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

[Jun Koi]

On Sat, Jul 4, 2015 at 1:06 AM, Peter Maydell <address@hidden> wrote:

On 3 July 2015 at 18:02, Jun Koi <address@hidden> wrote:
> I am looking at how the main thread interrupts TCG thread. Inside
> cpu-exec.c, in function cpu_tb_exec(), I can see that it executes one TB,
> and when this is over, it can check for events from other threads (like main
> thread). This is the reason why TCG thread can be interrupted.
>
> Is my understanding correct?
>
> 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?

Thanks.