What about machines with dual graphics cards, like one intel on board and another nvidia?

Would that be OK?

I ask because many laptops used to have similar setups.

Yes, this works for many laptops but it depends on how the laptop is internally wired. We have many members that use their iGPU for Linux, and the dGPU for the VM.

I haven't tried in about 6 months but I had issues with reusing that same dGPU if I wanted to use it for Linux gaming when the VM is offline. I thought it would be easy to bind and unbind it as needed but had issues with doing so. Is this a possible/recommended setup, is there another alternative or I shouldn't bother trying?

In theory yes, but I concluded my attempts on a Dell 9570 with “forget it”.

I fear I'll get a similar result.

Wouldn't be it possible to let Linux/Xorg give up GPU for a time Windows is used? Which could still let me access it via VNC from Windows. My native platform is Xubuntu on AMD Renoir laptop.

That is already possible without the Looking Glass. Just regular libvirt GPU passthrough.

I would say the use case here would be a machine that does not have a monitor connected, for example some Bitcoin mining server in the attic. With Looking Glass the game screen can get streamed to a ultrabook in your living room, with the server GPU performance

It's technically possible, but not stable. On my system, it's not stable, so I prefer to have two cards, but YMMV.

If this uses vfio then no, the host never sees the GPU, it's blind to it from boot.

What is stopping GPU hot plugging from working?

There was this recent improvement at least: https://phoronix.com/scan.php?page=news_item&px=Linux-5.14-A...

edit: seems there are some howtos - eg https://github.com/joeknock90/Single-GPU-Passthrough

Two reasons (in the context of a standard desktop user, not headless server):

1. Desktop environment, which uses the video card; this is not a big deal, since one can terminate the session

2. video drivers correctly and fully releasing the card; this is possible, but in my opinion, not stable.

Not really a problem, you can unbind the VFIO driver

> linux /vmlinuz-5.10.27-gentoo root=/dev/nvme0n1p2 ro amd_iommu=on iommu=pt pcie_acs_override=downstream,multifunction pci-stub.ids=10de:1b82,10de:10f0

the host never sees the card.

thats using the pci-stub driver, not VFIO

Does an iGPU fill the same criteria?

Yes it does. Intel usually is the host adapter.

The host can use a $15 5450, doesn’t have to be fancy.