target/arm: Don't trap WFI/WFE for M profile

M profile cores can never trap on WFI or WFE instructions. Check for
M profile in check_wfx_trap() to ensure this.

The existing code will do the right thing for v7M cores because
the hcr_el2 and scr_el3 registers will be all-zeroes and so we
won't attempt to trap, but when we start setting ARM_FEATURE_V8
for v8M cores the v8A handling of SCTLR.nTWE and .nTWI will not
give the right results.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 1501692241-23310-3-git-send-email-peter.maydell@linaro.org

target/arm/op_helper.c

@@ -370,6 +370,11 @@ static inline int check_wfx_trap(CPUARMState *env, bool is_wfe)
     int cur_el = arm_current_el(env);
     uint64_t mask;
 
+    if (arm_feature(env, ARM_FEATURE_M)) {
+        /* M profile cores can never trap WFI/WFE. */
+        return 0;
+    }
+
     /* If we are currently in EL0 then we need to check if SCTLR is set up for
      * WFx instructions being trapped to EL1. These trap bits don't exist in v7.
      */