fix(gaming): hide virtio-gpu render node so RADV binds the passthrough GPU

[Benehiko]

Problem

Proton games on the passthrough gaming VM regressed to CPU (software) rendering — vulkaninfo reports only llvmpipe, never the AMD GPU. This is not a passthrough regression: the GPU passes through fine (Navi31 bound to amdgpu, all rings healthy). The trigger is a Mesa 26 RADV change.

Passthrough gaming VMs attach a virtio-gpu-pci device alongside the VFIO GPU so SPICE/KMS always has a scanout surface (the passthrough GPU usually has no monitor → no native scanout). This creates a second DRM render node in the guest.

Since Mesa 26, radv enumerates every DRM render node during winsys init and, when a virtio-gpu render node is present, routes the amdgpu device through the virtio vdrm winsys. On the VFIO-passed GPU that path fails:

MESA: info: could not get caps: Function not implemented
MESA: error: vdrm_device_connect failed
radv/amdgpu: failed to initialize device

radv aborts the whole ICD → Vulkan falls back to llvmpipe → Proton/DXVK games run on the CPU. MESA_VK_DEVICE_SELECT does not help — the abort happens before device selection.

Fix

Install a udev rule from vee-firstboot that makes the virtio-gpu render node unreadable (MODE="0000"):

SUBSYSTEM=="drm", KERNEL=="renderD*", DRIVERS=="virtio-pci", MODE="0000"

radv skips the hidden node and binds the amdgpu node. The virtio card (scanout) node stays intact, so SPICE/KMS display and Sunshine KMS capture keep working. Matching by parent driver + render kernel name is robust to non-deterministic renderD*/card* numbering, needs no Mesa rebuild, and survives pacman -Syu.

Verified live (lotmonster-gaming, RX 7900 XTX)

• vulkaninfo --summary → AMD Radeon RX 7900 GRE (RADV NAVI31) ✓ (was llvmpipe)
• vkcube → Selected GPU 0: ... type: DiscreteGpu ✓
• Scanout card nodes + Sunshine capture intact ✓

Docs

• docs/gpu-passthrough-gaming.md: replaced the earlier "rebuild Mesa with -D amdgpu-virtio=false" (~30 min) guidance with the udev rule.
• site/content/gpu-passthrough/gaming-setup.md: replaced the non-working MESA_VK_DEVICE_SELECT workaround with the udev rule.

Notes

• The non-gaming passthrough.go template has no guest provisioning (bring-your-own-OS), so the rule is documented for manual install there.

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