rutabaga_gfx/cross_domain: handle CMD_WRITE on Eventfd items

[aford173]

The cross_domain write handler only matched
CrossDomainItem::WaylandWritePipe, falling into the catch-all for every other item type after the unconditional remove() at the top of the function had already dropped the entry from the table. PipeWire (and other clients that share host-created eventfds via SCM_RIGHTS for per-period wakeups) sends CMD_WRITE on those eventfd identifiers — the first such write returns InvalidCrossDomainItemType after removing the item, and every subsequent write on the same identifier returns InvalidCrossDomainItemId, masquerading on the guest as the opaque VIRTIO_GPU_RESP_ERR_UNSPEC (0x1200).

Reproduced inside a libkrun guest on x86_64 by routing PipeWire's ALSA-shim audio through a host PipeWire daemon. With a paired BT speaker as the sink, speaker-test -D pipewire -c2 -t wav -l 1 produces, per stream, ~10 entries of:

[    0.682819] [drm:virtio_gpu_dequeue_ctrl_func] *ERROR* response 0x1200 (command 0x207)
[    0.723762] [drm:virtio_gpu_dequeue_ctrl_func] *ERROR* response 0x1200 (command 0x207)
[    0.767615] [drm:virtio_gpu_dequeue_ctrl_func] *ERROR* response 0x1200 (command 0x207)
[    0.807779] [drm:virtio_gpu_dequeue_ctrl_func] *ERROR* response 0x1200 (command 0x207)
[    0.852469] [drm:virtio_gpu_dequeue_ctrl_func] *ERROR* response 0x1200 (command 0x207)
[    0.896552] [drm:virtio_gpu_dequeue_ctrl_func] *ERROR* response 0x1200 (command 0x207)
[    0.936504] [drm:virtio_gpu_dequeue_ctrl_func] *ERROR* response 0x1200 (command 0x207)
[    0.980567] [drm:virtio_gpu_dequeue_ctrl_func] *ERROR* response 0x1200 (command 0x207)
[    1.024476] [drm:virtio_gpu_dequeue_ctrl_func] *ERROR* response 0x1200 (command 0x207)
[    1.064636] [drm:virtio_gpu_dequeue_ctrl_func] *ERROR* response 0x1200 (command 0x207)

with audio still playing — PipeWire has socket-based fallback timing that doesn't depend on eventfd ack, so the failures are cosmetic for playback. They are not cosmetic for clients that strictly require the eventfd handshake (PipeWire's ALSA shim under heavier loads, and the buffer-pool wakeup path used by V4L2 capture streams).

Add an Eventfd arm that mirrors the WaylandWritePipe semantics: write_volatile performs the 8-byte counter increment, and the item is re-inserted into the table unless the guest signaled hang_up.

Verified post-fix: zero CMD_WRITE failures, zero 0x1200 entries in guest dmesg, audio playback unchanged. Camera capture (gst-launch pipewiresrc → MJPEG) also exercises this path for buffer-pool wakeups and runs cleanly with valid 98%-non-zero JPEG frames.

[slp]

Nice one, thanks. Please address the formatting issue so we can merge it.

[aford173]

@slp - The formatting has changed. Let me know if you want any other changes.