> From: Paolo Bonzini [mailto:pbonz...@redhat.com]
> >>>
> >>> I tried this approach and it didn't work.
> >>> I think iothread sets u16.high flag after resetting it in
> >>> cpu_handle_interrupt.
> >>
> >> But why is this a problem? The TB would exit immediately and go again
> >> to cpu_handle_interrupt. cpu_handle_interrupt returns true and
> >> cpu_handle_exception causes the exception via cpu_exec_nocache.
> >
> > I've tested your variant more thoroughly.
> > It seems, that iothread calls cpu_exec between
> > atomic_set(&cpu->icount_decr.u16.high, 0);
> > in cpu_handle_interrupt and cpu_exec_nocache in cpu_handle_exception.
> > I see no other reason, because this happens not for the every time.
> > And cpu_handle_interrupt is not called again, because cpu_handle_exception
> > returns true.
> > Therefore we have an infinite loop, because no other code here resets cpu-
> >icount_decr.u16.high.
>
> Then returning true unconditionally is wrong in the cpu_exec_nocache
> case. What if you do:
>
> diff --git a/accel/tcg/cpu-exec.c b/accel/tcg/cpu-exec.c
> index 61297f8f4a..fb5446be3e 100644
> --- a/accel/tcg/cpu-exec.c
> +++ b/accel/tcg/cpu-exec.c
> @@ -470,7 +470,19 @@ static inline void cpu_handle_debug_exception(CPUState
> *cpu)
>
> static inline bool cpu_handle_exception(CPUState *cpu, int *ret)
> {
> - if (cpu->exception_index >= 0) {
> + if (cpu->exception_index < 0) {
> +#ifndef CONFIG_USER_ONLY
> + if (replay_has_exception()
> + && cpu->icount_decr.u16.low + cpu->icount_extra == 0) {
> + /* try to cause an exception pending in the log */
> + cpu_exec_nocache(cpu, 1, tb_find(cpu, NULL, 0, curr_cflags()),
> true);
> + }
> +#endif
> + if (cpu->exception_index < 0) {
> + return;
return false, I guess?
This approach allows iterating in case of races
and QEMU does not hangs anymore at replay.
> + }
> + }
> +
> if (cpu->exception_index >= EXCP_INTERRUPT) {
> /* exit request from the cpu execution loop */
> *ret = cpu->exception_index;
> @@ -505,16 +517,6 @@ static inline bool cpu_handle_exception(CPUState *cpu,
> int *ret)
> }
> #endif
> }
> -#ifndef CONFIG_USER_ONLY
> - } else if (replay_has_exception()
> - && cpu->icount_decr.u16.low + cpu->icount_extra == 0) {
> - /* try to cause an exception pending in the log */
> - cpu_exec_nocache(cpu, 1, tb_find(cpu, NULL, 0, curr_cflags()), true);
> - *ret = -1;
> - return true;
> -#endif
> - }
> -
> return false;
> }
>
Pavel Dovgalyuk
