Re: qemu-system-s390x hang in tcg

[Richard Henderson]

On 6/29/22 16:16, Sven Schnelle wrote:
> Thanks, that was very helpful. I added debugging and it turned out
> that the TB is left because of a pending irq. The code then calls
> s390_cpu_exec_interrupt:
>
> bool s390_cpu_exec_interrupt(CPUState *cs, int interrupt_request)
> {
>      if (interrupt_request & CPU_INTERRUPT_HARD) {
>          S390CPU *cpu = S390_CPU(cs);
>          CPUS390XState *env = &cpu->env;
>
>          if (env->ex_value) {
>              /* Execution of the target insn is indivisible from
>                 the parent EXECUTE insn.  */
>              return false;
>          }
>          if (s390_cpu_has_int(cpu)) {
>              s390_cpu_do_interrupt(cs);
>              return true;
>          }
>          if (env->psw.mask & PSW_MASK_WAIT) {
>              /* Woken up because of a floating interrupt but it has already
>               * been delivered. Go back to sleep. */
>              cpu_interrupt(CPU(cpu), CPU_INTERRUPT_HALT);
>          }
>      }
>      return false;
> }
>
> Note the 'if (env->ex_value) { }' check. It looks like this function
> just returns false in case tcg is executing an EX instruction. After
> that the information that the TB should be exited because of an
> interrupt is gone. So the TB's are never exited again, although the
> interrupt wasn't handled. At least that's my assumption now, if i'm
> wrong please tell me.

Ah, yes, I see.

We wanted to treat ex_value != 0 as if interrupts are disabled, because we have no way of 
stacking that value for re-execution after the interrupt (which itself could use EXECUTE).

One solution might be to zap ex_value and arrange to re-execute the EXECUTE instruction 
after the interrupt.

Another solution is to generate an exit from any TB translating ex_value, so that 
interrupts are re-examined.  This is probably cleanest.  I'll prepare a patch.


r~