I don't think it's just rumours. Microsoft, Apple and Red Hat have said that full mitigation requires disabling hyperthreading. While Intel don't recommend doin

I don't think it's just rumours. Microsoft, Apple and Red Hat have said that full mitigation requires disabling hyperthreading. While Intel don't recommend doing so, their justification for this is that most people don't actually need process protection anyway. They also note that "it’s important to understand that doing so does not alone provide protection against MDS", which is true - you need to disable hyperthreading and apply additional performance-sapping mitigations which require both microcode updates and operating system support. The hyperthreading-based variants seem to be a lot more powerful than the ones fixed by the other mitigations though.