[Linux] Enabling Resizable BAR causes massive FPS drop

Recently I've been playing with enabling Resizable BAR as I've heard it can provide FPS boost(at least that's what AMD and Nvidia marketing wants me to believe).

After enabling it in BIOS my FPS in OpenMW was more than cut in a half.

My system:

Arch Linux
Linux 5.9.9
Mesa 20.2.2
Ryzen 3700X
Radeon 5700 XT Nitro+
G.SKILL 32GB (2x16GB) 3600MHz CL16 TridentZ RGB Neo
X570 Aorus Elite

OpenMW: 0.46
Mods installed: Morrowind Rebirth 5.3

Resizable BAR disabled: Zrzut_ekranu_z_2020-11-20_20-25-43 Zrzut_ekranu_z_2020-11-20_20-25-26

Resizable BAR enabled: Zrzut_ekranu_z_2020-11-20_20-21-51 Zrzut_ekranu_z_2020-11-20_20-21-41

Is it something that should be fixed by amdgpu team, mesa team or OpenMW team? Is this even something that could be fixed, or is Resizable BAR not what I believe it is and should do?

Since only amdgpu driver on Linux supports Resizable BAR(or so I've heard), hence the tag.

Edited by Kierek