target/i386: Check CR0.TS before enter_mmx

When CR0.TS=1, execution of x87 FPU, MMX, and some SSE instructions will
cause a Device Not Available (DNA) exception (#NM). System software uses
this exception event to lazily context switch FPU state.

Before this patch, enter_mmx helpers may be generated just before #NM
generation, prematurely resetting FPU state before the guest has a
chance to save it.

Signed-off-by: Matt Borgerson <contact@mborgerson.com>
Message-ID: <CADc=-s5F10muEhLs4f3mxqsEPAHWj0XFfOC2sfFMVHrk9fcpMg@mail.gmail.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

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

@@ -1803,16 +1803,18 @@ static void disas_insn_new(DisasContext *s, CPUState *cpu, int b)
         }
         break;
 
-    case X86_SPECIAL_MMX:
-        if (!(s->prefix & (PREFIX_REPZ | PREFIX_REPNZ | PREFIX_DATA))) {
-            gen_helper_enter_mmx(cpu_env);
-        }
+    default:
         break;
     }
 
     if (!validate_vex(s, &decode)) {
         return;
     }
+    if (decode.e.special == X86_SPECIAL_MMX &&
+        !(s->prefix & (PREFIX_REPZ | PREFIX_REPNZ | PREFIX_DATA))) {
+        gen_helper_enter_mmx(cpu_env);
+    }
+
     if (decode.op[0].has_ea || decode.op[1].has_ea || decode.op[2].has_ea) {
         gen_load_ea(s, &decode.mem, decode.e.vex_class == 12);
     }