target/i386: reintroduce debugging mechanism

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

target/i386/tcg/decode-new.c.inc

@@ -1689,6 +1689,9 @@ static void disas_insn_new(DisasContext *s, CPUState *cpu, int b)
     X86DecodeFunc decode_func = decode_root;
     uint8_t cc_live;
 
+#ifdef CONFIG_USER_ONLY
+    if (limit) { --limit; }
+#endif
     s->has_modrm = false;
 
  next_byte:

target/i386/tcg/translate.c

@@ -2969,6 +2969,9 @@ static void gen_sty_env_A0(DisasContext *s, int offset, bool align)
     tcg_gen_qemu_st_i128(t, s->tmp0, mem_index, mop);
 }
 
+static bool first = true;
+static unsigned long limit;
+
 #include "decode-new.h"
 #include "emit.c.inc"
 #include "decode-new.c.inc"
@@ -3124,15 +3127,39 @@ static bool disas_insn(DisasContext *s, CPUState *cpu)
 
     prefixes = 0;
 
+    if (first) {
+        const char *limit_str = getenv("QEMU_I386_LIMIT");
+        limit = limit_str ? atol(limit_str) : -1;
+        first = false;
+    }
+    bool use_new = true;
+#ifdef CONFIG_USER_ONLY
+    use_new &= limit > 0;
+#endif
+
  next_byte:
     s->prefix = prefixes;
     b = x86_ldub_code(env, s);
     /* Collect prefixes.  */
     switch (b) {
     default:
+#ifndef CONFIG_USER_ONLY
+        use_new &= b <= limit;
+#endif
+        if (use_new && 0) {
+            disas_insn_new(s, cpu, b);
+            return true;
+        }
         break;
     case 0x0f:
         b = x86_ldub_code(env, s) + 0x100;
+#ifndef CONFIG_USER_ONLY
+        use_new &= b <= limit;
+#endif
+        if (use_new && 0) {
+            disas_insn_new(s, cpu, b);
+            return true;
+        }
         break;
     case 0xf3:
         prefixes |= PREFIX_REPZ;