Re:Reading privileged memory with a side-channel
by bonzini on Friday January 05, 2018 @03:10PM
(#16081242)
> There is no fundamental reason why speculative instructions should be allowed to mutate the cache.
There is: hundreds of instructions can be in flight speculatively at the same time, especially if you take hyperthreading into account. Good luck rolling them all back.
The question is not whether the cache should be mutated during speculative execution. It's what kinds of speculative execution are allowed, and in some cases it's not even clear if fences should be placed by the programmer (whack-a-mole style), the compiler (not sure how) or the processor (probably not). It's non-obvious enough that how to solve it is to some extent a research problem.
There is: hundreds of instructions can be in flight speculatively at the same time, especially if you take hyperthreading into account. Good luck rolling them all back.
The question is not whether the cache should be mutated during speculative execution. It's what kinds of speculative execution are allowed, and in some cases it's not even clear if fences should be placed by the programmer (whack-a-mole style), the compiler (not sure how) or the processor (probably not). It's non-obvious enough that how to solve it is to some extent a research problem.