Re: RFC: Task switch emulation fails for VM86 mode

From: Paolo Bonzini
Date: Mon Jul 10 2017 - 12:22:18 EST

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.