KVM: VMX: Skip GUEST_CR3 VMREAD+VMWRITE if the VMCS is up-to-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.
Tested-by: Reto Buerki <reet@codelabs.ch>
Tested-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/vmx.c

@@ -3009,10 +3009,12 @@ void vmx_set_cr3(struct kvm_vcpu *vcpu, unsigned long cr3)
 		/* Loading vmcs02.GUEST_CR3 is handled by nested VM-Enter. */
 		if (is_guest_mode(vcpu))
 			update_guest_cr3 = false;
-		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 /* vmcs01.GUEST_CR3 is already up-to-date. */
+			update_guest_cr3 = false;
 		ept_load_pdptrs(vcpu);
 	}