Re: [Qemu-devel] [PATCH] timer: fix qemu_poll_ns early timeout on windows

[Paolo Bonzini]

Il 18/04/2014 10:29, Alex Bligh ha scritto:
> I suspect the easiest fix (certainly for debug purposes) would be to make all
> polls for non-zero time at least 10ms long. IE after the assignment of 'ms',
> do something like
>
> #ifdef BROKEN_POLL
>     if (ms < 10) {
>         ms = 10;
> #endif
>     }
>
> If you're actually saying g_poll has 10 millisecond resolution, or rounds down,
> then the appropriate course of action would be to change the rounding line to
>
>     ms = (ns + 10*SCALE_MS - 1) / (10*SCALE_MS);
>
> where g_poll is broken like this.
>
> As I say, I have no Windows box to test this on, but I hope that's helpful.

Unfortunately that would probably break guests.

Windows's default poll resolution is indeed around 10 ms, and that's 
where glib comes from.  But it can be lowered with timeBeginPeriod and 
timeEndPeriod, which is what QEMU does (note: the functions are 
basically nops with WINE, but if you remove them things actually break 
on Windows).

But it's unlikely that glib will change their code, which also makes 
sense in the common case where you don't call 
timeBeginPeriod/timeEndPeriod.  So I think Stanislav's patch makes sense.

Paolo