Re: [Qemu-devel] Re: [PATCH] flush TB on singlestep command

[Jan Kiszka]

Stefan Weil wrote:
> Jan Kiszka schrieb:
>> Alexander Graf wrote:
>>   
>>> On 21.04.2010, at 12:04, Jun Koi wrote:
>>>
>>>     
>>>> On Tue, Apr 20, 2010 at 8:44 PM, Alexander Graf <address@hidden> wrote:
>>>>       
>>>>> On 20.04.2010, at 13:38, Jan Kiszka wrote:
>>>>>
>>>>>         
>>>>>> Alexander Graf wrote:
>>>>>>           
>>>>>>> On 20.04.2010, at 09:18, Jan Kiszka wrote:
>>>>>>>
>>>>>>>             
>>>>>>>> Jun Koi wrote:
>>>>>>>>               
>>>>>>>>> Thank you for the explanation of this code.
>>>>>>>>>
>>>>>>>>> Qemu has a command named singlestep, which reduces the translated code
>>>>>>>>> block to be only one instruction.
>>>>>>>>> This new patch flushes TBs both when singlestep is on and off.
>>>>>>>>>
>>>>>>>>> Signed-off-by: Jun Koi <address@hidden>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> diff --git a/monitor.c b/monitor.c
>>>>>>>>> index 5659991..2b2005b 100644
>>>>>>>>> --- a/monitor.c
>>>>>>>>> +++ b/monitor.c
>>>>>>>>> @@ -1187,13 +1187,26 @@ static void do_log(Monitor *mon, const QDict 
>>>>>>>>> *qdict)
>>>>>>>>>    cpu_set_log(mask);
>>>>>>>>> }
>>>>>>>>>
>>>>>>>>> +/* flush all the TBs to force new code generation */
>>>>>>>>> +static void flush_all_tb(void)
>>>>>>>>> +{
>>>>>>>>> +    CPUState *env;
>>>>>>>>> +
>>>>>>>>> +    for (env = first_cpu; env != NULL; env = env->next_cpu) {
>>>>>>>>> +        tb_flush(env);
>>>>>>>>> +    }
>>>>>>>>> +}
>>>>>>>>> +
>>>>>>>>>                 
>>>>>>>> The smaller your patch are, the more people pick on it. :)
>>>>>>>>
>>>>>>>> I was about to suggest moving this close to tb_flush, but then I
>>>>>>>> realized that the env argument of that service is misleading. In fact,
>>>>>>>> it already flushes the one and only translation buffer pool.
>>>>>>>>
>>>>>>>>               
>>>>>>>>> static void do_singlestep(Monitor *mon, const QDict *qdict)
>>>>>>>>> {
>>>>>>>>>    const char *option = qdict_get_try_str(qdict, "option");
>>>>>>>>> +
>>>>>>>>>    if (!option || !strcmp(option, "on")) {
>>>>>>>>>        singlestep = 1;
>>>>>>>>> +        flush_all_tb();
>>>>>>>>>    } else if (!strcmp(option, "off")) {
>>>>>>>>>        singlestep = 0;
>>>>>>>>> +        flush_all_tb();
>>>>>>>>>    } else {
>>>>>>>>>        monitor_printf(mon, "unexpected option %s\n", option);
>>>>>>>>>    }
>>>>>>>>>
>>>>>>>>>                 
>>>>>>>> Let's just pass mon->mon_cpu to tb_flush and skip the redundant loop.
>>>>>>>>               
>>>>>>> That doesn't help, no? singlestep is a global variable. Flushing only 
>>>>>>> the current vcpu would still not affect the others, while the 
>>>>>>> singlestep switch would.
>>>>>>>             
>>>>>> tb_flush uses env only to dump some state when a problem occurred.
>>>>>>
>>>>>>           
>>>>>>> According to your above comment the cache is global, but I don't think 
>>>>>>> we should rely on that.
>>>>>>>             
>>>>>> It might make sense to define some tb_flush_all() as tb_flush(first_cpu)
>>>>>> for now to establish the infrastructure. Then we are prepared for the
>>>>>> day the tb_flush implementation may change.
>>>>>>           
>>>>> Right. But then the call to tb_flush_all here is still correct.
>>>>>         
>>>> So what is the final solution do you want?
>>>>
>>>> I still think that having flush_all_tb() like in the last patch is good 
>>>> enough.
>>>>       
>>> I agree. And I like the patch as is.
>>>
>>> Acked-by: Alexander Graf <address@hidden>
>>>
>>>     
>> Sorry, nack for keeping this service in /monitor.c/. But a bonus ack if
>> you avoid the needless loop when moving it to exec.c, adding a comment
>> that current tb_flush has global, env-invariant scope.
>>
>> Thanks,
>> Jan
> 
> flush_all_tb() is now called for singlestep on and off, that's fine.
> But it's called always - no way to disable this call. That's not good.
> Sometimes I don't want to flush all TBs when I switch singlestep mode
> (that's the reason why I suggested a separate monitor command which
> flushes all TBs - I still think that would be the best solution).

Mind to tell us the use case?

> 
> What about this syntax for the singlestep monitor command:
> 
> singlestep [on|off][,flush]
>       Run the emulation in single step mode. In that mode, QEMU uses
>       one translation block per target CPU instruction.
>       If called with option off, the emulation returns to normal mode.
>       If called with the optional parameter flush, existing translation
>       blocks are flushed.
> 
> Or, if you prefer to flush by default:
> 
> singlestep [on|off][,noflush]
>       Run the emulation in single step mode. In that mode, QEMU uses
>       one translation block per instructions.
>       ...

If we need this knob, then this version please (not wanting to flush is
likely the corner case).

> 
> Please update qemu-monitor.hx, too (that should be done in any case).

Right, and the qemu-options.hx needs update as well to explain that
"singlestep" has nothing to do with debugger single-stepping.

Jan

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