target/arm/translate-a64: Don't underdecode system instructions

The "system instructions" and "system register move" subcategories
of "branches, exception generating and system instructions" for A64
only apply if bits [23:22] are zero; other values are currently
unallocated. Correctly UNDEF these unallocated encodings.

Reported-by: Laurent Desnogues <laurent.desnogues@gmail.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Laurent Desnogues <laurent.desnogues@gmail.com>
Message-id: 20190125182626.9221-2-peter.maydell@linaro.org

diff --git a/target/arm/translate-a64.c b/target/arm/translate-a64.c
index 4d28a27..e6df303 100644 (file)
--- a/target/arm/translate-a64.c
+++ b/target/arm/translate-a64.c
@@ -2144,7 +2144,11 @@ static void disas_b_exc_sys(DisasContext *s, uint32_t insn)
         break;
     case 0x6a: /* Exception generation / System */
         if (insn & (1 << 24)) {
-            disas_system(s, insn);
+            if (extract32(insn, 22, 2) == 0) {
+                disas_system(s, insn);
+            } else {
+                unallocated_encoding(s);
+            }
         } else {
             disas_exc(s, insn);
         }