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Windows games on ChromeOS with Proton

Steam is now available on ChromeOS⁠ and many users are already enjoying some of their favorite games. But you may be wondering how the magic happens. Borealis is a VM running Arch Linux⁠, so Linux titles run the same way that they run on any Linux distribution. For Windows games, the story is more complex. Windows games run through a compatibility layer called Proton, which Steam downloads automatically when a user installs a Windows game on a Linux system.

So… what exactly is Proton?

Scrolling through the Proton repository⁠, you’ll quickly notice that the vast majority of Proton consists of git submodules. Proton is a collection of modules that grew organically in the open-source community out of a desire to bring widely-available cross-platform support to the world. Proton’s real value-add is sorting out how to get all of these modules working together properly and with good performance. This is no easy goal since the support offered by Proton is fraught with corner cases. Achieving synergy of these modules requires applying and maintaining meaningful modifications on top of upstream open source projects.

Proton support is still growing and can vary a lot even between similar games. The dedicated Linux gaming community has created ProtonDB⁠ to keep track of supported games. Most of the time, titles marked as Platinum and Gold work well on most Linux systems. Being an Arch Linux VM, Borealis often supports most of these titles as well, when device hardware specs allow.

Windows games can be installed by Steam on ChromeOS after either enabling Proton in the Steam Play tab of the Steam settings or enabling Proton for a specific game in the Compatibility tab of the game properties. Proton is automatically installed by Steam alongside game installations that require it.

Standardizing Proton dependencies

In general, software will work best when the runtime system has libraries with ABIs and APIs that match the build system’s. Unfortunately, there are no strong ABI or API guarantees in Linux across distros or library versions. Package managers traditionally work around this issue via dependency tracking. However, this approach breaks down when the list of dependencies is as long and as sensitive as Proton’s. Therefore, a big challenge for Proton has been ensuring consistent behavior across devices.

The Steam Linux Runtime⁠ (SLR), introduced in Proton 5.13, aims to solve this. It’s a Flatpak-style sandbox⁠ that bundles a specific set of libraries known to work well with Proton. This set of libraries is referred to as the Steam runtime⁠. When a Proton game is run, a tool called pressure-vessel⁠ combines this runtime with the graphics stack installed on the host system to round out the sandbox required by Proton games. The advantage of using a Flatpak sandbox over containers like Docker is that Flatpak sandboxes are meant to be created and run by an unprivileged user⁠ on the host system. This avoids some security concerns that would have come from using a true Linux container to run Proton.

Windows applications in Linux

Proton uses a fork⁠ of Wine⁠ to run Windows games in Linux. This Valve-owned fork maintains patches on top of upstream Wine to improve the compatibility of Proton. Most of these patches are eventually improved on and merged upstream.

For performance reasons, Wine is very much not an emulator or a virtual machine. Wine does not attempt to maintain or simulate the internal state that would be required to implement Windows at run time. Instead, Wine aims to maintain as little state as possible and to translate incoming Windows API calls into POSIX-compliant calls on the fly. Wine developers leverage publicly-available Windows API documentation and black-box testing techniques in their effort to approximate Windows API calls as closely as possible.

One of the unavoidable pieces of state that Wine requires to work correctly is a directory structure that resembles the standard directory structure available in Windows since applications generally expect to be able to open and load files from pre-specified paths. This directory structure created by Wine is called a wineprefix⁠. When Steam first runs a Proton game, it sets up this wineprefix automatically. The result is called the game’s compatdata directory⁠. It is within this directory structure that Proton games execute when they run within the sandbox created by pressure-vessel.

To minimize the chance of games stepping on each other when they read or write files, each game gets its own compatdata directory. The effect is similar to each game running within its own computer⁠, maximizing Proton compatibility. Unfortunately, due to duplicate files and directories, this also wastes some disk space. However, there are ongoing efforts to de-duplicate across compatdata directories.

Direct3D supported by Vulkan

Finally, one of the most integral parts of gaming: graphics. We’ve previously discussed the importance of shifting toward low-overhead graphics APIs like Vulkan⁠. ChromeOS is not the only Linux-based platform shifting toward the improved performance that Vulkan offers, though. Many open-source projects are doing the same. Proton leverages two such projects: DXVK⁠ and VKD3D-Proton⁠, the latter being a fork of a project driven by WineHQ⁠.

Both DXVK and VKD3D-Proton translate Direct3D⁠ API calls into Vulkan calls on the fly within the Borealis VM. Their difference is in the Direct3D version that each supports. DXVK supports Direct3D versions 9 to 11 and DXGI. VKD3D-Proton supports Direct3D 12. These two are different projects because Direct3D 12 is substantially more low-level⁠ than Direct3D 9-11 and requires different design considerations. Given the Direct3D version differences, it is more performant to design translation layers independently.

While DXVK, and VKD3D-Proton are quite performant, there is a non-zero performance cost to translate all the Direct3D API state when a Proton game runs. A good way to counteract this is with caching. Both DXVK and VKD3D-Proton implement caching schemes to help mitigate the cost of translation. DXVK’s⁠ and VKD3D-Proton’s⁠ caching schemes are functionally similar: both focus on caching the result of the translation of Direct3D graphics state and shaders to Vulkan.

Despite caching efforts by DXVK and VKD3D-Proton, it is still far more performant to avoid misses in the caches managed by device-specific graphics drivers. Proton games make heavy use of driver-managed shader caches to squeeze out as much performance as possible. To improve the performance of Proton games, Steam uses a tool called Fossilize⁠ to warm up Mesa’s Vulkan shader cache⁠. When a Steam user is presented with the “Processing Vulkan Shaders” stage before a Proton game starts, Fossilize is warming up the shader cache behind the scenes. Having a warm shader cache can greatly improve FPS and reduce unpleasant jank.

Proton has grown thanks to the hard work of a diverse group of contributors from all over the world. We’re excited to join this effort to increase support for gaming on Linux. Game on!