But as you have said, it's not as straighforward as I'd like. I haven't tried it yet but it's nice seeing others having success.
Are there any up to date guides and best practices for this?
https://www.reddit.com/r/linux_gaming/comments/2z2y7h/gpu_pa... is another fairly concise guide (a little out of date though).
https://bbs.archlinux.org/viewtopic.php?id=162768 is the main point of discussion for it - too much to read through entirely, but the top posts include a lot of info, and it contains a lot of valuable information on quirks for specific hardware if you search. Some of the stuff is outdated though (eg the kernel includes a lot of the patches now), and it's targetted at Arch, but most instructions should apply to any distro.
Another key point is, you need VT-d/IOMMU support on your motherboard and CPU, which isn't universal (only in the last few generations of intel CPUs, with some excptions that don't support it. And motherboard support can vary between OEMs). Check https://docs.google.com/spreadsheet/ccc?key=0Aryg5nO-kBebdFo... to see if others have had any luck with your hardware, or before making any purchases.
This could work with two nvidia based cards ? And when the VM isn't On, the host could use the main GPU ?
Someone share more info about this!
You can't switch a GPU between host and VM without rebooting, you have to assign one GPU to the VM through kernel boot options, which will hide it from the host.
A lot of people also just use a dedicated (Nvidia/AMD) GPU for the windows VM for gaming and heavy duty things, and only use their onboard Intel GPU for the linux host, which is sufficient for normal usage if you're doing all your gaming through the VM anyway.
See my post above for some links with more info.