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    Subject[PATCH 2/2] KVM: VMX: Skip GUEST_CR3 VMREAD+VMWRITE if the VMCS is up-to-date
    Date
    Skip the VMWRITE to update GUEST_CR3 if CR3 is not available, i.e. has
    not been read from the VMCS since the last VM-Enter. If vcpu->arch.cr3
    is stale, kvm_read_cr3(vcpu) will refresh vcpu->arch.cr3 from the VMCS,
    meaning KVM will do a VMREAD and then VMWRITE the value it just pulled
    from the VMCS.

    Note, this is a purely theoretical change, no instances of skipping
    the VMREAD+VMWRITE have been observed with this change.

    Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
    ---
    arch/x86/kvm/vmx/vmx.c | 8 +++++---
    1 file changed, 5 insertions(+), 3 deletions(-)

    diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
    index b530950a9c2b..6de09f60edf3 100644
    --- a/arch/x86/kvm/vmx/vmx.c
    +++ b/arch/x86/kvm/vmx/vmx.c
    @@ -3003,10 +3003,12 @@ void vmx_set_cr3(struct kvm_vcpu *vcpu, unsigned long cr3)

    if (is_guest_mode(vcpu))
    skip_cr3 = true;
    - else if (enable_unrestricted_guest || is_paging(vcpu))
    - guest_cr3 = kvm_read_cr3(vcpu);
    - else
    + else if (!enable_unrestricted_guest && !is_paging(vcpu))
    guest_cr3 = to_kvm_vmx(kvm)->ept_identity_map_addr;
    + else if (test_bit(VCPU_EXREG_CR3, (ulong *)&vcpu->arch.regs_avail))
    + guest_cr3 = vcpu->arch.cr3;
    + else
    + skip_cr3 = true; /* vmcs01.GUEST_CR3 is up-to-date. */
    ept_load_pdptrs(vcpu);
    }

    --
    2.22.0
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