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

[Paolo Bonzini <pbonzini@redhat.com>]

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().

Paolo