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Why does Linux run so well on ARM
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this post was submitted on 20 Aug 2023
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Microsoft absolutely could have made something comparable to Rosetta 2 for userspace if they were competent.
Rosetta 2 isn't an emulator, but a binary recompiler. It takes
amd64instructions, decodes them, and generates equivalentaarch64instructions. Theaarch64instructions are then executed directly by the processor, performing the same tasks that the original binary would do on an Intel processor.It's extremely difficult to do properly, but it's nothing inherently special to MacOS or Apple's ARM chips. ARMv8 has an attribute to enable strongly-ordered memory accesses, and it also supports native 4 KiB page sizes. Beyond those two solved concerns, there isn't any actual hardware barrier preventing binary translation. Individual
amd64instructions can be translated into one or more equivalentaarach64instructions, and complex instructions or those using large registers like those in AVX-512 can be shimmed and implemented in software. An offset table can be used to deal with indirect jumps, and direct jumps can just be rewritten in the generated code. And as Apple has proven, it's even possible to support JIT-compiled code by intercepting jumps into executable pages and recompiling them before executing.It's expensive in terms of time and engineering skills, but Microsoft had more than enough control over their own proprietary kernel to build something similar into Windows back when they first released it for ARM.
Rosetta certainly does emulate* x86. It can dynamically recompile x86 instructions to ARM instructions, otherwise applications that include an x86 JIT wouldn't work at all on ARM Macs.
* I know people will be pedantic about this... but other emulators (Dolphin, PCSX2 etc) have included a recompiler for ages and no one seemed to have a problem calling them emulators.
As far as people hypothetically being pedantic about it being an emulator, it does meet the dictionary definition of "hardware or software that permits programs written for one computer to be run on another computer." Personally, I don't see as one, though. It's more like WINE with a binary translator slapped onto it.
Dynamic recompilation is a part of modern emulators, but it's only a tiny piece. Software like Dolphin or Yuzu don't just provide a way to run non-native instruction sets, but provide a full environment mimicking the guest hardware. Things like low-level emulation of hardware components, high-level emulation (shims) of guest operating system APIs, and a virtual memory space for the programs to operate in.
The only significant thing Rosetta does is recompile the instructions of the guest program. All the APIs and abstractions the
amd64-compiled program uses are available natively. If I recall correctly there are shims for bridging between the calling convention of the host and the recompiled-amd64functions, but they don't do much more work than that.Another one of my reasons for not considering it to be an emulator is because it primarily goes for ahead-of-time cached recompilation. It definitely does JIT translation as you mentioned, but as a way to support
amd64JIT-generated code. In contrast, Dolphin and other emulators* rely on cached JIT recompiling or interpreting for everything related to executing the guest instructions.* Notable exceptions are Cxbx (Xbox -> Windows) and vita2hos (PS Vita -> Switch), which are emulators for platforms with compatible instructions sets. They work like WINE instead of JIT-recompiling or interpreting code, which is pretty cool.