It’s more like a built-in hardware emulation mode than anything else. Modern ARM chips use out of order execution as the default, whereas x86 uses ordered execution as the default. M-series and Snapdragon X chips have a little flag that can be passed to tell the hardware to run in in-order mode instead of out-of-order mode.
Depends on how it’s implemented. If they have a version of Proton that translates all x86 windows syscalls to ARM Linux, some operations could be extremely efficient.
There’s definitely got to be more overhead overall, though. Especially for devices with memory page sizes other than 4K, like the M-series Apple chips do (they use 16K as their page size), likely a VM will need to be sandwiched in there to ensure memory alignment. It’ll more fully be emulation and not just translation.
Even Rosetta still gives up 10%+ efficiency compared to a native compilation of the same program. I’m not saying it’s not viable, but in a resource constrained (especially battery-constrained) device 10% is a lot.
Steam for Android ready to play my PC games from my phone sounds awesome, not gonna lie.
In the shorter-term the issue is the lack of sufficiently powerful commercially-available RISC-V hardware for the level of gaming people expect out of a Steam Deck or VR headset, which ARM already has a number of SOCs capable of.
I don’t doubt that the work will continue but Valve isn’t likely to pour time or money into it until they think the hardware is there.
The architecture was originally developed for desktop PCs, but they discovered it was incredibly efficient at the time (late 80s, early 90s), so Apple partnered with ARM to develop it for the Newton.
The first commercial device with an ARM chip that I remember fondly was a Gameboy Advance.