[Qemu-devel] Re: [PATCH] fix halt emulation with icount and CONFIG_IOTHREAD (v2)

[Paolo Bonzini <pbonzini@redhat.com>]

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