[gok]

Important to differentiate between ARM the company, the instruction set architecture(s) and the specific implementation of those ISAs. The licensable nature of ARM means there very likely are (possibly undiscovered) implementations of the ARM ISAs floating around which are susceptible to Meltdown.

[palotasb]

I was under the impression that they generally license the IP cores (or at least some IP blocks) to implement the ISA and downstream vendors don't implement those differently.