target/arm: Restore M-profile CONTROL.SPSEL before any tailchaining

On exception return for M-profile, we must restore the CONTROL.SPSEL
bit from the EXCRET value before we do any kind of tailchaining,
including for the derived exceptions on integrity check failures.
Otherwise we will give the guest an incorrect EXCRET.SPSEL value on
exception entry for the tailchained exception.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NRichard Henderson <richard.henderson@linaro.org>
Message-id: 20180720145647.8810-4-peter.maydell@linaro.org

target/arm/helper.c

@@ -7131,6 +7131,16 @@ static void do_v7m_exception_exit(ARMCPU *cpu)
         }
     }
 
+    /*
+     * Set CONTROL.SPSEL from excret.SPSEL. Since we're still in
+     * Handler mode (and will be until we write the new XPSR.Interrupt
+     * field) this does not switch around the current stack pointer.
+     * We must do this before we do any kind of tailchaining, including
+     * for the derived exceptions on integrity check failures, or we will
+     * give the guest an incorrect EXCRET.SPSEL value on exception entry.
+     */
+    write_v7m_control_spsel_for_secstate(env, return_to_sp_process, exc_secure);
+
     if (sfault) {
         env->v7m.sfsr |= R_V7M_SFSR_INVER_MASK;
         armv7m_nvic_set_pending(env->nvic, ARMV7M_EXCP_SECURE, false);
@@ -7152,12 +7162,6 @@ static void do_v7m_exception_exit(ARMCPU *cpu)
         return;
     }
 
-    /* Set CONTROL.SPSEL from excret.SPSEL. Since we're still in
-     * Handler mode (and will be until we write the new XPSR.Interrupt
-     * field) this does not switch around the current stack pointer.
-     */
-    write_v7m_control_spsel_for_secstate(env, return_to_sp_process, exc_secure);
-
     switch_v7m_security_state(env, return_to_secure);
 
     {