linux-kernel - Re: RFC: Task switch emulation fails for VM86 mode

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Date:   Mon, 10 Jul 2017 18:21:50 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Nadav Amit <nadav.amit@...il.com>, Wanpeng Li <kernellwp@...il.com>
Cc:     Radim Krcmar <rkrcmar@...hat.com>, kvm <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Jan Kiszka <jan.kiszka@....de>
Subject: Re: RFC: Task switch emulation fails for VM86 mode

On 10/07/2017 17:48, Nadav Amit wrote:
>>>
>>> Any proposal is a great appreciated. :)
> I don’t 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 “task-switch assist” 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 “task-switch assist”
> 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.

Paolo