From 55c2a21fe83cf2189370863e653fc9fcb76df8d6 Mon Sep 17 00:00:00 2001
From: Paolo Bonzini <pbonzini@redhat.com>
Date: Wed, 21 Feb 2018 21:21:02 -0500
Subject: [PATCH] target-i386: Fix addr16 prefix

While ADDSEG will only be false in 16-bit mode for LEA, it can be
false even in other cases when 16-bit addresses are obtained via
the 67h prefix in 32-bit mode. In this case, gen_lea_v_seg forgets
to add a nonzero FS or GS base if CS/DS/ES/SS are all zero. This
case is pretty rare but happens when booting Windows 95/98, and
this patch fixes it.

The bug is visible since commit d6a291498, but it was introduced
together with gen_lea_v_seg and it probably could be reproduced
with a "addr16 gs movsb" instruction as early as in commit
ca2f29f555805d07fb0b9ebfbbfc4e3656530977.

Backports commit e2e02a820741ec4d96b8f313b06a2a7ed5e94fbd from qemu
---
 qemu/target-i386/translate.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/qemu/target-i386/translate.c b/qemu/target-i386/translate.c
index ce56279c..de4803e7 100644
--- a/qemu/target-i386/translate.c
+++ b/qemu/target-i386/translate.c
@@ -556,15 +556,15 @@ static void gen_lea_v_seg(DisasContext *s, TCGMemOp aflag, TCGv a0,
         break;
     case MO_16:
         /* 16 bit address */
-        if (ovr_seg < 0) {
-            ovr_seg = def_seg;
-        }
         tcg_gen_ext16u_tl(tcg_ctx, cpu_A0, a0);
-        /* ADDSEG will only be false in 16-bit mode for LEA.  */
-        if (!s->addseg) {
-            return;
-        }
         a0 = cpu_A0;
+        if (ovr_seg < 0) {
+            if (s->addseg) {
+                ovr_seg = def_seg;
+            } else {
+                return;
+            }
+        }
         break;
     default:
         tcg_abort();