KVM: nVMX: Invoke ept_save_pdptrs() if and only if PAE paging is enabled

Invoke ept_save_pdptrs() when restoring L1's host state on a "late"
VM-Fail if and only if PAE paging is enabled.  This saves a CALL in the
common case where L1 is a 64-bit host, and avoids incorrectly marking
the PDPTRs as dirty.

WARN if ept_save_pdptrs() is called with PAE disabled now that the
nested usage pre-checks is_pae_paging().  Barring a bug in KVM's MMU,
attempting to read the PDPTRs with PAE disabled is now impossible.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200415203454.8296-2-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/nested.c

@@ -4250,7 +4250,7 @@ static void nested_vmx_restore_host_state(struct kvm_vcpu *vcpu)
 	 * VMFail, like everything else we just need to ensure our
 	 * software model is up-to-date.
 	 */
-	if (enable_ept)
+	if (enable_ept && is_pae_paging(vcpu))
 		ept_save_pdptrs(vcpu);
 
 	kvm_mmu_reset_context(vcpu);

arch/x86/kvm/vmx/vmx.c

@@ -2937,12 +2937,13 @@ void ept_save_pdptrs(struct kvm_vcpu *vcpu)
 {
 	struct kvm_mmu *mmu = vcpu->arch.walk_mmu;
 
-	if (is_pae_paging(vcpu)) {
-		mmu->pdptrs[0] = vmcs_read64(GUEST_PDPTR0);
-		mmu->pdptrs[1] = vmcs_read64(GUEST_PDPTR1);
-		mmu->pdptrs[2] = vmcs_read64(GUEST_PDPTR2);
-		mmu->pdptrs[3] = vmcs_read64(GUEST_PDPTR3);
-	}
+	if (WARN_ON_ONCE(!is_pae_paging(vcpu)))
+		return;
+
+	mmu->pdptrs[0] = vmcs_read64(GUEST_PDPTR0);
+	mmu->pdptrs[1] = vmcs_read64(GUEST_PDPTR1);
+	mmu->pdptrs[2] = vmcs_read64(GUEST_PDPTR2);
+	mmu->pdptrs[3] = vmcs_read64(GUEST_PDPTR3);
 
 	kvm_register_mark_dirty(vcpu, VCPU_EXREG_PDPTR);
 }