The Scourge of x86 Emulation
FEX Emu explains why emulating x86's strict Total Store Ordering memory model on ARM's relaxed architecture is a persistent performance nightmare. Early ARMv8.0 solutions forced every x86 load and store into acquire/release instructions, crippling CPUs like AmpereOne. Newer extensions like LRCPC and Apple's hardware TSO toggle offer relief, but alignment issues and split-locks remain unsolved.
Using acquire-release semantics for every load for x86 emulation actually imposes some really strict limitations on ARM CPUs in that the load instructions can no longer be ordered around each other at all.
- pdw
The intro of this article repeats the common assertion that
> ARM is the most relaxed, allowing significant hardware optimizations; and x86 is the most strict, enforcing a very strong coherency model that doesn’t allow a lot of room for optimization
but I've seen some compelling arguments that a relaxed model doesn't necessarily have much of a benefit, https://fgiesen.wordpress.com/2026/08/25/memory-ordering-in-...
- dagmx
For reference , Fex is a translation framework for x86 to ARM much like Apple’s Rosetta2 and Microsoft’s Prism.
Valve sponsor development as it’s also the way the new Steam Frame supports x86 games. It’s also being used (as a fork) in Crossover Beta to replace the use of Rosetta2.
- sureglymop
Slightly related but this project (FEX) is amazing. I've been running Armada OS on multiple ARM handhelds and they're viable little linux machines now except with amazing battery life.
Most problems I run into are anti cheat related (EAC, etc.) but those can be circumvented for now. Feels kind of insane how far x86 emulation has come.
- modeless
As noted in the article, Apple solved this problem six years ago by simply adding an x86-compatible memory ordering mode to their chip when x86 emulation became important. Yet another way Apple's chips lead the industry.
- asksomeoneelse
Great article ! This is the kind of content I always hope to find on HN's front page.
I really wonder how things are organized at Apple to allow for vertical integration to work so well. That feature alone must have involved so many people from so many different teams.