saphid--chromium-webxr-steam-frame/docs/technical-notes.md

Technical notes

What we found getting WebXR working on the Steam Frame, for anyone picking this up or taking it upstream. Tested with Chromium 156.0.8071.0 (arm64), SteamOS 0.3.0 (build 20260922.6101926) and SteamVR 2.17.10.

Why stock Chromium on Linux has no immersive WebXR

Chromium 154 was the first release to compile OpenXR on Linux (enable_openxr includes Linux). But content/services/isolated_xr_device/xr_runtime_provider.cc only creates an OpenXR device when ENABLE_OPENXR && IS_WIN. Nothing on Linux calls the OpenXR code, so the linker drops it. Flathub's arm64 Chromium contains no OpenXR loader code at all, and flags such as --force-webxr-runtime=openxr can't bring it back.

The two Gerrit changes this repo builds fill that gap:

• CL 8132979 creates the OpenXR device on Linux, using the Vulkan graphics binding (XR_KHR_vulkan_enable2). device::features::kOpenXR stays off by default, so it needs --enable-features=OpenXR.
• CL 8441736 runs the XR device service in its own sandbox (xr_compositing, with the XrProcessPolicy seccomp policy and a file broker), instead of requiring --no-sandbox.

The build uses patch set 44 of CL 8132979, which sits on top of CL 8441736.

The SO_PEERCRED crash

With both CLs, the XR utility process crashed inside xrCreateInstance on system call 0xd1 (getsockopt on arm64). SteamVR's IPC client calls getsockopt(fd, SOL_SOCKET, SO_PEERCRED, …) to check which process is on the other end of its socket, and XrProcessPolicy refuses every getsockopt. patches/0001-xr-sandbox-allow-getsockopt-SO_PEERCRED.patch allows exactly SOL_SOCKET/SO_PEERCRED and still returns EPERM for everything else.

Why the seccomp filter is still off

With the patch, xrCreateInstance gets further but fails with Unable to init path manager: VRInitError_Init_Internal. SteamVR's client reads /proc/self/status to find its own process ID. Inside the seccomp sandbox, file opens go through Chrome's broker process, so /proc/self is the broker's, and SteamVR registers the broker's PID instead of the XR process's. Granting the broker /proc/self doesn't help, because the broker can only answer for itself.

Fixing this properly needs a change in Chromium: for example, having the broker client rewrite /proc/self to /proc/<caller pid>, or opening the needed /proc files before the sandbox seals. Until then the launcher passes --disable-seccomp-filter-sandbox. The namespace sandbox still works: SteamOS allows unprivileged user namespaces, so the setuid chrome_sandbox isn't needed.

What a working session looks like

• The page's isSessionSupported("immersive-vr") resolves true.
• After the Allow VR? prompt, requestSession("immersive-vr") succeeds. The first frame has a viewer pose with 2 views and a 2880 × 1440 framebuffer (1440 × 1440 per eye).
• SteamVR's log (~/.local/share/Steam/logs/vrserver.txt) shows the app move from VRApplication_OpenXRInstance to VRApplication_OpenXRScene, followed by controller binding files being created for system.generated.openxr.chromium 156.chrome.
• If nobody is wearing the headset, SteamVR keeps it in standby: the session stays at XR_SESSION_STATE_SYNCHRONIZED, the page sees visibilityState: "hidden", and only the first frame runs. Put the headset on to see it.

Graphics

Chromium's GPU process uses ANGLE on OpenGL, which runs on zink over the Turnip Vulkan driver (Adreno 750). Chromium's own Vulkan backend stays off; that doesn't stop the session. The OpenXR runtime uses Vulkan.

Panels and the Steam library

The Frame's compositor (gamescope) gives every Steam app ID its own SteamVR overlay, named valve.steam.desktopgame.<appid>, which appears as a panel in the headset. A non-Steam shortcut gets an app ID like any game, so launching Chromium XR from the library gives it a panel. frame/steam-shortcut.py adds the shortcut through the Steam client's DevTools port (127.0.0.1:8080, page SharedJSContext) with SteamClient.Apps.AddShortcut, so Steam doesn't need restarting. (steam steam://addnonsteamgame/<path> adds nothing.)

Steam preloads its in-game overlay, gameoverlayrenderer.so, into everything it launches through LD_PRELOAD. In Chromium it segfaults the zygote during library initialisation. The GPU process then fails to launch (GPU process launch failed: error_code=1002), and after a few tries Chromium quits with GPU process isn't usable. Goodbye. about 30 seconds after starting. The chromium-xr launcher removes the overlay from LD_PRELOAD before starting Chromium, keeping anything else that was preloaded.

Debugging

• chromium-xr --remote-debugging-port=9223 URL opens DevTools on the Frame's loopback. It has no password, so close the browser when you're done. If a VPN such as userspace Tailscale forwards traffic to loopback, other devices can reach it.
• chrome://gpu and chrome://webxr-internals show the graphics setup and the XR runtime Chromium picked.
• SteamVR's logs are in ~/.local/share/Steam/logs/. vrserver.txt shows the session starting and which app SteamVR bound it to.