Quick answer: The experimental xess-xmx-linux project runs Intel XeSS super resolution and frame generation on Arc XMX units inside Proton games instead of the slower DP4a path. It was built on a OneXPlayer 3 with an Arc B390, and PC Gamer cites Cyberpunk gains of about 29% from XMX upscaling alone and 145% with XeSS frame generation.
Intel still hasn't shipped its own XeSS library for Proton, but an independent open-source project is already running the real XMX path on Linux. The xess-xmx-linux repo puts XeSS super resolution and XeSS frame generation on Intel Arc's matrix (XMX) units inside Proton games, instead of the slower DP4a fallback stock Wine/Proton gets.
According to Phoronix, a shim DLL answers XeSS driver queries the way Intel's Windows driver does, and a patched Mesa ANV driver compiles XeSS's own kernels with Intel's IGC on the machine. Frame generation works as on Windows, including multi-frame generation at 2x/3x/4x on XMX. Games with FSR 3.1+ can also route that stack onto XeSS with a launch option.
The project was developed on a OneXPlayer 3 with an Arc B390 running SteamOS. Users have also reported it on Arc B580 and A310 hardware, and on MSI Claw handhelds. PC Gamer cites a Cyberpunk 2077 gain of about 29% from switching upscaling to XMX alone, and about 145% when XeSS frame generation is added. Forza Horizon 6 shows a smaller ~16% bump from the XMX upscaler path before frame generation.
It needs the native Steam client, not Flatpak Steam, and it isn't official Intel or Valve support. Intel has said it's working on native Linux XeSS for Proton; this project uses the Windows XeSS libraries in the meantime.