Re: [Qemu-devel] [PATCH v3 4/4] tcg: rework tb_invalidated_flag

[Sergey Fedorov <serge.fdrv@gmail.com>]

On 21/04/16 18:55, Alex Bennée wrote:
> Sergey Fedorov <serge.fdrv@gmail.com> writes:
>
>> On 18/04/16 20:51, Sergey Fedorov wrote:
>>> On 18/04/16 20:17, Alex Bennée wrote:
>>>> Sergey Fedorov <serge.fdrv@gmail.com> writes:
>>>>> On 18/04/16 17:09, Alex Bennée wrote:
>>>>>> Sergey Fedorov <sergey.fedorov@linaro.org> writes:
>>>>>>> diff --git a/cpu-exec.c b/cpu-exec.c
>>>>> (snip)
>>>>>>> @@ -507,14 +510,12 @@ int cpu_exec(CPUState *cpu)
>>>>>>>                  }
>>>>>>>                  tb_lock();
>>>>>>>                  tb = tb_find_fast(cpu);
>>>>>>> -                /* Note: we do it here to avoid a gcc bug on Mac OS X when
>>>>>>> -                   doing it in tb_find_slow */
>>>>>> Is this still true? Would it make more sense to push the patching down
>>>>>> to the gen_code?
>>>>> This comment comes up to the commit:
>>>>>
>>>>>     commit 1538800276aa7228d74f9d00bf275f54dc9e9b43
>>>>>     Author: bellard <bellard@c046a42c-6fe2-441c-8c8c-71466251a162>
>>>>>     Date:   Mon Dec 19 01:42:32 2005 +0000
>>>>>
>>>>>         workaround for gcc bug on PowerPC
>>>>>
>>>>>
>>>>> It was added more than ten years ago. Anyway, now this code is here not
>>>>> because of the bug: we need to reset 'next_tb' which is a local variable
>>>>> in cpu_exec(). Personally, I don't think it would be neater to hide it
>>>>> into gen_code(). Do you have some thoughts on how we could benefit from
>>>>> doing so? BTW, I had a feeling that it may be useful to reorganize
>>>>> cpu_exec() a bit, although I don't have a solid idea of how to do this
>>>>> so far.
>>>> I'm mainly eyeing the tb_lock/unlock which would be nice to push further
>>>> down the call chain if we can, especially if the need to lock
>>>> tb_find_fast can be removed later on.
>>> Yes, it would be nice to possibly have all tb_lock/unlock() calls (or at
>>> least their pairs) in the same block. There is a lot to be thought over :)
>> It's not so simple because tb_find_fast() is also called in replay mode
>> to find a TB for cpu_exec_nocache()... I'm not sure it's worth touching
>> it now.
> If the locking is pushed into tb_find_fast or further down is this an
> issue?

We would have to pass 'next_tb' (or 'last_tb' and 'tb_exit' after
cleaning it up) if we move TB chaining code to tb_find_fast(). But
tb_find_fast() is also called in replay mode to find a TB for
cpu_exec_nocache() where we don't bother with TB chaining... Do you
think it would be fine to make those changes?

Kind regards,
Sergey