On 26/01/2017 15:32, Pavel Dovgalyuk wrote:
>> From: Paolo Bonzini [mailto:pbonz...@redhat.com]
>> On 26/01/2017 14:37, Pavel Dovgalyuk wrote:
>>>> Simpler:
>>>>
>>>> use_icount &&
>>>> ((int32_t)cpu->icount_decr.u32 < 0 ||
>>>> cpu->icount_decr.u16.low + cpu->icount_extra == 0)
>>> Right.
>>>
>>>> But I'm not sure that you need to test u32. After all you're not
>>> Checking u32 is needed, because sometimes it is less than zero.
>>
>> If cpu->icount_decr.u32 is less than zero, the next translation block
>> would immediately exit with TB_EXIT_ICOUNT_EXPIRED, causing
>>
>> cpu->exception_index = EXCP_INTERRUPT;
>> *last_tb = NULL;
>> cpu_loop_exit(cpu);
>>
>> from cpu_loop_exec_tb's "case TB_EXIT_ICOUNT_EXPIRED".
>>
>> And the same is true for cpu->icount_decr.u16.low + cpu->icount_extra ==
>> 0, so I don't understand why this part of the patch is necessary.
>
> I removed that lines because we have to check icount=0 not only when it is
> expired,
> but also when all instructions were executed successfully.
> If there are no instructions to execute, calling tb_find (and translation
> then)
> may cause an exception at the wrong moment.
Ok, that makes sense for cpu->icount_decr.u16.low + cpu->icount_extra == 0.
But for decr.u32 < 0, the same reasoning of this comment is also true:
/* Something asked us to stop executing
* chained TBs; just continue round the main
* loop. Whatever requested the exit will also
* have set something else (eg exit_request or
* interrupt_request) which we will handle
* next time around the loop. But we need to
* ensure the tcg_exit_req read in generated code
* comes before the next read of cpu->exit_request
* or cpu->interrupt_request.
*/
(and by the way, this means that you need an smp_rmb in case
TB_EXIT_ICOUNT_EXPIRED).
Thanks,
Paolo
