Fix syscall handling in the kernel loop

Author: shengwen-tw

arch/v7m_entry.S

@@ -42,8 +42,6 @@ ENTRY(PendSV_Handler)
     /* Disable interrupts */
     irq_disable
 
-    neg   r7,  r7 /* Negate _r7 to indicate that the returning is triggered by SysTick */
-
     /* Save user state */
     mrs   r0,  psp  /* Load psp into the r0 */
 
@@ -87,11 +85,16 @@ ENTRY(SVC_Handler)
     msr  control, r4
 
     /* Load kernel state */
-    pop   {r4, r5, r6, r7, r8, r9, r10, r11, ip, lr}
-    msr   psr_nzcvq, ip /* Load psr from the ip */
+    pop  {r4, r5, r6, r7, r8, r9, r10, r11, ip, lr}
+    msr  psr_nzcvq, ip /* Load psr from the ip */
+
+    /* Set syscall request flag */
+    push {r0-r3, lr}
+    bl   set_syscall_flag 
+    pop  {r0-r3, lr}
 
     /* Exception return (back to the kernel) */
-    bx    lr
+    bx   lr
 ENDPROC(SVC_Handler)
 
 ENTRY(__preempt_disable)

arch/v7m_port.c

@@ -148,29 +148,6 @@ void __platform_init(void)
     os_env_init(&stack_empty[31]);
 }
 
-bool check_systick_event(void *sp)
-{
-    uint32_t lr = ((uint32_t *) sp)[8];
-    long *syscall_num = NULL;
-
-    /* EXC_RETURN[4]: 0 = FPU used / 1 = FPU unused */
-    if (lr & 0x10) {
-        syscall_num = (long *) &((struct context *) sp)->_r7;
-    } else {
-        syscall_num = (long *) &((struct context_fpu *) sp)->_r7;
-    }
-
-    /* If r7 is negative, then the kernel is returned from the SysTick
-     * interrupt. Otherwise the kernel is returned via supervisor call.
-     */
-    if (*syscall_num < 0) {
-        *syscall_num *= -1; /* Restore the syscall number */
-        return true;
-    }
-
-    return false;
-}
-
 unsigned long get_syscall_num(void *sp)
 {
     uint32_t lr = ((uint32_t *) sp)[8];

include/arch/port.h

@@ -76,13 +76,6 @@ void __stack_init(uint32_t **stack_top,
                   uint32_t return_handler,
                   uint32_t args[4]);
 
-/**
- * @brief  Check if the kernel is returned from user space via systick
- * @param  sp: The stack pointer points to the top of the thread stack.
- * @retval bool: true if it is returned via systick; otherwise false.
- */
-bool check_systick_event(void *sp);
-
 /**
  * @brief  Get syscall number
  * @param  sp: The stack pointer points to the top of the thread stack.

kernel/kernel.c

@@ -70,6 +70,9 @@ struct list_head ready_list[KTHREAD_PRI_MAX + 1];
 static bool need_resched_flag;
 static uint32_t preempt_cnt;
 
+/* System call */
+static bool syscall_flag;
+
 /* Tasks and threads */
 static struct task_struct tasks[TASK_MAX];
 
@@ -3066,6 +3069,21 @@ static void setup_syscall(struct thread_info *thread,
 /* Syscall table */
 static struct syscall_info syscall_table[] = {SYSCALL_TABLE_INIT};
 
+void set_syscall_flag(void)
+{
+    syscall_flag = true;
+}
+
+void reset_syscall_flag(void)
+{
+    syscall_flag = false;
+}
+
+bool get_syscall_flag(void)
+{
+    return syscall_flag;
+}
+
 static void syscall_handler(void)
 {
     unsigned long syscall_num = get_syscall_num(running_thread->stack_top);
@@ -3303,11 +3321,13 @@ void sched_start(void)
     list_del(&threads[0].list);
 
     while (1) {
-        if (!check_systick_event(running_thread->stack_top)) {
+        /* Syscall request */
+        if (get_syscall_flag()) {
+            reset_syscall_flag();
             syscall_handler();
         }
 
-        /* Rescheduling */
+        /* Rescheduling request */
         if (need_resched()) {
             reset_need_resched();
             __schedule();