linux-kernel - Re: [PATCH 3/4] KVM: async_pf: Force a nested vmexit if the injected #PF is async

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Message-ID: <CANRm+CzN4v50eUTKPJupEK0Y7VVao2xzTF6XNm_UO+XA-Ry4Yg@mail.gmail.com>
Date:   Wed, 14 Jun 2017 21:02:24 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Radim Krčmář <rkrcmar@...hat.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kvm <kvm@...r.kernel.org>, Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [PATCH 3/4] KVM: async_pf: Force a nested vmexit if the injected
 #PF is async_pf

2017-06-14 20:52 GMT+08:00 Radim Krčmář <rkrcmar@...hat.com>:
> 2017-06-14 09:07+0800, Wanpeng Li:
>> 2017-06-14 2:55 GMT+08:00 Radim Krčmář <rkrcmar@...hat.com>:
>> > Using vcpu->arch.cr2 is suspicious as VMX doesn't update CR2 on VM
>> > exits;  isn't this going to change the CR2 visible in L2 guest after a
>> > nested VM entry?
>>
>> Sorry, I don't fully understand the question. As you know this
>> vcpu->arch.cr2 which includes token is set before async pf injection,
>
> Yes, I'm thinking that setting vcpu->arch.cr2 is a mistake in this case.
>
>> and L1 will intercept it from EXIT_QUALIFICATION during nested vmexit,
>
> Right, so we do not need to have the token in CR2, because L1 is not
> going to look at it.
>
>> why it can change the CR2 visible in L2 guest after a nested VM entry?
>
> Sorry, the situation is too convoluted to be expressed in one sentence:
>
> 1) L2 is running with CR2 = L2CR2
> 3) VMX exits (say, unrelated EXTERNAL_INTERRUPT) and L0 stores L2CR2 in
>    vcpu->arch.cr2
> 2) APF for L1 has completed
> 4) L0 KVM wants to inject APF and sets vcpu->arch.cr2 = APFT
> 5) L0 KVM does a nested VM exit to L1, EXIT_QUALIFICATION = APFT
> 6) L0 KVM enters L1 with CR2 = vcpu->arch.cr2 = APFT
> 7) L1 stores APFT as L2's CR2
> 8) L1 handles APF, maybe reschedules, but eventually comes back to this
>    L2's thread
> 9) after some time, L1 enters L2 with CR2 = APFT
> 10) L2 is running with CR2 = APTF
>
> The original L2CR2 is lost and we'd introduce a bug if L2 wanted to look
> at it, e.g. it was in a process of handling its #PF.

Good point. What's your proposal? :)

Regards,
Wanpeng Li