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

Frederic Weisbecker
2009-12-18T23:15:53Z

On Fri, Dec 18, 2009 at 12:05:03PM -0800, Roland McGrath wrote:
> > Please find the trivial test-case below. It hangs, because
> > PTRACE_SINGLESTEP doesn't trigger the trap.
>
> 2.6.33-rc1 x86-64 works for me with either -m64 or -m32 version of that test.
>
> > (not sure this matters, but I did the testing under kvm)
>
> 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.

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.

Adding Avi and Jan in Cc.