From f5e796e604e6454c8fbc6639040fe0fd5afd4973 Mon Sep 17 00:00:00 2001
From: patacongo <patacongo@42af7a65-404d-4744-a932-0658087f49c3>
Date: Thu, 10 Mar 2011 18:38:37 +0000
Subject: [PATCH] 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 | 16 ++++++--
 arch/x86/src/qemu/qemu_saveusercontext.S    | 41 ++++++++++++++++-----
 2 files changed, 44 insertions(+), 13 deletions(-)

diff --git a/arch/x86/src/qemu/qemu_fullcontextrestore.S b/arch/x86/src/qemu/qemu_fullcontextrestore.S
index 7d25e9072ed..9a1e1e64e4f 100644
--- a/arch/x86/src/qemu/qemu_fullcontextrestore.S
+++ b/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
  **************************************************************************/
@@ -151,5 +161,5 @@ up_fullcontextrestore:
 	popl	%eax
 	iret
 	.size up_fullcontextrestore, . - up_fullcontextrestore
-    .end
+	.end
 
diff --git a/arch/x86/src/qemu/qemu_saveusercontext.S b/arch/x86/src/qemu/qemu_saveusercontext.S
index ffa1cf7f627..e9a98b33154 100644
--- a/arch/x86/src/qemu/qemu_saveusercontext.S
+++ b/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
  **************************************************************************/
@@ -132,15 +142,26 @@ up_saveusercontext:
 
 	movl	%ebp, (4*REG_EBP)(%eax)
 
-    /* Get and save the interrupt state */
-
-    pushf
-    pop		%ecx
-	movl	%ecx, (4*REG_EFLAGS)(%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.
+	 */
  
-	/* And return 0.  'ret' will remove the EIP from the top of the stack. */
+	movl	$1, (4*REG_EAX)(%eax)
 
-	xorl    %eax, %eax
+	/* Get and save the interrupt state */
+
+	pushf
+	pop		%ecx
+	movl	%ecx, (4*REG_EFLAGS)(%eax)
+
+	/* 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
 	.size	up_saveusercontext, . - up_saveusercontext
-    .end
+	.end