Jun Koi writes:
> On Sat, Jul 4, 2015 at 1:12 AM, Peter Maydell <peter.mayd...@linaro.org>
> wrote:
> On 3 July 2015 at 18:10, Jun Koi <junkoi2...@gmail.com> wrote:
>> On Sat, Jul 4, 2015 at 1:06 AM, Peter Maydell <peter.mayd...@linaro.org>
>> wrote:
>>> On 3 July 2015 at 18:02, Jun Koi <junkoi2...@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.
Cheers,
Lluis
--
"And it's much the same thing with knowledge, for whenever you learn
something new, the whole world becomes that much richer."
-- The Princess of Pure Reason, as told by Norton Juster in The Phantom
Tollbooth
