linux-kernel - Re: [PATCH 2/4] KVM: nVMX: Defer APICv updates while L2 is active until L1 is active

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Date:   Mon, 18 Apr 2022 15:35:57 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Gaoning Pan <pgn@....edu.cn>,
        Yongkang Jia <kangel@....edu.cn>
Subject: Re: [PATCH 2/4] KVM: nVMX: Defer APICv updates while L2 is active
 until L1 is active

On Mon, Apr 18, 2022, Maxim Levitsky wrote:
> On Sat, 2022-04-16 at 03:42 +0000, Sean Christopherson wrote:
> When L2 uses APICv/AVIC, we just safely passthrough its usage to the real hardware.
> 
> If we were to to need to inhibit it, we would have to emulate APICv/AVIC so that L1 would
> still think that it can use it - thankfully there is no need for that.

What if L1 passes through IRQs and all MSRs to L2?  E.g. if L2 activates Auto EOI
via WRMSR, then arguably it is KVM's responsibility to disable APICv in vmcs02 _and_
vmcs01 in order to handle the Auto EOI properly.  L1 isn't expecting a VM-Exit, so
KVM can't safely punt to L1 even if conceptually we think that it's L1's problem.

It's a contrived scenario, but technically possible.