On 02/17/2011 04:15 AM, Marcelo Tosatti wrote:
Yes, using qemu_icount_delta directly in tcg_wait_io_event timedwait
is explicit (partially the reason for confusion with my patch).
So the reasoning for the patch is:
With icount vm_timer timers expire on virtual CPU time. If a CPU halts,
you cannot expect passage of realtime to trigger vm_timers expiration.
But if a CPU is halted, all_cpu_threads_idle() will still be true even
if you signal the condition variable, and you'll be looping in the while
condition. That's why I say that
while (x) {
cond_timedwait (cond, mutex);
}
(i.e. without checking the return value of cond_timedwait, and without
polling something else upon return) is a broken idiom that can only work
around missing signals/broadcasts.
So instead vm_timer expiration is converted to realtime, and used as
halt timeout.
But vm_timer timers are only used by "-icount auto", which works in my
tests [*]. It is "-icount N" which is broken and unfortunately your
patch does not fix it. The CRIS image on qemu.org triggers the watchdog
(and if I eliminate the watchdog I see that the CPU is hung).
[*] Actually, it works but doesn't calibrate very well. It shows
25 bogomips, sometimes 50, compared to 250 without iothread.
Paolo
