[Qemu-devel] Re: [patch 01/11] qemu: create helper for event notification

[Anthony Liguori <aliguori@us.ibm.com>]

mtosatti@redhat.com wrote:
> Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
>
> Index: trunk/vl.c
> ===================================================================
> --- trunk.orig/vl.c
> +++ trunk/vl.c
> @@ -1183,9 +1183,8 @@ void qemu_mod_timer(QEMUTimer *ts, int64
>              qemu_rearm_alarm_timer(alarm_timer);
>          }
>          /* Interrupt execution to force deadline recalculation.  */
> -        if (use_icount && cpu_single_env) {
> -            cpu_exit(cpu_single_env);
> -        }
> +        if (use_icount)
> +            qemu_notify_event();
>      }
>  }
>
> @@ -1338,8 +1337,6 @@ static void host_alarm_handler(int host_
>                                 qemu_get_clock(vm_clock))) ||
>          qemu_timer_expired(active_timers[QEMU_TIMER_REALTIME],
>                             qemu_get_clock(rt_clock))) {
> -        CPUState *env = next_cpu;
> -
>  #ifdef _WIN32
>          struct qemu_alarm_win32 *data = ((struct qemu_alarm_timer*)dwUser)->priv;
>          SetEvent(data->host_alarm);
> @@ -1349,16 +1346,7 @@ static void host_alarm_handler(int host_
>  #endif
>          alarm_timer->flags |= ALARM_FLAG_EXPIRED;
>
> -        if (env) {
> -            /* stop the currently executing cpu because a timer occured */
> -            cpu_exit(env);
> -#ifdef USE_KQEMU
> -            if (env->kqemu_enabled) {
> -                kqemu_cpu_interrupt(env);
> -            }
> -#endif
> -        }
> -        event_pending = 1;
> +        qemu_notify_event();
>      }
>  }
>   

I always thought next_cpu was used here to cover an explicit race condition.

If you're using TCG, and you get a single after running the loop, but 
before assigning cpu_single_env, then you'll set the interrupt exit 
request on the old CPU state.  You'll eventually exit I guess but you 
potentially have to run through multiple VCPUs.

I'd feel more comfortable if we preserved the behavior here that we had 
before.

-- 
Regards,

Anthony Liguori