target/arm: Allow explicit writes to CONTROL.SPSEL in Handler mode

In ARMv7M the CPU ignores explicit writes to CONTROL.SPSEL
in Handler mode. In v8M the behaviour is slightly different:
writes to the bit are permitted but will have no effect.

We've already done the hard work to handle the value in
CONTROL.SPSEL being out of sync with what stack pointer is
actually in use, so all we need to do to fix this last loose
end is to update the condition we use to guard whether we
call write_v7m_control_spsel() on the register write.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 1512153879-5291-3-git-send-email-peter.maydell@linaro.org

1 files changed, 4 insertions, 1 deletions

diff --git a/target/arm/helper.c b/target/arm/helper.c
index 88394d4..f21c142 100644
--- a/target/arm/helper.c
+++ b/target/arm/helper.c
@@ -10091,8 +10091,11 @@ void HELPER(v7m_msr)(CPUARMState *env, uint32_t maskreg, uint32_t val)
          * thread mode; other bits can be updated by any privileged code.
          * write_v7m_control_spsel() deals with updating the SPSEL bit in
          * env->v7m.control, so we only need update the others.
+         * For v7M, we must just ignore explicit writes to SPSEL in handler
+         * mode; for v8M the write is permitted but will have no effect.
          */
-        if (!arm_v7m_is_handler_mode(env)) {
+        if (arm_feature(env, ARM_FEATURE_V8) ||
+            !arm_v7m_is_handler_mode(env)) {
             write_v7m_control_spsel(env, (val & R_V7M_CONTROL_SPSEL_MASK) != 0);
         }
         env->v7m.control[env->v7m.secure] &= ~R_V7M_CONTROL_NPRIV_MASK;