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

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Date:   Mon, 10 Jul 2017 12:26:16 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.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

Paolo Bonzini <pbonzini@...hat.com> wrote:

> 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.

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.