I don't think the current mitigations under discussion here are about keeping the system working. In fact most encrypted VM configs tend to stop booting as a preferred way to handle security issues.
Maybe we should avoid the "trusted" term here. We're only really using it because USB is using it and we're now using a common framework like Greg requested. But I don't think it's the right way to think about it.
We usually call the drivers "hardened". The requirement for a hardened driver is that all interactions through MMIO/port/config space IO/MSRs are sanitized and do not cause memory safety issues or other information leaks. Other than that there is no requirement on the functionality. In particular DOS is ok since a malicious hypervisor can decide to not run the guest at any time anyways.
Someone loading an malicious driver inside the guest would be out of scope. If an attacker can do that inside the guest you already violated the security mechanisms and there are likely easier ways to take over the guest or leak data.
The goal of the device filter mechanism is to prevent loading unhardened drivers that could be exploited without them being themselves malicious.
-Andi
