> Software developers creating platform-independent code are advised to avoid using r18 if at all possible. Most compilers provide a mechanism to prevent specific registers from being used for general allocation; portable hand-coded assembler should avoid it entirely. It should not be assumed that treating the register as callee-saved will be sufficient to satisfy the requirements of the platform. Virtualization code must, of course, treat the register as they would any other resource provided to the virtual machine.
From: https://github.com/ARM-software/abi-aa/blob/2bcab1e3b22d5517...
On Darwin, x18 is used as a scratch register in context switches on hardware where Meltdown mitigations are needed.
As such, it is cleaned on each ctx switch on that hardware.
On M1, it's currently usable by applications, but that is not part of the ABI contract and might change at any time without notice.
On Windows, x18 is the TEB (thread environment block) register. It must as such _not_ be touched by apps either.
I’m confused. x18 is an ARM register, but I thought Meltdown only affected x86 chips. Were iOS devices vulnerable to Meltdown too? Or did you mean not x18 specifically but some equivalent OS-reserved x86 register?
However, due to specific features of the Arm architecture, it was patchable without a significant performance hit.
(see this section: https://siguza.github.io/KTRR/#meltdownspectre-mitigations-1..., which made the need for actually doing a page table switch avoidable)