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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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Apple designed the silicon to have an "x86 mode" for the memory model ordering, as well as an undocumented mode that makes certain arm instructions set flags similarly to x86. There's a good write up of the reasons here: https://dougallj.wordpress.com/2022/11/09/why-is-rosetta-2-fast/
Apple hit a sweet spot with this. x86_64 applications run at acceptable speed (making the transition easy for people who buy the hardware) while not being SO good that there's zero reason for developers to start porting their software.
Small correction: the flag setting modes aren't undocumented. They're standardized extensions. ARMv8.4 added
FEAT_FlagM, and ARMv8.5 addedFEAT_FlagM2.https://developer.arm.com/downloads/-/exploration-tools/feature-names-for-a-profile
IIRC, the only nonstandard ARM extension used by Rosetta 2 in Apple's processors is TSO, and that's also implemented by other manufacturers. It's also not a hard requirement to run
amd64under ARM. You can emulate it very slowly or restrict the application to a single core.Apologies for the tangent, but I needed to make sure nobody could defend Microsoft's prior failings by saying "but Apple has secret hardware sauce".
So while not technically "secret sauce," it's certainly "special sauce." Good point.