Re: [Qemu-devel] [PATCH v8 2/9] icount: exit cpu loop on expire

[Paolo Bonzini]

On 26/01/2017 15:32, Pavel Dovgalyuk wrote:
>> From: Paolo Bonzini [mailto:address@hidden
>> 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