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

[Paolo Bonzini]

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