KVM: x86/mmu: Don't WARN on a NULL shadow page in TDP MMU check

Treat a NULL shadow page in the "is a TDP MMU" check as valid, non-TDP
root.  KVM uses a "direct" PAE paging MMU when TDP is disabled and the
guest is running with paging disabled.  In that case, root_hpa points at
the pae_root page (of which only 32 bytes are used), not a standard
shadow page, and the WARN fires (a lot).

Fixes: 0b873fd7 ("KVM: x86/mmu: Remove redundant is_tdp_mmu_enabled check")
Cc: David Matlack <dmatlack@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210622072454.3449146-1-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/tdp_mmu.h

@@ -94,11 +94,13 @@ static inline bool is_tdp_mmu(struct kvm_mmu *mmu)
 	if (WARN_ON(!VALID_PAGE(hpa)))
 		return false;
 
+	/*
+	 * A NULL shadow page is legal when shadowing a non-paging guest with
+	 * PAE paging, as the MMU will be direct with root_hpa pointing at the
+	 * pae_root page, not a shadow page.
+	 */
 	sp = to_shadow_page(hpa);
-	if (WARN_ON(!sp))
-		return false;
-
-	return is_tdp_mmu_page(sp) && sp->root_count;
+	return sp && is_tdp_mmu_page(sp) && sp->root_count;
 }
 #else
 static inline bool kvm_mmu_init_tdp_mmu(struct kvm *kvm) { return false; }