I don't see why that would have to add latency to all (or any) memory access. The addresses generated by programs (except in real mode, when everything has access to everything anyway so we don't care about these issues then) are virtual addresses, so they have to be translated to get the actual memory address.
The permission information for a page is stored in the same place as the physical address translation information for that page. The processor fetches it at the same time it fethes the physical base address of the page.
They should also have the current permission level of the program readily available. That should be enough to let them do something about Meltdown without any performance impact. They could do something as simple as if the page is a supervisor page and the CPU is not in supervisor mode don't actually read the memory. Just substitute fixed data.
Note that AMD is reportedly not affected by Meltdown. From what I've read that is because they in fact do the protection check before trying to access the memory, even during speculation, and they don't suffer any performance loss from that.
Note that since Meltdown is only an issue when the kernel memory read is on the path that does NOT become the real path (because if it becomes the real path, then the program is going to get a fault anyway for an illegal memory access), the replacing of the memory accesses with fixed data cannot harm any legitimate program.
Spectre is going to be the hard one for the CPU people to fix, I think. I think they may have to offer hardware memory protection features that can be used in user mode code to protect parts of that code from other parts of that code, so that things that want to run untrusted code in a sandbox in their own processes can do so in a separate address space that is protected similar to the way kernel space is protected from user space.
It may be more complicated than that, though, because Spectre also does some freaky things that take advantage of branch prediction information not being isolated between processors. I haven't read enough to understand the implications of that. I don't know if that can be defeated just be better memory protection enforcement.
L1 caches are generally virtually indexed for exactly this reason: to allow a L1 cache read to happen in parallel with the TLB lookup. (They're also usually, I believe, physically tagged, so we have to check for collisions at some point, but making sure there's no side channel information at that point is, obviously given recent events, hard.)
Spectre is, as you say, harder - but more because the line of what sort of state should be separate isn't as clear-cut as we might like it to be (i.e. it's not neccessarily just "processes" as the OS sees it - e.g. JVM/JavaScript interpreter state should allow for an effective sandbox between the executing interpreter/JVM process and what the running JVM/JavaScript code can see). And worse, those are precisely the cases where one probably cares most about separation given that's where untrusted code is typically run.
But hardware assistance could help - in simple terms, I'd imagine that allowing a swap out of more of the internal processor state (to the extent that one process "training" the branch-predictor doesn't impact how the branch predictor acts in another process) would be pretty effective. That might be expensive in terms of performance per-transistor/per-watt however (though probably not absolute performance).
If there are bugs that can be exposed through various machine code patterns, the compiler can centralize the restrictions of what may be executed, enforce runtime checks, or prevent certain instructions from being used at all. Security or optimization updates would affect all running programs automatically. Granted, these current speculative vulnerabilities would be much more difficult to statically detect.
But it would follow the crazy gentoo dream of having everything optimized for your environment better, allow much better compatibility across systems, and prevent entire classes of privilege escalation issues.
So... basically re-inventing Java? :)
"Raw machine code bytes" aren't distributed but occur through the privileged JVM and its just-in-time compiler, the byte-code verifier enforces restrictions on what data-access patterns and where instructions can be used, the JVM for a particular OS has optimizations for that environment, and sandboxing (while imperfect) blocks some classes of privilege escalation issues.
Don't get me wrong, I'm not saying Java is perfect or that the underlying goal isn't good, I'm just happily amused by this sense of "everything old is new again."
Of course, Java certainly does have some higher level weaknesses as in the introspection API kerfuffle a while back, and is too locked into its Object Obsessed design for it to be a truly general purpose object code format.
A privileged process (the microcode) enforces restrictions and converts it to micro-ops which execute on the real processor.
I just don't want to see performance being decimated as a trade off for security, if at all possible.
Having said that, AMD CPUs are the existence proof that you can be immune to meltdown with no significant overhead.
Spectre is a completely different issue though.
Perhaps that because they haven't taken the speed short-cuts that Intel took...?
But yeah, protecting against it means implementing memory protection in more places in the CPU. More gates and the possibility of becoming a bottleneck.
I believe one solution would be to put permission checks before the memory access, which would add serialized latency to all memory access. Another would be to have the speculative execution system flush cache lines that were loaded but ultimately ignored, which would be complex but probably not be as much of a speed hit.
(edit: yeah, a simple "flush" is insufficient, it would have to be closer to an isolated transaction with rollback of the access's effects on the cache system.)