Isn't the problem rather that we have instructions to control the cache at all? As far as my research goes, things like CLFLUSH were introduced at the same time as SSE2, which was what, the Pentium III era? We were apparently doing fine without them before that, even with 4+ CPU systems.

I'm currently trying to get the spectre.c PoC to work without using CLFLUSH or similar, but it doesn't look too promising yet. Then again, I only started an hour ago and lack the ability to think in those creative and twisted ways that lead to the discovery of this whole issue in the first place.