[Qemu-devel] Re: MIPS, io-thread, icount and wfi

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2011-01-18 01:19, Edgar E. Iglesias wrote:
> On Mon, Jan 17, 2011 at 11:03:08AM +0100, Edgar E. Iglesias wrote:
>> Hi,
>>
>> I'm running an io-thread enabled qemu-system-mipsel with icount.
>> When the guest (linux) goes to sleep through the wait insn (waiting
>> to be woken up by future timer interrupts), the thing deadlocks.
>>
>> IIUC, this is because vm timers are driven by icount, but the CPU is
>> halted so icount makes no progress and time stands still.
>>
>> I've locally disabled vcpu halting when icount is enabled, that
>> works around my problem but of course makes qemu consume 100% host cpu.
>>
>> I don't know why I only see this problem with io-thread builds?
>> Could be related timing and luck.
>>
>> Would be interesting to know if someone has any info on how this was
>> intended to work (if it was)? And if there are ideas for better
>> workarounds or fixes that don't disable vcpu halting entirely.
> 
> Hi,
> 
> I've found the problem. For some reason io-thread builds use a
> static timeout for wait loops. The entire chunk of code that
> makes sure qemu_icount makes forward progress when the CPU's
> are idle has been ifdef'ed away...
> 
> This fixes the problem for me, hopefully without affecting
> io-thread runs without icount.
> 
> commit 0f4f3a919952500b487b438c5520f07a1c6be35b
> Author: Edgar E. Iglesias <edgar@axis.com>
> Date:   Tue Jan 18 01:01:57 2011 +0100
> 
>     qemu-timer: Fix timeout calc for io-thread with icount
>     
>     Make sure we always make forward progress with qemu_icount to
>     avoid deadlocks. For io-thread, use the static 1000 timeout
>     only if icount is disabled.
>     
>     Signed-off-by: Edgar E. Iglesias <edgar@axis.com>
> 
> diff --git a/qemu-timer.c b/qemu-timer.c
> index 95814af..db1ec49 100644
> --- a/qemu-timer.c
> +++ b/qemu-timer.c
> @@ -110,7 +110,6 @@ static int64_t cpu_get_clock(void)
>      }
>  }
>  
> -#ifndef CONFIG_IOTHREAD
>  static int64_t qemu_icount_delta(void)
>  {
>      if (!use_icount) {
> @@ -124,7 +123,6 @@ static int64_t qemu_icount_delta(void)
>          return cpu_get_icount() - cpu_get_clock();
>      }
>  }
> -#endif
>  
>  /* enable cpu_get_ticks() */
>  void cpu_enable_ticks(void)
> @@ -1077,9 +1075,17 @@ void quit_timers(void)
>  
>  int qemu_calculate_timeout(void)
>  {
> -#ifndef CONFIG_IOTHREAD
>      int timeout;
>  
> +#ifdef CONFIG_IOTHREAD
> +    /* When using icount, making forward progress with qemu_icount when the
> +       guest CPU is idle is critical. We only use the static io-thread timeout
> +       for non icount runs.  */
> +    if (!use_icount) {
> +        return 1000;
> +    }
> +#endif
> +
>      if (!vm_running)
>          timeout = 5000;
>      else {
> @@ -1110,8 +1116,5 @@ int qemu_calculate_timeout(void)
>      }
>  
>      return timeout;
> -#else /* CONFIG_IOTHREAD */
> -    return 1000;
> -#endif
>  }
>  
> 
> 

This logic and timeout values were imported on iothread merge. And I bet
at least the timeout value of 1s (vs. 5s) can still be found in
qemu-kvm. Maybe someone over there can remember the rationales behind
choosing this value.

Jan

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