Re: [Qemu-devel] implementing architectural timers using QEMU timers

[Max Filippov]

On Mon, Jan 16, 2017 at 9:19 AM, Paolo Bonzini <address@hidden> wrote:
> On 12/01/2017 13:19, Pavel Dovgalyuk wrote:
>>> From: Peter Maydell [mailto:address@hidden
>>> On 12 January 2017 at 11:28, Pavel Dovgalyuk <address@hidden> wrote:
>>>>> From: Max Filippov [mailto:address@hidden
>>>>> Ok, looks like what happens in my case is that instruction that
>>>>> sets CCOMPARE and thus changes remaining icount does not
>>>>> cause exit from the cpu_exec. So merely ending TB on
>>>>> QEMU_CLOCK_VIRTUAL timer update is not enough, I need to
>>>>> throw an exception of some kind? Or does the timer code need
>>>>> to take care of that?
>>>>
>>>> Yes, it seems that you should end the block with an exception,
>>>> to allow icount loop recalculate the timeouts.
>>>
>>> Really? The ARM translate.c doesn't generate an exception.
>>> It just does
>>>  gen_io_end();
>>>  gen_lookup_tb();
>>>
>>> (so we force a lookup of the next TB, but don't throw an
>>> exception of any kind).
>>
>> Maybe I missing something. As far as I understand, changing the virtual timer
>> notifies the iothread and os_host_main_loop_wait kicks the CPU thread.
>>
>> But within that period of time before changing the timer and kicking the 
>> thread
>> CPU may proceed some instructions and the timer will be expired if it was set
>> to one of the soonest instructions.
>
> My understanding (which may be wrong!) was that after gen_io_end you
> would exit with TB_EXIT_ICOUNT_EXPIRED and cpu->icount_decr.u16.high =
> -1, but indeed I don't see anything that calls cpu_interrupt in that
> path.
>
> Maybe something like this:
>
> diff --git a/include/exec/gen-icount.h b/include/exec/gen-icount.h
> index 050de59..c20d193 100644
> --- a/include/exec/gen-icount.h
> +++ b/include/exec/gen-icount.h
> @@ -73,6 +73,9 @@ static inline void gen_io_end(void)
>  {
>      TCGv_i32 tmp = tcg_const_i32(0);
>      tcg_gen_st_i32(tmp, cpu_env, -ENV_OFFSET + offsetof(CPUState, 
> can_do_io));
> +    /* Make the next TB exit immediately with TB_EXIT_ICOUNT_EXPIRED.  */
> +    tcg_gen_st16_i32(-1, cpu_env,
> +                     -ENV_OFFSET + offsetof(CPUState, icount_decr.u16.high));
>      tcg_temp_free_i32(tmp);
>  }

With -1 moved into TCG temporary that works for me as well.

-- 
Thanks.
-- Max