Re: RFC: Task switch emulation fails for VM86 mode

From: Nadav Amit
Date: Mon Jul 10 2017 - 15:26:39 EST

Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:

> On 10/07/2017 17:48, Nadav Amit wrote:
>>>> Any proposal is a great appreciated. :)
>> I donât see a (very) easy solution. The code was (apparently) never built to
>> deal with a task switch during an instruction emulation.
>> AFAIU kvm_task_switch() expects information about the task-switch from the
>> CPU âtask-switch assistâ mechanisms, and this information (or even the fact
>> that a task-switch is needed due to an exception) are unavailable from the
>> instruction emulator. The instruction emulator itself does not know to
>> emulate task-switches, e.g., during far CALL and JMP.
>> A complete solution is therefore complicated and requires some work. Your
>> specific problem may be addressed by detecting the injection of an exception
>> while having invalid guest state in vm86 in vmx_queue_exception() or in
>> handle_invalid_guest_state(), and emulating the âtask-switch assistâ
>> mechanism.
> I agree, the right solution would be to read the IDT in
> vmx_queue_exception if vmx->emulation_required, and inject the exception
> manually. It would be an extension of what
> kvm_inject_realmode_interrupt already does.

I take it back. While everything I said is true, there is no reason for the
guest state to be invalid in vm86, at least in the unit-test. It appears
that the task-switch emulation updates rflags (and vm86 flag) only after the
segments are loaded, causing vmx->emulation_required to be set, when in fact
emulation is not needed.

And indeed adding in the end of handle_task_switch():

vmx->emulation_required = emulation_required(vcpu);

solves the problem for me.