Tutorial

This guide is a bit outdated: https://www.reddit.com/r/homelab/comments/b5xpua/the_ultimate_beginners_guide_to_gpu_passthrough/

Here's how I got my GTX1080 to work:
I'll be doing everything as root. You'll need to add sudo in front of the commands if you aren't a root user.

Grub:
nano /etc/default/grub
GRUB_CMDLINE_LINUX_DEFAULT="quiet intel_iommu=on iommu=pt pcie_acs_override=downstream,multifunction nofb nomodeset video=efifb:off initcall_blacklist=sysfb_init"
I needed the ACS patch because of everything I wanted to pass through.¨'

Bare minimum to work:
GRUB_CMDLINE_LINUX_DEFAULT="quiet intel_iommu=on iommu=pt nofb nomodeset video=efifb:off"

nofb nomodeset video=efifb:off Disables the frame buffer Proxmox uses for output. This applies if you are passing through your primary GPU to a VM.

initcall_blacklist=sysfb_init Use this if you get 2 blinking boxes on your primary display.

Don't forget to do:
update-grub

nano /etc/modules Same as the guide:
vfio
vfio_iommu_type1
vfio_pci
vfio_virqfd

Run:
update-initramfs -u


IOMMU interrupt remapping. Same as the guide:
echo "options vfio_iommu_type1 allow_unsafe_interrupts=1" > /etc/modprobe.d/iommu_unsafe_interrupts.conf


KVM Config:
echo "options kvm ignore_msrs=1" > /etc/modprobe.d/kvm.conf
This'll stop all of those "pve kernel: kvm [1849]: ignored rdmsr" errors from cluttering the Syslog:
echo "options kvm report_ignored_msrs=0 > /etc/modprobe.d/kvm.conf


Blacklisting Drivers:
echo "blacklist radeon" >> /etc/modprobe.d/blacklist.conf
echo "blacklist nouveau" >> /etc/modprobe.d/blacklist.conf
echo "blacklist nvidia" >> /etc/modprobe.d/blacklist.conf
echo "blacklist nvidiafb" >> /etc/modprobe.d/blacklist.conf

Not entirely convinced this actually did anything for me, but I did it as a step to trying to fix my GPU audio problems:
echo "blacklist snd_hda_intel" >> /etc/modprobe.d/blacklist.conf


Adding the GPU. Same as the guide:
To get the ID: lspci -v
lspci -n -s whatever it said next to VGA compatible controller
echo "options vfio-pci ids=GPU ID,GPU Audio ID disable_vga=1"> /etc/modprobe.d/vfio.conf

Run:
reboot
To reboot the host.


VM settings:
Download VirtIO drivers to your VM and install them. DO NOT SKIP!!
https://fedorapeople.org/groups/virt/virtio-win/direct-downloads/stable-virtio/virtio-win.iso

You can leave Display as Default or VirtIO-GPU if you want native VNC.

Ignore that whole args: -cpu 'host,+kvm_pv_unhalt,+kvm_pv_eoi,hv_vendor_id=NV43FIX,kvm=off' line. As far as I can tell, it does fuck all. just set your cpu: host and be done with it.
You will NEED to dump your ROM file. No way around it. You can mess with grub and those args all you want. You will still get error 43.

The guide's version of the ROM file dump and modification no longer works.
I recommend using Filezilla to transfer files between Proxmox and a Windows machine. Or you can figure out how to do everything in Linux with Okteta hex editor and the cp and rsync commands.

1. Download https://www.techpowerup.com/download/nvidia-nvflash/
(Using Windows for this part is easiest. I never figured out how to do this in Linux)
You'll want the Linux version. You can do this in Windows as well, but I'll be using the Linux version in this guide since you'll need to use Proxmox shell anyway.

2. Place the file from the zip "nvflash_5.792_linux.zip\x64" to anywhere you like on the Proxmox host. I just used cd ~/ as root user.

3. The file needs perms to run chmod +x nvflash

4. The GPU needs to not be running or drivers loaded for it.
Run these commands:
modprobe -r nvidia_uvm
modprobe -r nvidia_drm
modprobe -r nvidia_modeset
modprobe -r nvidia

5. Now we can finally dump the vBios ROM.
./nvflash --save vbios.rom
You can change vbios to whatever name you want for the ROM.

Now to modify the bios:
1. Download any hex editor you like.
I recommend https://mh-nexus.de/en/hxd

2. Open the ROM with hex edit. Search for VIDEO
https://gitlab.com/risingprismtv/single-gpu-passthrough/-/wikis/images/okteta/okteta_search.png

3. Now, place your cursor before the first U before the VIDEO we just searched, like this, and select everything before it and delete it:
(CTRL + SHIFT +Home to select everything before your cursor)
https://gitlab.com/risingprismtv/single-gpu-passthrough/-/wikis/images/okteta/okteta_header_removal.png

4. Save and place the modified ROM to /usr/share/kvm/

5. Add the ROM to your VM config
nano /etc/pve/qemu-server/101.conf Replace 101 with your VM's ID
Edit the line where your GPU is:
hostpci0: 0000:01:00,pcie=1,romfile=ROM_File.rom,x-vga=1
Your GPU and Host PCI IDs are likely different!

5. Start the VM and install Graphics drivers as you would on a regular PC.

Reference for GPU ROM file patch:
https://gitlab.com/risingprismtv/single-gpu-passthrough/-/wikis/6)-Preparation-and-placing-of-the-ROM-file

Bonus. Fixing the glitchy GPU audio with MSI mode and adding a tiny bit of performance.
If anyone knows how to get this to work without it. That would be appreciated. I could not figure it out.

1. Scetchy download time YAY. MSI Util V3
https://www.mediafire.com/file/ewpy1p0rr132thk/MSI_util_v3.zip/file
MD5 hash for zip-file: 8424509737CEDBDE4BA9E9A780D5CE96

2. More scetchyness! Run as administrator.

3. Tick the MSI box for your GPU and High Definition Audio Controller (This is the GPU sound card if you will.) Also do this for any SoundBlaster Sound cards you may have if they support MSI mode. I didn't run into any issues without MSI mode with mine but might as well.

4. Set interrupt priority as High

5. Hit Apply at the top right and reboot your VM
https://gyazo.com/8d218fd9ad33030f643119fd6518b543

You can check if MSI mode is enabled for your VM in proxmox with:
lspci -vv

It'll say something like this on your GFX card:
Capabilities: [68] MSI: Enable+ Count=1/1 Maskable- 64bit+

+ = Enabled, - = Disabled, 0 = Not supported

Reference for MSI mode audio fix:
https://forums.guru3d.com/threads/windows-line-based-vs-message-signaled-based-interrupts-msi-tool.378044/