AMD HVM: Fix enter-realmode stub to initialise 32-bit GPRs.

This fixes booting Ubuntu live ISO, which assumes the upper half of
%esp is zero on entry.

Signed-off-by: Keir Fraser <keir@xensource.com>

tools/firmware/hvmloader/hvmloader.c

@@ -62,15 +62,21 @@ asm(
     "    mov  %ax,%fs                \n"
     "    mov  %ax,%gs                \n"
     "    mov  %ax,%ss                \n"
-    "    ljmp $0x8,$0x0              \n"
+    /* Initialise all 32-bit GPRs to zero. */
+    "    xor  %eax,%eax              \n"
+    "    xor  %ebx,%ebx              \n"
+    "    xor  %ecx,%ecx              \n"
+    "    xor  %edx,%edx              \n"
+    "    xor  %esp,%esp              \n"
+    "    xor  %ebp,%ebp              \n"
+    "    xor  %esi,%esi              \n"
+    "    xor  %edi,%edi              \n"
     /* Enter real mode, reload all segment registers and IDT. */
+    "    ljmp $0x8,$0x0              \n"
     "trampoline_start: .code16       \n"
-    "    mov  %cr0,%eax              \n"
-    "    and  $0xfe,%al              \n"
     "    mov  %eax,%cr0              \n"
     "    ljmp $0,$1f-trampoline_start\n"
-    "1:  xor  %ax,%ax                \n"
-    "    mov  %ax,%ds                \n"
+    "1:  mov  %ax,%ds                \n"
     "    mov  %ax,%es                \n"
     "    mov  %ax,%fs                \n"
     "    mov  %ax,%gs                \n"