linux-kernel - Re: [PATCH 1/2] KVM: nVMX: Always write vmcs02.GUEST

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Date:   Thu, 26 Sep 2019 16:39:28 -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>
Subject: Re: [PATCH 1/2] KVM: nVMX: Always write vmcs02.GUEST_CR3 during
 nested VM-Enter

On Thu, Sep 26, 2019 at 2:43 PM Sean Christopherson
<sean.j.christopherson@...el.com> wrote:
>
> Write the desired L2 CR3 into vmcs02.GUEST_CR3 during nested VM-Enter
> isntead 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 L2 is being put into 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-by: Reto Buerki <reet@...elabs.ch>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> ---
>  arch/x86/kvm/vmx/nested.c | 8 ++++++++
>  arch/x86/kvm/vmx/vmx.c    | 9 ++++++---
>  2 files changed, 14 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index 41abc62c9a8a..971a24134081 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -2418,6 +2418,14 @@ static int prepare_vmcs02(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12,
>                                 entry_failure_code))
>                 return -EINVAL;
>
> +       /*
> +        * Immediately write vmcs02.GUEST_CR3.  It will be propagated to vmcs12
> +        * on nested VM-Exit, which can occur without actually running L2, e.g.
> +        * if L2 is entering HLT state, and thus without hitting vmx_set_cr3().
> +        */
> +       if (enable_ept)
> +               vmcs_writel(GUEST_CR3, vmcs12->guest_cr3);
> +

This part of the change I can definitely confirm, since we have a
similar change in our fetid pile of commits that have yet to be
upstreamed. Thanks for taking care of this one!

>         /* Late preparation of GUEST_PDPTRs now that EFER and CRs are set. */
>         if (load_guest_pdptrs_vmcs12 && nested_cpu_has_ept(vmcs12) &&
>             is_pae_paging(vcpu)) {
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index d4575ffb3cec..b530950a9c2b 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -2985,6 +2985,7 @@ void vmx_set_cr3(struct kvm_vcpu *vcpu, unsigned long cr3)
>  {
>         struct kvm *kvm = vcpu->kvm;
>         unsigned long guest_cr3;
> +       bool skip_cr3 = false;
>         u64 eptp;
>
>         guest_cr3 = cr3;
> @@ -3000,15 +3001,17 @@ void vmx_set_cr3(struct kvm_vcpu *vcpu, unsigned long cr3)
>                         spin_unlock(&to_kvm_vmx(kvm)->ept_pointer_lock);
>                 }
>
> -               if (enable_unrestricted_guest || is_paging(vcpu) ||
> -                   is_guest_mode(vcpu))
> +               if (is_guest_mode(vcpu))
> +                       skip_cr3 = true;
> +               else if (enable_unrestricted_guest || is_paging(vcpu))
>                         guest_cr3 = kvm_read_cr3(vcpu);
>                 else
>                         guest_cr3 = to_kvm_vmx(kvm)->ept_identity_map_addr;
>                 ept_load_pdptrs(vcpu);
>         }
>
> -       vmcs_writel(GUEST_CR3, guest_cr3);
> +       if (!skip_cr3)
> +               vmcs_writel(GUEST_CR3, guest_cr3);

Is this part of the change necessary, or is it just an optimization to
save a redundant VMWRITE?

>  }
>
>  int vmx_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
> --
> 2.22.0
>