Radeons 5600XT from multiple vendors (Perhaps all, but can confirm on my own MSI Gaming X and Sapphire Pulse based on other report) return these supported ReBAR capabilities via lspci -vvv under Linux:
03:00.0 VGA compatible controller : Advanced Micro Devices, Inc. [AMD/ATI] Navi 10 [Radeon RX 5600 OEM/5600 XT / 5700/5700 XT] [1002:731f] (rev ca) (prog-if 00 [VGA controller]) Capabilities: [200 v1] Physical Resizable BAR BAR 0: current size: 8GB, supported: 256MB 512MB 1GB BAR 2: current size: 2MB, supported: 2MB 4MB 8MB 16MB 32MB 64MB 128MB 256MB
The 1 GiB is the highest one, and is what Windows 11 uses when ReBAR is On on BIOS Setup:
But, if you paid attention to lspci output above, it reports than the CURRENT BAR0 size is 8 GiB. After some googling I found than an AMD engineer upstreamed a commit to some Linux component saying than GPUs with Device ID 0x731F (Which seems to be ALL Radeons 5600XT) ReBAR Capabilities announcement is broken, but they can force the 8 GiB value just fine:
The AMD engineer mentioned than “This is most likely fixable with a VBIOS update”, but as far that I could google this never materialized, so 5600XT is stuck with 1 GiB ReBAR in Windows because that is the maximum announced. In comparison, the 5700XT I saw do properly announce 8 GiB ReBAR support.
Core i5 12600K
MSI PRO Z690-A WIFI DDR4
I’m testing with both MSI BIOSes (Currently sticking to 1.70) and Dasharo (Coreboot + edk2/Tianocore). Actually, I was intending to ask for the 5600XT special case fix to be implemented directly there.
The VBIOS matches this one: VGA Bios Collection: MSI RX 5600 XT 6 GB | TechPowerUp
Which is the latest available. Older VBIOS should clock the RAM lower (For those unaware, 5600XTs nominal clock speeds were increased post launch via VBIOS updates), so it is a downgrade.
Maybe try to ask msi support and persuade them if you could, same thing happened in asus z68/p67 mobo i forgor in this forum but he asked to fix the uefi stuff and asus give a revised bios for him, and he could fully boot to uefi (before it cant just blackscreen)
Have you tried to crossflash other vbioses from sapphire or xfx or his? the one that supports 8gb of rebar? since your board is rebar capable so no rebar mod needed. its the vbios itself… so yeah hoping msi did support to the card and give u a fixed vbios would be good.
Msi did provide latest vbios update through msi center right?