Common issues (and fixes)

Resizable BAR doesn't work on NVIDIA RTX 30 series GPU

A VBIOS update is required to support Resizable BAR. NVIDIA provides instructions on how to update it GeForce RTX 30 Series Performance Accelerates With Resizable BAR Support.

Games crash after enabling Resizable BAR

Increase pagefile size.

Driver shows Resizable BAR disabled while GPU-Z shows it working

Reinstall driver. Typically using DDU isn't required.

I have an AMD GPU and disabling CSM makes my display go blank till OS boots

Restore original VBIOS.

In some rare cases a VBIOS GOP update may be needed which in this case you can follow the guide AMD and Nvidia GOP update (No requests, DIY). Use the updated version by Sylar76. If you have a modified VBIOS you will need to update it with "magic GOP" instead of usual GOP.

How do I enable Resizable BAR on unsupported AMD GPUs ?

You can use Nimez drivers and select Enable with ASIC in NzSetupCli -> Tweak Toolkit Management -> PCI-E Resizable Bar Service Mode.

If you don't want to use modded drivers you can apply the following .reg file (reboot after applying).

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Control\Class\{4d36e968-e325-11ce-bfc1-08002be10318}\0000]
"KMD_RebarControlMode"=dword:00000001
"KMD_RebarControlSupport"=dword:00000001
"KMD_EnableReBarForLegacyASIC"=dword:00000001

Unsupported 4GB cards only work when BAR size is set to 2GB or smaller. 4GB BAR causes graphics APIs (DX/Vulkan/OpenGL) to not work on unsupported 4GB cards. Even with 1 or 2GB BAR you might still have some issues in certain games with 4GB cards.

RX 5600 XT doesn't resume from sleep on Windows

Set BAR size to 1GB or lower. This issue is due to a workaround used to get full 8GB BAR on the faulty VBIOS implementation.

ReBarUEFI can be used to enable full 8GB BAR with RX 5600 XT even on motherboards officially supporting Resizable BAR. @FusionDragon was able to use it on a Gigabyte Aorus B550M Pro-P (rev 1.0) without any issues besides broken sleep.

Windows won't boot after disabling CSM

You need to convert your legacy MBR installation to UEFI GPT.

Do I need a BAR size larger than VRAM ?

Not sure why this question often is asked but you only need a BAR size that fits all your VRAM. So 8GB is enough for 8GB VRAM and 16GB is enough for 12GB VRAM. GPUs also don't typically support BARs more than what's needed to fit all their VRAM

Stutters on Ryzen system

See https://gitlab.freedesktop.org/drm/amd/-/issues/1864. It's a firmware bug currently there is no fix for it.

LGA1151 system won't POST or displays graphical corruption even after patching

See #4, the workaround is just use a BAR size of 4GB or larger.

LGA1151 system won't POST with 4GB+ BAR even after patching

Connect monitor to iGPU, for some reason the BIOS doesn't detect displays connected to 64-bit BAR GPU and won't POST. Once OS boots the GPU will start outputting so you can just use a dummy connector on the iGPU too.

No video output during boot before OS loads

Try disabling the Above 4G Decoding Patch option, it might be hidden which can still be modified following instructions in the wiki.

Can't boot Windows or GPU gives error in Device Manager

Make sure to following DSDT Patching guide

Can I use this without 4G Decoding ?

Yes but you will likely be limited to 1GB BAR size. You can reduce the TOLUD option in your BIOS to free up more space for PCI BARs, this might make even 2GB BAR work but that's the largest you can get without 4G Decoding.

Some games may have issues when using BAR smaller than VRAM especially on AMD GPUs

Is Memory Remap in the same as 4G Decoding ?

It isn't and you should leave Memory Remap at the default setting.