Linus Torvalds (torvalds.delete@this.linux-foundation.org) on September 21, 2021 10:04 am wrote:

> And it doesn't even fix the permission problems. We considered using segmentation to fix Meltdown,
> but even the segment range check wasn't necessarily done early enough, so while it helped the
> data leak in some situations, it had the same fundamental issues that a TLB lookup did.

By description MMU first check privilege level and only after success gives access to data. No matter which kind of privilege checking system is used if MMU fist gives data and checks permissions later, that data can be leaked through side-channels.

It's a fundamental problem that Meltdown cpus did wrong.