target/s390x: special handling when starting a CPU with WAIT PSW

When we try to start a CPU with a WAIT PSW, we have to take care that
TCG will actually try to continue executing instructions.

We must therefore really only unhalt the CPU if we don't have a WAIT
PSW. Also document the special order for restart interrupts, which
load a new PSW and change the state to operating.

To keep KVM working, simply don't have a look at the WAIT bit when
loading the PSW. Otherwise the behavior of a restart interrupt when
a CPU stopped would be changed.
Signed-off-by: NDavid Hildenbrand <david@redhat.com>
Message-Id: <20170928203708.9376-31-david@redhat.com>
Reviewed-by: NRichard Henderson <richard.henderson@linaro.org>
Signed-off-by: NCornelia Huck <cohuck@redhat.com>

target/s390x/cpu.c

@@ -337,8 +337,15 @@ unsigned int s390_cpu_set_state(uint8_t cpu_state, S390CPU *cpu)
         break;
     case CPU_STATE_OPERATING:
     case CPU_STATE_LOAD:
-        /* unhalt the cpu for common infrastructure */
-        s390_cpu_unhalt(cpu);
+        /*
+         * Starting a CPU with a PSW WAIT bit set:
+         * KVM: handles this internally and triggers another WAIT exit.
+         * TCG: will actually try to continue to run. Don't unhalt, will
+         *      be done when the CPU actually has work (an interrupt).
+         */
+        if (!tcg_enabled() || !(cpu->env.psw.mask & PSW_MASK_WAIT)) {
+            s390_cpu_unhalt(cpu);
+        }
         break;
     default:
         error_report("Requested CPU state is not a valid S390 CPU state: %u",

target/s390x/sigp.c

@@ -232,8 +232,12 @@ static void sigp_restart(CPUState *cs, run_on_cpu_data arg)
     case CPU_STATE_STOPPED:
         /* the restart irq has to be delivered prior to any other pending irq */
         cpu_synchronize_state(cs);
-        do_restart_interrupt(&cpu->env);
+        /*
+         * Set OPERATING (and unhalting) before loading the restart PSW.
+         * load_psw() will then properly halt the CPU again if necessary (TCG).
+         */
         s390_cpu_set_state(CPU_STATE_OPERATING, cpu);
+        do_restart_interrupt(&cpu->env);
         break;
     case CPU_STATE_OPERATING:
         cpu_inject_restart(cpu);