KVM: VMX: Move nested EPT out of kvm_x86_ops.get_tdp_level() hook

Separate the "core" TDP level handling from the nested EPT path to make
it clear that kvm_x86_ops.get_tdp_level() is used if and only if nested
EPT is not in use (kvm_init_shadow_ept_mmu() calculates the level from
the passed in vmcs12->eptp).  Add a WARN_ON() to enforce that the
kvm_x86_ops hook is not called for nested EPT.

This sets the stage for snapshotting the non-"nested EPT" TDP page level
during kvm_cpuid_update() to avoid the retpoline associated with
kvm_x86_ops.get_tdp_level() when resetting the MMU, a relatively
frequent operation when running a nested guest.

No functional change intended.
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200502043234.12481-10-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/vmx.c

@@ -3023,15 +3023,23 @@ void vmx_set_cr0(struct kvm_vcpu *vcpu, unsigned long cr0)
 	vmx->emulation_required = emulation_required(vcpu);
 }
 
-static int get_ept_level(struct kvm_vcpu *vcpu)
+static int vmx_get_tdp_level(struct kvm_vcpu *vcpu)
 {
-	if (is_guest_mode(vcpu) && nested_cpu_has_ept(get_vmcs12(vcpu)))
-		return vmx_eptp_page_walk_level(nested_ept_get_eptp(vcpu));
+	WARN_ON(is_guest_mode(vcpu) && nested_cpu_has_ept(get_vmcs12(vcpu)));
+
 	if (cpu_has_vmx_ept_5levels() && (cpuid_maxphyaddr(vcpu) > 48))
 		return 5;
 	return 4;
 }
 
+static int get_ept_level(struct kvm_vcpu *vcpu)
+{
+	if (is_guest_mode(vcpu) && nested_cpu_has_ept(get_vmcs12(vcpu)))
+		return vmx_eptp_page_walk_level(nested_ept_get_eptp(vcpu));
+
+	return vmx_get_tdp_level(vcpu);
+}
+
 u64 construct_eptp(struct kvm_vcpu *vcpu, unsigned long root_hpa)
 {
 	u64 eptp = VMX_EPTP_MT_WB;
@@ -7859,7 +7867,7 @@ static struct kvm_x86_ops vmx_x86_ops __initdata = {
 
 	.set_tss_addr = vmx_set_tss_addr,
 	.set_identity_map_addr = vmx_set_identity_map_addr,
-	.get_tdp_level = get_ept_level,
+	.get_tdp_level = vmx_get_tdp_level,
 	.get_mt_mask = vmx_get_mt_mask,
 
 	.get_exit_info = vmx_get_exit_info,