_OS_MAILBOX_THREAD is disabled, a reply is
+         * returned inside mailbox_rx_client_reply(). ns_mailbox_set_reply_isr()
+         * is defined as dummy function.
+         */
+        ns_mailbox_set_reply_isr(idx);
+
+        /* Wake up the owner of this mailbox message */
+        set_queue_slot_woken(idx);
+
+        task_handle = mailbox_queue_ptr->queue[idx].reply.owner;
+        if (task_handle) {
+            tfm_ns_mailbox_os_wake_task_isr(task_handle);
+        }
+
+        complete_slots |= (1UL << idx);
+
+        replied_status &= ~(0x1UL << idx);
+        if (!replied_status) {
+            break;
+        }
+    }
+
+    set_queue_slot_all_empty(complete_slots);
+
+    /*
+     * Wake up the NS mailbox thread in case it is waiting for
+     * empty slots.
+     */
+    if (mailbox_queue_ptr->is_full) {
+        if (ns_mailbox_thread_handle) {
+            tfm_ns_mailbox_os_wake_task_isr(ns_mailbox_thread_handle);
+        }
+    }
+
+    return MAILBOX_SUCCESS;
+}
+
+static inline int32_t mailbox_req_queue_init(uint8_t queue_depth)
+{
+    msgq_handle = tfm_ns_mailbox_os_mq_create(sizeof(struct ns_mailbox_req_t),
+                                              queue_depth);
+    if (!msgq_handle) {
+        return MAILBOX_GENERIC_ERROR;
+    }
+
+    return MAILBOX_SUCCESS;
+}
+
+int32_t tfm_ns_mailbox_init(struct ns_mailbox_queue_t *queue)
+{
+    int32_t ret;
+
+    if (!queue) {
+        return MAILBOX_INVAL_PARAMS;
+    }
+
+    /*
+     * Further verification of mailbox queue address may be required according
+     * to non-secure memory assignment.
+     */
+
+    memset(queue, 0, sizeof(*queue));
+
+    /* Initialize empty bitmask */
+    queue->empty_slots =
+            (mailbox_queue_status_t)((1UL << (NUM_MAILBOX_QUEUE_SLOT - 1)) - 1);
+    queue->empty_slots +=
+            (mailbox_queue_status_t)(1UL << (NUM_MAILBOX_QUEUE_SLOT - 1));
+
+    mailbox_queue_ptr = queue;
+
+    /* Platform specific initialization. */
+    ret = tfm_ns_mailbox_hal_init(queue);
+    if (ret != MAILBOX_SUCCESS) {
+        return ret;
+    }
+
+    ret = mailbox_req_queue_init(NUM_MAILBOX_QUEUE_SLOT);
+
+#ifdef TFM_MULTI_CORE_TEST
+    tfm_ns_mailbox_tx_stats_init(queue);
+#endif
+
+    return ret;
+}