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SubjectRe: [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@intel.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@vger.kernel.org
> Reported-by: Reto Buerki <reet@codelabs.ch>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.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
>

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