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

[Marcelo Tosatti]

On Wed, Feb 16, 2011 at 10:32:25AM +0100, Paolo Bonzini wrote:
> On 02/15/2011 09:56 PM, Marcelo Tosatti wrote:
> >Note: to be applied to uq/master.
> >
> >In icount mode, halt emulation should take into account the nearest
> >event when sleeping.
> 
> I agree with Jan that this patch is not the best solution, if not incorrect.
> 
> However, in the iothread, the main loop can kick the VCPU thread
> instead of running cpu_exec_all like it does in non-iothread mode.
> Something like this:
> 
> diff --git a/vl.c b/vl.c
> index b436952..7835317 100644
> --- a/vl.c
> +++ b/vl.c
> @@ -1425,7 +1425,9 @@ static void main_loop(void)
>      qemu_main_loop_start();
> 
>      for (;;) {
> -#ifndef CONFIG_IOTHREAD
> +#ifdef CONFIG_IOTHREAD
> +        qemu_cpu_kick(first_cpu);
> +#else
>          nonblocking = cpu_exec_all();
>          if (vm_request_pending()) {
>              nonblocking = true;
> 
> I don't like this 100% because it relies on the fact that there is
> only one TCG execution thread.  In a multithreaded world you would:
> 
> 1) have each CPU register its own instruction counter;
> 
> 2) have each CPU register its own QEMU_CLOCK_REALTIME timer based on
> qemu_icount_delta() and arm it just before going to sleep; the timer
> kicks the CPU.
> 
> 3) remove all icount business from qemu_calculate_timeout.
> 
> Item (3) is what makes me prefer my patch above (if it works) to
> Marcelo's.  Marcelo's patch is tying even more
> qemu_calculate_timeout to the icount.  So if anything, a patch
> tweaking the timedwait like Marcelo's should use something based on
> qemu_icount_delta().

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.

So instead vm_timer expiration is converted to realtime, and used as
halt timeout.