Address @thockin feedback

pkg/api/pod/util.go

@@ -259,8 +259,8 @@ func DropDisabledAlphaFields(podSpec *api.PodSpec) {
 
 	DropDisabledRunAsGroupField(podSpec)
 
-	if !utilfeature.DefaultFeatureGate.Enabled(features.RuntimeClass) && podSpec.RuntimeClassName != "" {
-		podSpec.RuntimeClassName = ""
+	if !utilfeature.DefaultFeatureGate.Enabled(features.RuntimeClass) && podSpec.RuntimeClassName != nil {
+		podSpec.RuntimeClassName = nil
 	}
 }
 

pkg/apis/core/types.go

@@ -2571,12 +2571,14 @@ type PodSpec struct {
 	// More info: https://github.com/kubernetes/community/blob/master/keps/sig-network/0007-pod-ready%2B%2B.md
 	// +optional
 	ReadinessGates []PodReadinessGate
-	// RuntimeClassName refers to a RuntimeClass object with the same name, which should be used to
-	// run this pod.  If no RuntimeClass resource matches the named class, the pod will not be run.
+	// RuntimeClassName refers to a RuntimeClass object in the node.k8s.io group, which should be used
+	// to run this pod.  If no RuntimeClass resource matches the named class, the pod will not be run.
+	// If unset or empty, the "legacy" RuntimeClass will be used, which is an implicit class with an
+	// empty definition that uses the default runtime handler.
 	// More info: https://github.com/kubernetes/community/blob/master/keps/sig-node/0014-runtime-class.md
 	// This is an alpha feature and may change in the future.
 	// +optional
-	RuntimeClassName string
+	RuntimeClassName *string
 }
 
 // HostAlias holds the mapping between IP and hostnames that will be injected as an entry in the

pkg/apis/core/validation/validation.go

@@ -259,7 +259,13 @@ var ValidatePriorityClassName = apimachineryvalidation.NameIsDNSSubdomain
 // ValidateRuntimeClassName can be used to check whether the given RuntimeClass name is valid.
 // Prefix indicates this name will be used as part of generation, in which case
 // trailing dashes are allowed.
-var ValidateRuntimeClassName = apimachineryvalidation.NameIsDNSSubdomain
+func ValidateRuntimeClassName(name string, fldPath *field.Path) field.ErrorList {
+	var allErrs field.ErrorList
+	for _, msg := range apimachineryvalidation.NameIsDNSSubdomain(name, false) {
+		allErrs = append(allErrs, field.Invalid(fldPath, name, msg))
+	}
+	return allErrs
+}
 
 // Validates that given value is not negative.
 func ValidateNonnegativeField(value int64, fldPath *field.Path) field.ErrorList {
@@ -3004,12 +3010,8 @@ func ValidatePodSpec(spec *core.PodSpec, fldPath *field.Path) field.ErrorList {
 		}
 	}
 
-	if len(spec.RuntimeClassName) > 0 {
-		if utilfeature.DefaultFeatureGate.Enabled(features.RuntimeClass) {
-			for _, msg := range ValidateRuntimeClassName(spec.RuntimeClassName, false) {
-				allErrs = append(allErrs, field.Invalid(fldPath.Child("runtimeClassName"), spec.RuntimeClassName, msg))
-			}
-		}
+	if spec.RuntimeClassName != nil && utilfeature.DefaultFeatureGate.Enabled(features.RuntimeClass) {
+		allErrs = append(allErrs, ValidateRuntimeClassName(*spec.RuntimeClassName, fldPath.Child("runtimeClassName"))...)
 	}
 
 	return allErrs

pkg/apis/core/validation/validation_test.go

@@ -6171,7 +6171,7 @@ func TestValidatePodSpec(t *testing.T) {
 			Containers:       []core.Container{{Name: "ctr", Image: "image", ImagePullPolicy: "IfNotPresent", TerminationMessagePolicy: "File"}},
 			RestartPolicy:    core.RestartPolicyAlways,
 			DNSPolicy:        core.DNSClusterFirst,
-			RuntimeClassName: "valid-sandbox",
+			RuntimeClassName: utilpointer.StringPtr("valid-sandbox"),
 		},
 	}
 	for i := range successCases {
@@ -6358,7 +6358,7 @@ func TestValidatePodSpec(t *testing.T) {
 			Containers:       []core.Container{{Name: "ctr", Image: "image", ImagePullPolicy: "IfNotPresent", TerminationMessagePolicy: "File"}},
 			RestartPolicy:    core.RestartPolicyAlways,
 			DNSPolicy:        core.DNSClusterFirst,
-			RuntimeClassName: "invalid/sandbox",
+			RuntimeClassName: utilpointer.StringPtr("invalid/sandbox"),
 		},
 	}
 	for k, v := range failureCases {

staging/src/k8s.io/api/core/v1/types.go

@@ -2865,12 +2865,14 @@ type PodSpec struct {
 	// More info: https://github.com/kubernetes/community/blob/master/keps/sig-network/0007-pod-ready%2B%2B.md
 	// +optional
 	ReadinessGates []PodReadinessGate `json:"readinessGates,omitempty" protobuf:"bytes,28,opt,name=readinessGates"`
-	// RuntimeClassName refers to a RuntimeClass object with the same name, which should be used to
-	// run this pod.  If no RuntimeClass resource matches the named class, the pod will not be run.
+	// RuntimeClassName refers to a RuntimeClass object in the node.k8s.io group, which should be used
+	// to run this pod.  If no RuntimeClass resource matches the named class, the pod will not be run.
+	// If unset or empty, the "legacy" RuntimeClass will be used, which is an implicit class with an
+	// empty definition that uses the default runtime handler.
 	// More info: https://github.com/kubernetes/community/blob/master/keps/sig-node/0014-runtime-class.md
 	// This is an alpha feature and may change in the future.
 	// +optional
-	RuntimeClassName string `json:"runtimeClassName,omitempty" protobuf:"bytes,29,opt,name=runtimeClassName"`
+	RuntimeClassName *string `json:"runtimeClassName,omitempty" protobuf:"bytes,29,opt,name=runtimeClassName"`
 }
 
 // HostAlias holds the mapping between IP and hostnames that will be injected as an entry in the