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VMWare, etc started making virtual machines an actual viable product, because they were able to set things up so that the majority of code in the client OS was running directly on the host CPU. Those problems are why CPUs added explicit support for virtualization in the early 2000s. So now what was previously a single memory access for the CPU involves a code loop with multiple (again, for the cpu) random memory accesses. Significant silicon resources are expended (the extremely high performance and physically colocated TLBs, caches, etc) to make paged memory efficient, and suddenly you have actual code running and walking what (to the cpu) is just regular ram.

It's hard to say exactly which bit would be the worst part, but likely memory protection - you could very easily end up in a state where your translator has to added non-hardware supported memory protection checks on every memory access, and that is slow. But at a more fundamental level your translator now needs to translate completely different mechanisms for how memory is accessed, how page tables are set up, how interrupts, are handled, etc. That basically breaks down completely for kernel mode code, for a variety of reasons, at one level a lot of the things rosetta style translators can do for speed break down because they can't just leverage memory protection any more.

Windows has its own translator so doesn't need rosetta.Īt a basic level the semantics of usermode x86 and usermode arm are identical, so you can basically say 'I just care about translating the x86 instructions to arm equivalents', and all the rest of the 'make this code run correctly' semantics of the hardware (memory protection, etc) then just work. Happily Apple actually provides direct support for linux in rosetta (as in it can parse linux binaries and libraries, etc can remap syscalls correctly, etc in linux VMs), and there's even a WWDC video on how to integrate it, so that you can install an arm linux kernel, and then run x86_64 linux binaries via rosetta. Rosetta is an emulator (sorry translator) for userspace.

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