Re: x86: do_debug && PTRACE_SINGLESTEP broken by 08d68323d1f0c34452e614263b212ca556dae47f

Avi Kivity
2009-12-20T08:32:12Z

On 12/19/2009 01:15 AM, Frederic Weisbecker wrote:
>
>> Apparently it does. You should hack some printks into do_debug() and see
>> how kvm is differing from real hardware. (Actually you can probably do
>> this with a notifier added by a module, not that you are shy about
>> recompiling!)
>>
>> Probably kvm's emulation of the hardware behavior wrt the DR6 bits is not
>> sufficiently faithful. Conceivably, kvm is being consistent with some
>> older hardware and we have encoded assumptions that only newer hardware
>> meets. But I'd guess it's just a plain kvm bug.
>>
>

A kvm bug is most likely.

> It looks like in kvm, before entering the guest, we restore its
> debug registers:
>
> vcpu_enter_guest():
> if (unlikely(vcpu->arch.switch_db_regs)) {
> set_debugreg(0, 7);
> set_debugreg(vcpu->arch.eff_db[0], 0);
> set_debugreg(vcpu->arch.eff_db[1], 1);
> set_debugreg(vcpu->arch.eff_db[2], 2);
> set_debugreg(vcpu->arch.eff_db[3], 3);
> }
>
>
> But what happens to dr6, I don't know.
>

That's done later, in vmx.c:vmx_vcpu_run():

if (vcpu->arch.switch_db_regs)
set_debugreg(vcpu->arch.dr6, 6);

Can you describe the failure? I'll try to construct a test case
reproducer and work with Jan to fix it.

--
error compiling committee.c: too many arguments to function