Update Control Plane replica validation for Single Node OpenShift

Installer can be used to provision Single Node OpenShift(SNO) only
for AWS/GCP/Azure.
SNO is not supported on all on-prem platforms yet. Even on the
platforms that support SNO installs, the install method currently
supported are assisted/agent/ZTP.

pkg/types/validation/installconfig.go

@@ -121,7 +121,7 @@ func ValidateInstallConfig(c *types.InstallConfig, usingAgentMethod bool) field.
 	}
 	allErrs = append(allErrs, validatePlatform(&c.Platform, usingAgentMethod, field.NewPath("platform"), c.Networking, c)...)
 	if c.ControlPlane != nil {
-		allErrs = append(allErrs, validateControlPlane(&c.Platform, c.ControlPlane, field.NewPath("controlPlane"))...)
+		allErrs = append(allErrs, validateControlPlane(&c.Platform, c.ControlPlane, c.IsSingleNodeOpenShift(), field.NewPath("controlPlane"))...)
 	} else {
 		allErrs = append(allErrs, field.Required(field.NewPath("controlPlane"), "controlPlane is required"))
 	}
@@ -596,13 +596,20 @@ func validateNetworkingClusterNetworkMTU(c *types.InstallConfig, fldPath *field.
 	return allErrs
 }
 
-func validateControlPlane(platform *types.Platform, pool *types.MachinePool, fldPath *field.Path) field.ErrorList {
+func validateControlPlane(platform *types.Platform, pool *types.MachinePool, isSingleNodeOpenShift bool, fldPath *field.Path) field.ErrorList {
 	allErrs := field.ErrorList{}
 	if pool.Name != types.MachinePoolControlPlaneRoleName {
 		allErrs = append(allErrs, field.NotSupported(fldPath.Child("name"), pool.Name, []string{types.MachinePoolControlPlaneRoleName}))
 	}
-	if pool.Replicas != nil && *pool.Replicas == 0 {
-		allErrs = append(allErrs, field.Invalid(fldPath.Child("replicas"), pool.Replicas, "number of control plane replicas must be positive"))
+	if pool.Replicas != nil {
+		switch {
+		case *pool.Replicas == 0:
+			allErrs = append(allErrs, field.Invalid(fldPath.Child("replicas"), pool.Replicas, "number of control plane replicas must be positive"))
+		case *pool.Replicas == 1 && !supportedSingleNodePlatform(isSingleNodeOpenShift, platform):
+			allErrs = append(allErrs, field.Invalid(fldPath.Child("replicas"), pool.Replicas, "Single Node OpenShift(SNO) not supported by this install method for this platform"))
+		case *pool.Replicas != 3:
+			allErrs = append(allErrs, field.Invalid(fldPath.Child("replicas"), pool.Replicas, "number of control plane replicas must equal 3"))
+		}
 	}
 	allErrs = append(allErrs, ValidateMachinePool(platform, pool, fldPath)...)
 	return allErrs
@@ -1345,3 +1352,17 @@ func validateReleaseArchitecture(controlPlanePool *types.MachinePool, computePoo
 
 	return allErrs
 }
+
+// supportedSingleNodePlatform indicates if the IPI Installer can be used to install SNO on
+// a platform.
+func supportedSingleNodePlatform(bootstrapInPlace bool, p *types.Platform) bool {
+	if p.AWS != nil || p.GCP != nil || p.Azure != nil {
+		// Single node OpenShift installations supported without `bootstrapInPlace`
+		return true
+	}
+	if bootstrapInPlace && p.None != nil {
+		// Single node OpenShift installations supported with `bootstrapInPlace`
+		return true
+	}
+	return false
+}