target-arm: In cpsr_write() ignore mode switches from User mode

The only case where we can attempt a cpsr_write() mode switch from
User is from the gdbstub; all other cases are handled in the
calling code (notably translate.c). Architecturally attempts to
alter the mode bits from user mode are simply ignored (and not
treated as a bad mode switch, which in v8 sets CPSR.IL). Make
mode switches from User ignored in cpsr_write() as well, for
consistency.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Sergey Fedorov <serge.fdrv@gmail.com>
Message-id: 1455556977-3644-6-git-send-email-peter.maydell@linaro.org

target-arm/helper.c

@@ -5316,6 +5316,7 @@ void cpsr_write(CPUARMState *env, uint32_t val, uint32_t mask,
     env->daif |= val & CPSR_AIF & mask;
 
     if (write_type != CPSRWriteRaw &&
+        (env->uncached_cpsr & CPSR_M) != CPSR_USER &&
         ((env->uncached_cpsr ^ val) & mask & CPSR_M)) {
         if (bad_mode_switch(env, val & CPSR_M)) {
             /* Attempt to switch to an invalid mode: this is UNPREDICTABLE.