The Host-Side Software is the second complementary part of the trusted application. It is the responsibility of the host-side software to manage the trusted application and its instances, and to handle the ‘positive’ functional flows which might involve events initiated from within the firmware and interactions with back-end servers. On the other hand, it should cover ‘negative’ flows in which errors occurred, in cases in which some event in the platform occurred and has an impact on the application (e.g., power flow), and to protect the trusted application from unauthorized applications to interact with it.
Validation of the host-side software isn’t gating for being able to get your trusted application signed, but finding functional or security gaps there might need changes in the trusted application. Therefore it should be the validation and security teams’ responsibility to cover it as part of the trusted application’s validation plan.