Fix QEMU context switch bug

git-svn-id: svn://svn.code.sf.net/p/nuttx/code/trunk@3361 42af7a65-404d-4744-a932-0658087f49c3

arch/x86/src/qemu/qemu_fullcontextrestore.S

@@ -61,14 +61,24 @@
 
 /* Trace macros, use like trace 'i' to print char to serial port. */
 
-	.macro	trace, ch
+	.macro	chout, addr, ch
 #ifdef CONFIG_DEBUG
-	mov		$0x3f8, %dx
+	mov		$\addr, %dx
 	mov		$\ch, %al
 	out		%al, %dx
 #endif
 	.endm
 
+	.macro	trace, ch
+#ifdef CONFIG_DEBUG
+	push	%eax
+	push	%edx
+	chout	0x3f8, \ch
+	pop		%edx
+	pop		%eax
+#endif
+	.endm
+
 /**************************************************************************
  * Public Functions
  **************************************************************************/

arch/x86/src/qemu/qemu_saveusercontext.S

@@ -57,14 +57,24 @@
 
 /* Trace macros, use like trace 'i' to print char to serial port. */
 
-	.macro	trace, ch
+	.macro	chout, addr, ch
 #ifdef CONFIG_DEBUG
-	mov		$0x3f8, %dx
+	mov		$\addr, %dx
 	mov		$\ch, %al
 	out		%al, %dx
 #endif
 	.endm
 
+	.macro	trace, ch
+#ifdef CONFIG_DEBUG
+	push	%eax
+	push	%edx
+	chout	0x3f8, \ch
+	pop		%edx
+	pop		%eax
+#endif
+	.endm
+
 /**************************************************************************
  * .text
  **************************************************************************/
@@ -132,13 +142,24 @@ up_saveusercontext:
 
 	movl	%ebp, (4*REG_EBP)(%eax)
 
+	/* Save EAX=1.  This will be the "apparent" return value from this
+	 * function when context is switch back to this thread.  The non-zero
+	 * return value is the indication that we have been resumed.
+	 */
+ 
+	movl	$1, (4*REG_EAX)(%eax)
+
 	/* Get and save the interrupt state */
 
 	pushf
 	pop		%ecx
 	movl	%ecx, (4*REG_EFLAGS)(%eax)
 
-	/* And return 0.  'ret' will remove the EIP from the top of the stack. */
+	/* And return 0 -- The zero return value is the indication that that
+	 * this is the original, "true" return from the function.
+	 *
+	 * 'ret' will remove the EIP from the top of the stack.
+	 */
 
 	xorl	%eax, %eax
 	ret