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

[Paolo Bonzini]

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);
 }
 

?

Paolo