When Spectre first came out, my prediction was basically that the only effective mitigation would be to move the untrusted code into another address space. This post reinforces that they have exhausted all other options:
> Our research reached the conclusion that, in principle, untrusted code can read a process’s entire address space using Spectre and side channels. Software mitigations reduce the effectiveness of many potential gadgets, but are not efficient or comprehensive. The only effective mitigation is to move sensitive data out of the process’s address space.
Probably the most effective hardware mitigation would be to shift the isolation to a page-level granularity, so that you could say that speculation is disabled for memory in specific pages.
> Our research reached the conclusion that, in principle, untrusted code can read a process’s entire address space using Spectre and side channels. Software mitigations reduce the effectiveness of many potential gadgets, but are not efficient or comprehensive. The only effective mitigation is to move sensitive data out of the process’s address space.
Probably the most effective hardware mitigation would be to shift the isolation to a page-level granularity, so that you could say that speculation is disabled for memory in specific pages.