Re: [PATCH 1/2] KVM: x86 emulator: don't update vcpu state if instruction is restarted.

Gleb Natapov
2010-08-01T09:24:39Z

On Sun, Aug 01, 2010 at 12:14:40PM +0300, Avi Kivity wrote:
> On 08/01/2010 12:01 PM, Gleb Natapov wrote:
> >>>
> >>>It uses rip _during_ pio. And pio emulation changes rip
> >>>only at the end of emulation.
> >>But non-emulated pio does a skip_emulated_instruction() immediately
> >>(or so the code in kvm-tpr-opt.c assumes:
> >>
> >Indeed, this is bug in non-emulated pio.
>
> But userspace depends on this bug.
We can fix that, or make it smarter. Look for io instruction at
rip/rip-2 and use rip accordingly for instance.

>
> >But the patch does not change
> >rip behaviour for emulated pio. vcpu->arch.emulate_ctxt.eip is updated
> >only at the end of emulation.
>
> That will lead to failures if the emulator is used for the
> kvm-tpr-opt pio (which may happen with big real mode).
>
IIRC it was always this way in emulator. I'd rather fix userspace than
break emulator.

--
Gleb.