> A potential concern is that when jumping between x86 emulation and ARM code, that the ARM code will pay unnecessary overhead due to all its accesses being TSO now. While this is a reasonable concern, the amount of ARM native code executing under emulation approaches 0%.

This may be true when FEX is executing as a usermode whole-process emulator on Linux, but it is not true when FEX is built for Windows(/Wine)'s ARM64EC mode. With ARM64EC a thread could be running a very small amount of emulated code while everything else is native.

I believe Microsoft Office is built as ARM64EC in order to support x86_64 plugins, in this case the entire suite itself (along with all the system DLLs) are native ARM64EC code and the only emulation would be for plugins. Kingdom Come Deliverance 2 has an ARM64EC build where the main game EXE is small and x86_64, but the actual game engine is in an ARM64EC DLL.

I don't know of a good solution for this though, enabling/disabling TSO needs a kernel syscall so is too slow to be doing constantly when entering/leaving emulation. With cases like KCD2 where the game itself is ARM64EC, maybe it could be faster to not use hardware TSO.