[Qemu-devel] guest os time tick issue

[Michael Qiu]

I'm using qemu to boot a mips kernel.
But it seems always hung like this:
"Calibrating delay loop..."
Actually, it is caused by following code in 

> int c0_compare_int_usable(void)
> {
>     unsigned int delta;
>     unsigned int cnt;
>
>     /*
>      * IP7 already pending?  Try to clear it by acking the timer.
>      */
>     if (c0_compare_int_pending()) {
>         write_c0_compare(read_c0_count());
>         compare_change_hazard();
>         if (c0_compare_int_pending())
>             return 0;
>     }
>
>     for (delta = 0x10; delta <= 0x400000; delta <<= 1) {
>         cnt = read_c0_count();
>         cnt += delta;
>         write_c0_compare(cnt);
>         compare_change_hazard();
>         if ((int)(read_c0_count() - cnt) < 0)
>             break; // In qemu, it seems always 1024. Is this a configurable value in qemu?
>         /* increase delta if the timer was already expired */
>     }
>
>     while ((int)(read_c0_count() - cnt) <= 0)
>         ;    /* Wait for expiry  */
>
>     compare_change_hazard();
>     if (!c0_compare_int_pending())
>         return 0;  // If i comment out this, everything will be fine.
>
>     write_c0_compare(read_c0_count());
>     compare_change_hazard();
>     if (c0_compare_int_pending())
>         return 0;
>
>     /*
>      * Feels like a real count / compare timer.
>      */
>     return 1;
> }

Seems the  hight line will check the timer interrupt, as in real world, there should be a pending timer interrupt pending untill next write to compare.

I just guest the qemu will do like this:

while(1)
{
  execute some guest code // is the number of instuctions fixed? If the guest code access some memory io memory, simulated hw will be executed.
  update background io // just like chardev, sdl relative...
  update count value in guest cpu // Is the value fixed?
}

Please correct me, thanks in advance