There are two problems:

Problem 1. net-istio manifest is deployed by default.  

Current KnativeServing CR configuration is:

  config:
    network:
      ingress.class: "kourier.ingress.networking.knative.dev"
  ingress:
    (empty as openshift-knative-operator/cmd/operator/kodata/ingress/ is empty)

But once we added net-istio manifest into the kodata/ingress, net-istio is deployed by default.

Note: Deploying net-istio controller by default has a problem. The manifest has Istio specific resource like PeerAuthentication so users must install Istio beforehand or we must remove the istio specific resources from manifest.

Problem 2. ingress.class in KnativeServing cannot be set to istio. 

If users set ingress.class like this:

  config:
    network:
      ingress.class: "istio.ingress.networking.knative.dev"

the current serverless-opreator reconciles it to Kourier. We want to make it customizable but Kourier is still default.