[PATCH v2] xen/x86: Zero out .bss for PV guests

[Boris Ostrovsky]

Baremetal kernels clear .bss early in the boot but Xen PV guests don't
execute that code. They have been able to run without problems because
Xen domain builder happens to give out zeroed pages. However, since this
is not really guaranteed, .bss should be explicitly cleared.

(Since we introduce macros for specifying 32- and 64-bit registers we
can get rid of ifdefs in startup_xen())

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
---
 arch/x86/xen/xen-head.S | 29 ++++++++++++++++++++++-------
 1 file changed, 22 insertions(+), 7 deletions(-)

diff --git a/arch/x86/xen/xen-head.S b/arch/x86/xen/xen-head.S
index b65f59a..2af87d1 100644
--- a/arch/x86/xen/xen-head.S
+++ b/arch/x86/xen/xen-head.S
@@ -35,16 +35,31 @@
 #define PVH_FEATURES (0)
 #endif
 
-	__INIT
-ENTRY(startup_xen)
-	cld
 #ifdef CONFIG_X86_32
-	mov %esi,xen_start_info
-	mov $init_thread_union+THREAD_SIZE,%esp
+#define REG(register)	%e##register
+#define WSIZE_SHIFT	2
+#define STOS		stosl
 #else
-	mov %rsi,xen_start_info
-	mov $init_thread_union+THREAD_SIZE,%rsp
+#define REG(register)	%r##register
+#define WSIZE_SHIFT	3
+#define STOS		stosq
 #endif
+
+	__INIT
+ENTRY(startup_xen)
+	cld
+
+	/* Clear .bss */
+	xor REG(ax),REG(ax)
+	mov $__bss_start,REG(di)
+	mov $__bss_stop,REG(cx)
+	sub REG(di),REG(cx)
+	shr $WSIZE_SHIFT,REG(cx)
+	rep STOS
+
+	mov REG(si),xen_start_info
+	mov $init_thread_union+THREAD_SIZE,REG(sp)
+
 	jmp xen_start_kernel
 
 	__FINIT
-- 
2.1.0