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saphidandClaude Opus 5.5
8b7c46a63a
Flathub Chromium can't enter immersive WebXR on Linux because upstream only wires the OpenXR device on Windows. Document why, and add scripts to cross-compile arm64 Chromium with the unmerged Linux OpenXR CLs and to install, launch and check it on the Frame. The first build is still running; immersive-vr support is unverified. Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
83 lines
3.6 KiB
Markdown
83 lines
3.6 KiB
Markdown
# WebXR in Chromium on the Frame
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Goal: open a web VR180 or 360 player (DeoVR and DL8 embeds, WebXR samples),
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press its VR button, and watch in 3D in the headset.
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## Why Flathub Chromium can't
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**Verified 2026-09-25** (Frame BUILD_ID 20260922.6101926, Flathub
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`org.chromium.Chromium` 154.0.8037.57 aarch64):
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- `navigator.xr` exists, but `isSessionSupported("immersive-vr")` is `false`.
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Flags don't change that, and neither does `--force-webxr-runtime=openxr`
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with SteamVR exposed to the Flatpak.
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- The binary has no OpenXR loader: no `XR_RUNTIME_JSON`,
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`xrGetInstanceProcAddr` or `XR_LOADER_DEBUG` strings. The only OpenXR
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strings are the `chrome://flags` entries.
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**Cause (verified against Chromium source, same date).** M154 is the first
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release that compiles OpenXR on Linux:
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- `device/vr/buildflags/buildflags.gni` adds `is_linux` to `enable_openxr`.
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- Flathub's tarball sets `checkout_openxr = true`.
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- Flathub's GN args don't turn it off.
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But `content/services/isolated_xr_device/xr_runtime_provider.cc` only creates
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the OpenXR device under `ENABLE_OPENXR && IS_WIN`. That's true on 154, 155 and
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`main`. Nothing on Linux calls the OpenXR code, so the linker drops it. The
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rest of the Linux port is in two unmerged CLs (bug 506004811):
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- [8441736](https://chromium-review.googlesource.com/c/chromium/src/+/8441736)
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runs the XR device service in a Linux sandbox that allows SteamVR's
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sockets, `/dev/shm` and `flock`.
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- [8132979](https://chromium-review.googlesource.com/c/chromium/src/+/8132979)
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wires the provider to `OpenXrPlatformHelperLinux`. `kOpenXR` stays off by
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default, so it needs `--enable-features=OpenXR`.
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8132979 sits on top of 8441736, so fetching `refs/changes/79/8132979/<ps>`
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gets both.
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**The Frame side is ready.** `~/.config/openxr/1/active_runtime.json` names
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SteamVR (`bin/linuxarm64/vrclient.so`, `VALVE_runtime_is_steamvr`). The
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Linux backend uses Vulkan (`XR_USE_GRAPHICS_API_VULKAN`).
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## Building it
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[`scripts/build-chromium-xr.sh`](../scripts/build-chromium-xr.sh)
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cross-compiles arm64 Linux Chromium on an x64 Linux host. It doesn't need
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sudo: the arm64 sysroot comes from Chromium's own script. It needs about
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90 GB of disk. It shallow-fetches the CL ref, runs `gclient sync --no-history`,
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installs the sysroot, builds `chrome` with `symbol_level=0` and proprietary
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codecs, and packs `chromium-xr-arm64.tar.xz`. Progress is logged to
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`~/chromium-xr/stage`. The build aborts if `/` drops below 12 GB free.
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First run: buildhost (12 cores, 31 GB RAM), started 2026-09-25.
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## Running it on the Frame
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[`scripts/chromium-xr.sh`](../scripts/chromium-xr.sh):
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```sh
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scripts/chromium-xr.sh install # scp from buildhost, unpack to ~/chromium-xr
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scripts/chromium-xr.sh launch [URL] # headset desktop, --enable-features=OpenXR
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scripts/chromium-xr.sh check # prints isSessionSupported('immersive-vr')
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```
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It runs natively, not as a Flatpak, so the XR sandbox and SteamVR's IPC work
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as the CL expects. It uses its own profile (`~/.config/chromium-xr`) and
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DevTools on loopback port 9223, so it doesn't collide with the Flatpak's 9222.
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**Verified 2026-09-25:**
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- Vulkan is there: Turnip (Mesa) on Adreno 750, API 1.4.359.
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- Unprivileged user namespaces work (`unshare -Ur true`), so Chromium's
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namespace sandbox shouldn't need the setuid `chrome_sandbox`.
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**Unverified (inferred):**
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- Chromium's GPU process may still fall back from Vulkan to GL on Turnip.
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- An immersive session started from a window on the nested desktop may not
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hand over cleanly to the SteamVR compositor.
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- If the sandbox fails to start, `--no-sandbox` is the fallback for a first
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test.
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