[Qemu-arm] [PATCH for-3.0] target/arm: Use correct mmu_idx for exception-return unstacking

[Peter Maydell]

For M-profile exception returns, the mmu index to use for exception
return unstacking is supposed to be that of wherever we are returning to:
 * if returning to handler mode, privileged
 * if returning to thread mode, privileged or unprivileged depending on
   CONTROL.nPRIV for the destination security state

We were passing the wrong thing as the 'priv' argument to
arm_v7m_mmu_idx_for_secstate_and_priv(). The effect was that guests
which programmed the MPU to behave differently for privileged and
unprivileged code could get spurious MemManage Unstack exceptions.

Reported-by: Adithya Baglody <address@hidden>
Signed-off-by: Peter Maydell <address@hidden>
---
 target/arm/helper.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/target/arm/helper.c b/target/arm/helper.c
index a2ac96084e7..0604a0efbe2 100644
--- a/target/arm/helper.c
+++ b/target/arm/helper.c
@@ -7171,9 +7171,11 @@ static void do_v7m_exception_exit(ARMCPU *cpu)
         uint32_t frameptr = *frame_sp_p;
         bool pop_ok = true;
         ARMMMUIdx mmu_idx;
+        bool return_to_priv = return_to_handler ||
+            !(env->v7m.control[return_to_secure] & R_V7M_CONTROL_NPRIV_MASK);
 
         mmu_idx = arm_v7m_mmu_idx_for_secstate_and_priv(env, return_to_secure,
-                                                        !return_to_handler);
+                                                        return_to_priv);
 
         if (!QEMU_IS_ALIGNED(frameptr, 8) &&
             arm_feature(env, ARM_FEATURE_V8)) {
-- 
2.17.1