GPU-accelerated composition (virtio-gpu 3D / virgl)

Summary

Add virtio-gpu 3D (virgl) support to offload window composition to the host GPU. Currently the compositor does CPU-side pixel-by-pixel alpha blending, which is slow for overlapping windows with transparency.

See plans/virtio-gpu-3d-composition.md for the full design.

Current state

• Virtio GPU 2D is working: surface blit, fill, flush operations
• Window composition (alpha blending, z-ordering) is done in software on the CPU
• Each frame redraws all visible pixels, even for static content
• No 3D context or virgl support

Desired behaviour

• Initialize a virtio-gpu 3D context with virgl
• Each window surface becomes a virgl texture
• Composition uses GPU shaders: textured quads with alpha blending
• Only re-compose when window content or layout changes (damage tracking)
• Significant reduction in CPU time spent on composition

Key components

1. Virgl context creation: Negotiate 3D capabilities with virtio-gpu, create a virgl rendering context
2. Texture management: Upload window surfaces as virgl textures, update on damage
3. Composition shader: Simple fragment shader that samples window textures and blends with alpha
4. Damage tracking: Only re-compose regions that changed (dirty rectangles)
5. Scanout: Present the composed framebuffer to the display

Key files

• panda-kernel/src/drivers/virtio_gpu/ — extend with 3D/virgl support
• Compositor code (wherever window composition happens)
• plans/virtio-gpu-3d-composition.md — detailed design document

Implementation notes

• Virtio-gpu 3D uses the VIRTIO_GPU_CMD_CTX_* and VIRTIO_GPU_CMD_SUBMIT_3D commands
• Virgl encodes OpenGL commands into a command stream sent to the host
• The host uses the virgl renderer library to execute GL commands
• QEMU must be started with virtio-gpu-gl device (not just virtio-gpu)
• Need to handle fallback to 2D composition if 3D is not available

Testing

• Test 3D context creation and capability negotiation
• Visual test: compose overlapping windows with transparency, compare screenshot
• Performance test: measure frame time for composition with and without GPU acceleration
• Fallback test: verify 2D composition still works when 3D is unavailable
• Run existing window/surface tests to verify no regressions

Documentation

• Document the virgl command stream format
• Document the composition pipeline (texture upload → shader → scanout)
• Document QEMU configuration for 3D support
• Update any GPU driver documentation

Start by reading plans/virtio-gpu-3d-composition.md and inspecting panda-kernel/src/drivers/virtio_gpu/. Locate the compositor code and existing window/surface tests, then use the design and listed 3D, visual, performance, and fallback tests to define completion. Done means virgl composition works when available and 2D composition remains functional otherwise.