All that follows is relating to muxless/non-MXM Optimus cards, i.e. those that have no display outputs and show as 3D Controller in lspci output. If you have a MXM / output-providing card (shows as VGA Controller in lspci), then this is not for you. For such devices, see @Misairu-G's guide

What works

• Compute workloads (via render nodes) on GVT device
  • You can find a sample program that dispatches a compute shader via render nodes here
• Compute workloads (via render nodes) on nvidia card (nouveau)
• Render workloads on the nvidia card with the nouveau driver, with a GVT device as the guest's primary GPU
  • I have only tested this with q35. i440fx is untested
  • Subsystem vendor & device Id's must be set with x-pci-sub-vendor-id and x-pci-sub-device-id on the qemu vfio-pci device for the nvidia card
  • Nvidia VBIOS must be provided to the guest via libvirt's <rom file=...> / qemu's romfile option on the device
    • VBIOS can be obtained under linux by extracting a BIOS update using coderobe/VBiosFinder
    • Alternatively, when using Windows the VBIOS can be obtained by dumping the system firmware with Universal Bios Backup Toolkit and using MMTool to extract the VBIOS fromthe "Option ROMs" section
      • This needs to be done under bare-metal Windows. I use a Windows-To-Go install of Server 2016 on a USB (UAS) SSD for this, to leave my machine's internal storage alone.
  • I've only tested this with OVMF/UEFI. It may or may not work with seabios VMs
  • My dumped VBIOS did not have EFI support, but I don't think that is necessary when the card has no outputs. rom-parser output for reference:

Valid ROM signature found @0h, PCIR offset 190h PCIR: type 0, vendor: 10de, device: 139b, class: 030200 PCIR: revision 3, vendor revision: 1 Last image ```

• Guest kernel must booted via OVMF directly, or via a bootloader that supports the EFI handover protocol
  • Failure to do this will result in nouveau failing to load vbios via the FIRMWARE interface
  • Mainline GRUB does not have EFI handover support. Fedora (and maybe other distros) have their own patches adding support for this in the form of the linuxefi command
• No local display output at present; remote display must be used.
• Render workloads on the nvidia card with the nvidia driver, with the GVT device as secondary, linked with PRIME
  • Requires a patch to the nvidia driver to fake loading VBIOS from ACPI. See below
• Render workloads on the GVT device, with the GVT device as the guest's primary GPU
  • Only tested on q35 + OVMF
  • No VBIOS needed, everything should Just Work™
  • No local display output at present; remote display must be used.

What works, with patching

• Binary nvidia driver (Linux)
  • For Optimus cards, it will only attempt to load VBIOS via ACPI _ROM method, which won't exist in the guest.
  • We can probably patch nv_acpi_rom_method in kernel/nvidia/nv-acpi.c to simply return our VBIOS from a hard-coded buffer, just for testing
    • [UPDATE] I tested this and it worked, the binary driver booted and the NVIDIA card can be set up with PRIME. glxinfo, patch. Note you'll need to dump your own VBIOS and inline it in the patch.
  • Long term we need to build a custom ACPI table (provided to qemu with the -acpitable option) that has _ROM implemented at the correct path. The _ROM implementationm would need to seek over a hard-coded buffer stored elsewhere and return the VBIOS in 4kb chunks as expected by nvidia driver

What doesn't work (yet)

• Windows guest
  • Will need custom ACPI table to get VBIOS, as detailed above
• Reverse PRIME to mirror GVT display to QXL device to use Qemu's built in spice server
  • The modesetting DDX must be used with QXL, as the qxl DDX lacks PRIME support
  • The QXL-backed modesetting instance can be set up as a PRIME output provider successfully, and modes will show as available on the newly created Virtual-X-Y output (where X and Y will vary depending on your xorg server's layout)
  • Attempting to set any mode on the Virtual-X-Y output will fail with the following errors from the kernel:

    qxl 0000:00:03.0: swiotlb buffer is full (sz: 299008 bytes)
    qxl 0000:00:03.0: DMA: Out of SW-IOMMU space for 299008 bytes
    

    • Next step may be to try with Qemu's emulated intel-iommu device and see if this helps
• QXL as primary xorg GPU with GVT/nvidia as secondary
  • QXL-backed modesetting provides only the Sink Output capability. Sink Offload is required for render offloading to a device with Source Offload
    • Maybe QXL could be improved to have the Sink Offload capability?
  • Attempting to set up nouveau as the Source Offload for the GVT card (which has Sink Offload) will crash Xorg with the following assertion failure: xorg-server-1.19.5/dix/dispatch.c:4035: AttachOutputGPU: Assertion 'new->current_master == pScreen' failed. This suggests that PRIME render offload requires the destination device (the one being used as a Sink Offload to be the primary Xorg GPU
• Bumblebee
  • Attempting to use bumblebee without a GVT device will fail as bumblebeed will bail out at startup upon not finding an Intel card
    • Patching out this check will allow bumblebeed to start.
  • Once bumblebeed has started, it is unable to successfully start the secondary X server, as the nvidia card has no outputs and Xorg bails out due to no outputs existing.
    • Nouveau does not support/have the AllowEmptyInitialConfiguration option that the proprietary nvidia driver has.

What will (probably) never work

• Using the nvidia card in the guest as the sole GPU
  • As non-MXM Optimus cards lack any outputs, xorg will not start as there are no outputs available
  • xf86-video-dummy cannot be used as the primary GPU as it lacks PRIME support

What's untested

• Wayland

What's needed

• A way to obtain a dump of nvidia VBIOS under Linux
  • Try coderobe/VBiosFinder
• Custom ACPI tables embedding nvidia VBIOS and exposing it via the _ROM method at the appropriate path