Depends on the use-case. For the standard hardware-accelerated guest GPU in virtualized environments, there's already QEMU's virtio-gpu device.[1]

For "carving up" there are technologies like SR-IOV (Single Root I/O Virtualization).[2]

For advanced usage, like prototyping new hardware (host driver), you could use PCIem to emulate a not-yet-existing SR-IOV-capable GPU. This would allow you to develop and test the host-side driver (the one that manages the VFs) in QEMU without needing the actual hardware.

Another advanced use-case could be a custom vGPU solution: Instead of SR-IOV, you could try to build a custom paravirtualized GPU from scratch. PCIem would let you design the low-level PCIe interface for this new device, while you write a corresponding driver on the guest. This would require significant effort but it'd provide you complete control.

[1] https://qemu.readthedocs.io/en/v8.2.10/system/devices/virtio...

[2] https://en.wikipedia.org/wiki/Single-root_input/output_virtu...