Re: [Qemu-devel] How to break cpu_tb_exec()?

[Richard Henderson <rth@twiddle.net>]

On 07/06/2015 12:26 PM, Lluís Vilanova wrote:
> Jun Koi writes:
>
>> On Sat, Jul 4, 2015 at 1:12 AM, Peter Maydell <peter.maydell@linaro.org> wrote:
>>      On 3 July 2015 at 18:10, Jun Koi <junkoi2004@gmail.com> wrote:
>>> On Sat, Jul 4, 2015 at 1:06 AM, Peter Maydell <peter.maydell@linaro.org>
>>> wrote:
>>>> On 3 July 2015 at 18:02, Jun Koi <junkoi2004@gmail.com> wrote:
>>>>> If this is true, then what if this TB is running infinitely, and do not
>>>>> return, or it is in a very long loop? In this case, TCG thread cannot
>>      be
>>>>> interrupted?
>>>>
>>>> Every TB starts with a little bit of generated code that checks
>>>> the 'tcg_exit_req' flag in the CPUState for the CPU (see the
>>>> gen_tb_start() function). If some other part of QEMU wants the
>>>> CPU to stop running guest code and return to the top level loop,
>>>> it calls cpu_exit() which sets this flag.
>>>
>>>
>>> But this does not answer my question yet: if we the flag is only
>>> enable when TB already enters the "long loop", then nothing can break
>>> this TB execution?
>
>>      We check the flag for every TB we execute. Therefore in any
>>      loop we must check the flag each time round the loop. So
>>      if another thread sets the flag, we will exit.
>
>>      (A TB is always ended by any kind of branch instruction,
>>      so you can't have a loop within a single TB. A tight loop
>>      turns into a TB that ends with "branch back to the start
>>      of this TB", but that will re-execute the flag-check code.)
>
>
>> Oh right, this tight loop is my main concern. It makes sense now.
>
> I'm not sure if "rep"-style x86 instructions loop inside the same TB, though.

They do not.


r~