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

Avi Kivity
2010-08-01T10:00:14Z

On 08/01/2010 12:24 PM, Gleb Natapov wrote:
> 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.

That requires everyone to update, or suffer major breakage.

>>> 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.

It wasn't a problem because the emulator wasn't (and still isn't) used
for this. But it has the potential to break badly once we make
emulate_invalid_guest_state=1 the default.

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