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Common issues (and fixes)
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.
Increase pagefile size.
Reinstall driver. Typically using DDU isn't required.
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.
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.
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.
You need to convert your legacy MBR installation to UEFI GPT.
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
See https://gitlab.freedesktop.org/drm/amd/-/issues/1864. It's a firmware bug currently there is no fix for it.
You can also try setting Above 4GB MMIO Limit to 512GB (39-bit) to see if it helps. This may also fix boot issues with macOS.
See #4, the workaround is just use a BAR size of 4GB or larger.
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.
Try disabling the Above 4G Decoding Patch option, it might be hidden which can still be modified by following instructions in Enabling hidden 4G decoding. This is also related to #4
This is caused by corruption from using UEFITool which happens on some firmwares. Redo the guide but use MMTool instead for all steps involving adding/replacing modules so you would use the pad file workaround for all patched modules.
Make sure to following DSDT Patching guide
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.
If you use Linux you can follow DSDT Patching | DSDT broken 4G Decoding fix (Haswell and up) to get working full rebar without 4G decoding, this doesn't even require flashing as you can load a custom DSDT. May also work on Windows for compute GPUs
Some games may have issues when using BAR smaller than VRAM especially on AMD GPUs
It isn't and you should leave Memory Remap at the default setting.
GPU-Z often reports these values incorrectly so no they shouldn't.
This can happen when the allocated BAR address of the GPU is higher than recommended. Set MMIOH Base to 256GB or the closest value above that you system supports (eg 1TB), you can try higher if you need to due to installed RAM amount. This can also fix boot issues with macOS.