[Qemu-devel] Stopping the VM due to an illegal instruct ion [was: QOM CPUState, part 13: Emptying CPU_COMMON ]

[Michael Walle]

Hi,

Am 2013-09-17 20:46, schrieb Richard Henderson:
> On 09/17/2013 10:46 AM, Michael Walle wrote:
> > Am Montag, 9. September 2013, 18:47:21 schrieb Peter Maydell:
> > > For lm32 you might consider raising EXCP_BREAKPOINT;
> > > then if you have a gdb connected to the debug stub
> > > the developer will get dropped into the debugger for
> > > the offending insn and can look at it.
> >
> > Are you sure, this will drop to the debugger? I guess the target cpu 
> > jumps to
> > the exception vector.
>
> He meant EXCP_DEBUG, I think.
>
> It will never affect the target cpu at all, of course, that would be 
> useless
> for debugging.  Issuing it will exit the cpu loop.
>
> If the debugger is attached, all of the expected things should 
> happen.
>
> Otherwise, it appears as if we'll still have set cpu->stopped, and
> we'll fail to run anymore instructions.  Even without the debugger
> you ought to be able to poke around with the monitor to see what's
> going on.


Raising EXCP_DEBUG results in a segmentation fault if qemu is not 
started with the -s option, because there is not gdbserver_state:

[Switching to Thread 0xb4b2eb70 (LWP 20930)]
0x081c88fa in gdb_set_stop_cpu (cpu=0x8730f88) at 
/home/mw/repo/qemu/gdbstub.c:1187
1187        gdbserver_state->c_cpu = cpu;
(gdb) bt
#0  0x081c88fa in gdb_set_stop_cpu (cpu=0x8730f88) at 
/home/mw/repo/qemu/gdbstub.c:1187
#1  0x081995f2 in cpu_handle_guest_debug (cpu=0x8730f88)
    at /home/mw/repo/qemu/cpus.c:485
#2  0x0819ad7f in tcg_exec_all () at /home/mw/repo/qemu/cpus.c:1218
#3  0x0819a283 in qemu_tcg_cpu_thread_fn (arg=0x8730f88)
    at /home/mw/repo/qemu/cpus.c:882
#4  0xb7dd7c39 in start_thread () from 
/lib/i386-linux-gnu/i686/cmov/libpthread.so.0
#5  0xb7d4478e in clone () from /lib/i386-linux-gnu/i686/cmov/libc.so.6
(gdb) p gdbserver_state
$1 = (GDBState *) 0x0
(gdb)


I then tried to raise EXCP_HALTED and setting cs->stopped and the 
runstate manually:


void HELPER(ill)(CPULM32State *env)
{
#ifndef CONFIG_USER_ONLY
    CPUState *cs = CPU(lm32_env_get_cpu(env));
    fprintf(stderr, "VM paused due to illegal instruction. "
            "Connect a debugger or switch to the monitor console "
            "to find out more.\n");
    qemu_system_vmstop_request(RUN_STATE_PAUSED);
    cs->stopped = true;
    raise_exception(env, EXCP_HALTED);
#endif
}

The above works as expected, but i don't know if that is the right way 
to do it.

btw, what is the difference between cs->stopped and cs->halted?

--
michael