Re: [Qemu-devel] question about syscall

[Antonio Ricci <ricciantonio@gmail.com>]

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Hi Muladi,

some times ago someone said me to do something like this:

/****************************************************/
a piece of code handling interrupt in translate.c:
case 0xcd: /* int N */
      val = ldub_code(s->pc++);
      /* asia: start */
      if (val == 0x80){
        gen_op_handle_syscall();
        break;
      }

      if (s->vm86 && s->iopl != 3) {
          gen_exception(s, EXCP0D_GPF, pc_start - s->cs_base);
      } else {
          gen_interrupt(s, val, pc_start - s->cs_base, s->pc - s->cs_base);
      }
      break;

/*********************************************************/


I can't understand why I have to intercept the case 0xcd. Can you explain
me?

I've tried to intercept sysenter (0x134) and  sysexit (0x135) events, but I
had no information about syscall from it.


Thanks for help,


Antonio
2009/2/7 Mulyadi Santosa <mulyadi.santosa@gmail.com>

> Hi
>
> On Sat, Feb 7, 2009 at 8:15 PM, Antonio Ricci <ricciantonio@gmail.com>
> wrote:
> > Hi all,
> > i'd want to understand how Qemu handles system calls while executing both
> > Linux and Windows operating system. I've searched on the web with no
> > results.
> > Do you know if there is some documentation about syscall handling in
> Qemu?
> > Thanks in advance for help
>
> I assume you ask about full system emulation. Try to check how Qemu
> intercept int 80h or SYSENTER/SYSCALL instruction (if the guest OS is
> Linux). Not sure what Windows does when doing system call, maybe by
> executing int instruction toward certain vectors too.
>
> As usual, related interrupt handler will be executed (and translated).
> So from Qemu point of view, it's strictly a matter of intercepting
> code and translating it. It's the guest OS that does the real work on
> syscall handling.
>
> In user space emulation only, system call is intercepted by the normal
> dynamic code translation, but syscall is handled by syscall() C
> function. The complication could arise if the guest OS is not the same
> type as the host OS. Some "interpretations" are needed here.
>
> regards,
>
> Mulyadi.
>
>
>

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