Doesn’t Spectre/Meltdown rely on hyperthreading? In that case, not doing that would stop it, no? Or am I misinformed and it would affect multi core single threaded chips as well?
There were multiple attack vectors and some of them were significantly better at leaking information with hyperthreading enabled but the basic cache side channel leak worked just fine in single core systems before kernel patches mitigated the impact by flushing hardware buffers as part of the context switch (at a significant performance cost)
If you had a machine that was gapped could you disable the flushing?
Any machine that runs only verified code (ie. no JavaScript in the browser, no downloading nifty things and running them) don't need the flushes. Problem is (as shown by the Solarwind hack) even code you should be able to trust can be backdoored. However: if that is the attack vector, spectre and its' offspring are not your real concern: there are much more efficient and effective ways to compromise your machine if a blob of binary ends of being backdoored.
Yes and no. Yes I think it will protect independent processes sharing a computer that's fully patched. No it will not protect you if you run multiple untrusted scripts in the same process. This is partly why Chrome moved to isolate tabs in their own process after the V8 team eventually admitted defeat in implementing spectre mitigations.