according what does the qemu know to exit current TB and find helper functions from environment?

[tugouxp]

Hi folks:

   a questions puzzles me during review the qemu code of 5.0.0, take vexpress emulation arm A9 on RTOS  for example.

the emulated RTOS has its own "printf" implementations, so during the qemu emulations, it would find 

   helper_le_stl_mmu()

         store_helper()

           ......................

              pl011_write();

                    writev();  //syscall

and finally, the helper would call the pl011_write to emulate the actual stdio output functions.

but, how did the TCB TB block know this during the translations? there may be others different "printf" implement ions on different  target, 

how the tcg knows that a special helper functions need to be called and others not, from the binary code disassembly language of the emulated target, there is no

obvious sign to make anyone know which instutions is special and need to emulation by helper, How get this?

Thank you.