> Intel CPUs allow this, but AMD CPUs do not.

Do we know this for sure ? I see now that the meltdown paper has a suggestion in chapter 7.1 that the CPU should perform the permission check on the page table before data is fetched to the cache, But dismisses that suggestion on the grounds it will haves very high performance impact.

Are you saying AMD does the check first anyway, and either takes the performance hit or has found a way to not take a performance hit ?