target/arm: handle A-profile semihosting at translate time

As for the other semihosting calls we can resolve this at translate
time.
Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Message-id: 20190913151845.12582-4-alex.bennee@linaro.org
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

target/arm/translate.c

@@ -10222,14 +10222,25 @@ static bool trans_CBZ(DisasContext *s, arg_CBZ *a)
 }
 
 /*
- * Supervisor call
+ * Supervisor call - both T32 & A32 come here so we need to check
+ * which mode we are in when checking for semihosting.
  */
 
 static bool trans_SVC(DisasContext *s, arg_SVC *a)
 {
-    gen_set_pc_im(s, s->base.pc_next);
-    s->svc_imm = a->imm;
-    s->base.is_jmp = DISAS_SWI;
+    const uint32_t semihost_imm = s->thumb ? 0xab : 0x123456;
+
+    if (!arm_dc_feature(s, ARM_FEATURE_M) && semihosting_enabled() &&
+#ifndef CONFIG_USER_ONLY
+        !IS_USER(s) &&
+#endif
+        (a->imm == semihost_imm)) {
+        gen_exception_internal_insn(s, s->base.pc_next, EXCP_SEMIHOST);
+    } else {
+        gen_set_pc_im(s, s->base.pc_next);
+        s->svc_imm = a->imm;
+        s->base.is_jmp = DISAS_SWI;
+    }
     return true;
 }