KVM: x86/mmu: Detect if unprotect will do anything based on invalid_list

Explicitly query the list of to-be-zapped shadow pages when checking to
see if unprotecting a gfn for retry has succeeded, i.e. if KVM should
retry the faulting instruction.

Add a comment to explain why the list needs to be checked before zapping,
which is the primary motivation for this change.

No functional change intended.
Reviewed-by: Yuan Yao <yuan.yao@intel.com>
Link: https://lore.kernel.org/r/20240831001538.336683-22-seanjc@google.comSigned-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/kvm/mmu/mmu.c

@@ -2721,12 +2721,15 @@ bool __kvm_mmu_unprotect_gfn_and_retry(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
 			goto out;
 	}
 
-	r = false;
 	write_lock(&kvm->mmu_lock);
-	for_each_gfn_valid_sp_with_gptes(kvm, sp, gpa_to_gfn(gpa)) {
-		r = true;
+	for_each_gfn_valid_sp_with_gptes(kvm, sp, gpa_to_gfn(gpa))
 		kvm_mmu_prepare_zap_page(kvm, sp, &invalid_list);
-	}
+
+	/*
+	 * Snapshot the result before zapping, as zapping will remove all list
+	 * entries, i.e. checking the list later would yield a false negative.
+	 */
+	r = !list_empty(&invalid_list);
 	kvm_mmu_commit_zap_page(kvm, &invalid_list);
 	write_unlock(&kvm->mmu_lock);