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. Thanks!!
