Poison the stack to uncover programming errors

Code can easily make the mistake of using uninitialized
values or, in assembly, mistakenly dereferencing stack pointers
when an address is desired.

Set the stack to a non-zero value which is also (by testing)
a pointer which will crash coreboot if used. This poisoning
has uncovered at least one bug.

Change-Id: I4affb9a14b96611e8bf83cb82636e47913025a5d
Signed-off-by: Ronald G. Minnich <rminnich@gmail.com>
Reviewed-on: http://review.coreboot.org/1221
Reviewed-by: Marc Jones <marcj303@gmail.com>
Tested-by: build bot (Jenkins)
Reviewed-by: Sven Schnelle <svens@stackframe.org>

src/arch/x86/lib/c_start.S

@@ -16,17 +16,21 @@ _start:
 
 	post_code(POST_ENTRY_C_START)		/* post 13 */
 
-	/** clear stack */
+	/** poison the stack. Code should not count on the
+	 * stack being full of zeros. This stack poisoning
+	 * recently uncovered a bug in the broadcast SIPI
+	 * code.
+	 */
 	cld
 	leal	_stack, %edi
 	movl	$_estack, %ecx
 	subl	%edi, %ecx
 	shrl	$2, %ecx   /* it is 32 bit aligned, right? */
-	xorl	%eax, %eax
+	movl	$0xDEADBEEF, %eax
 	rep
 	stosl
 
-	/** clear bss */
+	/** clear bss, which unlike the stack is zero by definition */
 	leal	_bss, %edi
 	movl	$_ebss, %ecx
 	subl	%edi, %ecx