Well we don't really know, because AMD hasn't said in enough detail, but it is entirely possible that they are totally immune to the Meltdown attack. The crux

Well we don't really know, because AMD hasn't said in enough detail, but it is entirely possible that they are totally immune to the Meltdown attack. The crux of it comes down to how their TLB, L1 and out-of-order engine interact. Can a load Y whose address depends on an earlier load X whose entry exists in the TLB but only with the S-bit (supervisor access only) end up actually using the true value of X before it is squashed? Clearly Intel allows it to occur, but it's not obvious that this has to be the way. The TLB, which presumably contains the S-bit, is already on the critical load-to-load dependency path (since the physical tag needs to be used to select the way) so it isn't obvious that the S-check can't happen at the same time and essentially squash the result of the load X before it ever shows up on the bypass network for consumption by Y. On the other hand, it might be slightly easier to let the X result show up up on the bypass network and do the S-check in parallel and then only flag the ROB-entry as squashed after the fact[1] - perhaps it saves a MUX in the critical path. This is very much like unlike Spectre, where it is more or less obvious by the way that almost everyone does branch prediction and speculative execution that you can probably pull of the attacks on modern cores (perhaps with the exception of branch predictors that do a full address check to access the BTB). Here AMD has also tried to claim to some invulnerability, but IMO these claims are much weaker since it seems unlikely that the predictors cannot be trained. AMD is probably just relying on the fact the the predictors are harder to train. [1] It's worth noting that everyone is saying that Intel only applies the security check at retirement - but we don't really know this: we only know they apply it "too late" in that the Y load can consume the result, but it could still be applied before retirement, as little as a few cycles later.