linux-kernel - Re: [PATCH v2 1/8] KVM: nVMX: Always write vmcs02.GUEST

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Date:   Fri, 27 Sep 2019 16:37:25 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Reto Buerki <reet@...elabs.ch>,
        Liran Alon <liran.alon@...cle.com>
Subject: Re: [PATCH v2 1/8] KVM: nVMX: Always write vmcs02.GUEST_CR3 during
 nested VM-Enter

On Fri, Sep 27, 2019 at 2:45 PM Sean Christopherson
<sean.j.christopherson@...el.com> wrote:
>
> Write the desired L2 CR3 into vmcs02.GUEST_CR3 during nested VM-Enter
> instead of deferring the VMWRITE until vmx_set_cr3().  If the VMWRITE
> is deferred, then KVM can consume a stale vmcs02.GUEST_CR3 when it
> refreshes vmcs12->guest_cr3 during nested_vmx_vmexit() if the emulated
> VM-Exit occurs without actually entering L2, e.g. if the nested run
> is squashed because nested VM-Enter (from L1) is putting L2 into HLT.
>
> Note, the above scenario can occur regardless of whether L1 is
> intercepting HLT, e.g. L1 can intercept HLT and then re-enter L2 with
> vmcs.GUEST_ACTIVITY_STATE=HALTED.  But practically speaking, a VMM will
> likely put a guest into HALTED if and only if it's not intercepting HLT.
>
> In an ideal world where EPT *requires* unrestricted guest (and vice
> versa), VMX could handle CR3 similar to how it handles RSP and RIP,
> e.g. mark CR3 dirty and conditionally load it at vmx_vcpu_run().  But
> the unrestricted guest silliness complicates the dirty tracking logic
> to the point that explicitly handling vmcs02.GUEST_CR3 during nested
> VM-Enter is a simpler overall implementation.
>
> Cc: stable@...r.kernel.org
> Reported-and-tested-by: Reto Buerki <reet@...elabs.ch>
> Tested-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> Reviewed-by: Liran Alon <liran.alon@...cle.com>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
Reviewed-by: Jim Mattson <jmattson@...gle.com>