kvm: mmu: fix is_tdp_mmu_check when the TDP MMU is not in use

In some cases where shadow paging is in use, the root page will
be either mmu->pae_root or vcpu->arch.mmu->lm_root.  Then it will
not have an associated struct kvm_mmu_page, because it is allocated
with alloc_page instead of kvm_mmu_alloc_page.

Just return false quickly from is_tdp_mmu_root if the TDP MMU is
not in use, which also includes the case where shadow paging is
enabled.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/tdp_mmu.c

@@ -49,7 +49,14 @@ bool is_tdp_mmu_root(struct kvm *kvm, hpa_t hpa)
 {
 	struct kvm_mmu_page *sp;
 
+	if (!kvm->arch.tdp_mmu_enabled)
+		return false;
+	if (WARN_ON(!VALID_PAGE(hpa)))
+		return false;
+
 	sp = to_shadow_page(hpa);
+	if (WARN_ON(!sp))
+		return false;
 
 	return sp->tdp_mmu_page && sp->root_count;
 }