target-arm: In v8, make illegal AArch32 mode changes set PSTATE.IL

In v8, the illegal mode changes which are UNPREDICTABLE in v7 are
given architected behaviour:
 * the mode field is unchanged
 * PSTATE.IL is set (so any subsequent instructions will UNDEF)
 * any other CPSR fields are written to as normal

This is pretty much the same behaviour we picked for our
UNPREDICTABLE handling, with the exception that for v8 we
need to set the IL bit.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Sergey Fedorov <serge.fdrv@gmail.com>
Message-id: 1455556977-3644-10-git-send-email-peter.maydell@linaro.org

diff --git a/target-arm/helper.c b/target-arm/helper.c
index 5926b15..ff5f895 100644 (file)
--- a/target-arm/helper.c
+++ b/target-arm/helper.c
@@ -5325,11 +5325,20 @@ void cpsr_write(CPUARMState *env, uint32_t val, uint32_t mask,
         (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.
-             * We choose to ignore the attempt and leave the CPSR M field
-             * untouched.
+            /* Attempt to switch to an invalid mode: this is UNPREDICTABLE in
+             * v7, and has defined behaviour in v8:
+             *  + leave CPSR.M untouched
+             *  + allow changes to the other CPSR fields
+             *  + set PSTATE.IL
+             * For user changes via the GDB stub, we don't set PSTATE.IL,
+             * as this would be unnecessarily harsh for a user error.
              */
             mask &= ~CPSR_M;
+            if (write_type != CPSRWriteByGDBStub &&
+                arm_feature(env, ARM_FEATURE_V8)) {
+                mask |= CPSR_IL;
+                val |= CPSR_IL;
+            }
         } else {
             switch_mode(env, val & CPSR_M);
         }