Re: [PATCH 3/8] qemu-thread-win32: cleanup, fix, document QemuEvent

[Paolo Bonzini <pbonzini@redhat.com>]

On 3/6/23 14:31, David Hildenbrand wrote:
>>
>> -    smp_mb_acquire();
>> -    if (value == EV_SET) {
>> -        /* If there was a concurrent reset (or even reset+wait),
>> -         * do nothing.  Otherwise change EV_SET->EV_FREE.
>> -         */
>> -        qatomic_or(&ev->value, EV_FREE);
>> -    }
>> +
> 
> [had the same thought on patch #2]
> 
> IIUC, the "read first" is an optimization to not unconditionally dirty 
> the cache-line. But I assume we don't particularly care about that 
> optimization on the reset path.

Thinking more about it, the intended usage of QemuEvent is either

     qemu_event_reset();
     if (!check()) {
          qemu_event_wait());
     }

or

     if (!check()) {
         qemu_event_reset();
         if (!check()) {
              qemu_event_wait());
         }
     }

If we don't care on the reset path we care much less on the wait path. 
Should I remove it and go straight to the cmpxchg, just for peace of mind?

Paolo