fix(gpu): run on rutabaga's 2D component when virgl is off

[ya-luotao]

Problem

The virtio-gpu device always builds a VirglRenderer rutabaga component, whatever the flags. With VIRGL_RENDERER_NO_VIRGL and no VENUS — i.e. a host with no 3D renderer at all, which is what a headless macOS build gets — virglrenderer has no vrend state and rejects every non-blob resource. From the guest's side, after a KMS modeset:

[drm:virtio_gpu_dequeue_ctrl_func] *ERROR* response 0x1200 (command 0x101)   RESOURCE_CREATE_2D
[drm:virtio_gpu_dequeue_ctrl_func] *ERROR* response 0x1200 (command 0x106)   ATTACH_BACKING
[drm:virtio_gpu_dequeue_ctrl_func] *ERROR* response 0x1202 (command 0x103)   SET_SCANOUT
[drm:virtio_gpu_dequeue_ctrl_func] *ERROR* response 0x1200 (command 0x105)   TRANSFER_TO_HOST_2D
[drm:virtio_gpu_dequeue_ctrl_func] *ERROR* response 0x1203 (command 0x104)   RESOURCE_FLUSH

so a scanout never reaches the krun_display backend, and the device advertises +virgl with 5 capsets that cannot work, which also makes guest mesa try the virgl driver.

Change

• is_2d_only(virgl_flags) = NO_VIRGL set and VENUS clear. In that mode create_rutabaga (and the fallback) build RutabagaComponentType::Rutabaga2D with no channels or export table; the device advertises only VERSION_1 | EDID and reports num_capsets = 0. Venus/virgl configurations are untouched (this is complementary to virtio-gpu venus: fix blob mapping on macOS/HVF and Linux/KVM #91, which fixes the Venus blob path).
• flush_resource read the whole resource with resource.width * 4 as the stride into a frame buffer sized by the SET_SCANOUT rectangle. When the two differ (Hyprland's GBM dumb buffers), rutabaga's transfer_2d returns InvalidIovec and the unwrap() took the gpu worker thread down. VirtioGpuScanout now records the rectangle, read_2d_resource copies the intersection with the destination stride, and a failed read-back answers ErrUnspec instead of panicking.

Verification

macOS/HVF, Arch Linux ARM guest, libkrunfw 6.12: modetest -M virtio_gpu -s 37@36:1920x1080 completes with zero errors in dmesg (ten before); the guest logs features: -virgl +edid -resource_blob …; Hyprland 0.56 on llvmpipe delivers 1920x1080 BGRX frames to a krun_display backend, and they render as a native window in https://github.com/ya-luotao/msb-omarchy. cargo check -p msb_krun_devices --features gpu,input passes.

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https://claude.ai/code/session_01A2DMienRTQjgR5CF3uQ2AZ