I don't think undoing all cache changes is enough, since it went to memory it generated memory bus traffic, which could be observed by another core (as extra latency). And speaking of another core, the cacheline might have come from another core, which could observe the effect on its own cache, even if it's later put back.
Yes. I doubt this will be what actually happens, but perhaps the safest fix is to remove speculative execution in its entirety, increasing clock frequency and/or core count to mitigate the resulting performance loss by as much as possible.
