The wedge work did pretty much this - it allowed multiple page tables to be used within one process, and memory tags indicated which regions should be accessibl

The wedge work did pretty much this - it allowed multiple page tables to be used within one process, and memory tags indicated which regions should be accessible to each "sthread". In this case, an sthread would run the JITed untrusted code. There are a lot of details to get right though, such as callgates between untrusted and trusted code so it's possible to call OS or process support functions. Anyway, although wedge was not intended to guard against spectre, it should do so nicely. Andrea had all this working in Linux.