Since about 5 or 6 years ago, PCIe Video Cards advertise a capability known as Resizeable BAR. PCI Devices like GPUs require some memory to use for PCI MMIO, which is directly visible on the CPU Address Space. As a side note, this Address Space is shared with RAM, and anyone that was around when having 4 GiB RAM with a 32 Bit OS was common (Earlier than 2010 or so) knows that you only saw about 3.25 GiB RAM or so because of sharing the Address Space with PCI MMIO, and those fortunate enough to have used SLI usually saw even less than that, like 2.87 GiB RAM.

GPUs has been using a 256 MiB PCI MMIO window regardless of how much VRAM they actually have since... nearly forever? At least since PCIe is a thing, since I recall than AGP Aperture Size was seteable in era accurate BIOSes. PCIe 3.0 specification introduced a feature known as Resizeable BAR, where the PCI Device can tell a compatible Firmware how much MMIO it actually wants. GPUs uses that to tell a ReBAR capable UEFI Firmware that it wants more MMIO (Usually as big as the GPU VRAM), or uses legacy 256 MiB otherwise.

It's hard to explain without going into a few low-level details of PCI Express, but let me try.

Most PCI devices expose some memory and/or I/O ports to the CPU. That memory (or I/O ports) is mapped to somewhere in the address space visible to the CPU. Besides the memory and I/O ports, all PCI devices also expose a separate set of configuration registers; among these registers, there are the Base Address Registers (BARs), which configure where the memory or I/O ports is mapped.

Here's an example output from "lspci -vv" for a GPU:

        Region 0: Memory at 7c00000000 (64-bit, prefetchable) [size=8G]
        Region 2: Memory at 7e00000000 (64-bit, prefetchable) [size=256M]
        Region 4: I/O ports at f000 [size=256]
        Region 5: Memory at fca00000 (32-bit, non-prefetchable) [size=1M]
        Expansion ROM at fcb00000 [disabled] [size=128K]

        Capabilities: [200 v1] Physical Resizable BAR
                BAR 0: current size: 8GB, supported: 256MB 512MB 1GB 2GB 4GB 8GB
                BAR 2: current size: 256MB, supported: 2MB 4MB 8MB 16MB 32MB 64MB 128MB 256MB

Why is this useful? This particular GPU has 8GB of VRAM; it's quite obvious that region 0 is a direct view into that VRAM. When using the maximum BAR size, the CPU can directly read and write anywhere into the VRAM; when using a smaller BAR, the CPU can only see a small window into the VRAM, and has to use less direct methods to access it.

(As an aside: go right now and do a "sudo lspci -vv" on your computer, if you see a Resizable BAR capability which isn't using the maximum size, you can probably gain a bit more speed for free by going into the BIOS and enabling "Resizable BAR" and/or "Above 4G decoding". If you can't find these options, well, AFAIU that's what this project is all about..)

ReBar's commercial name is AMD Smart Access Memory, it allows a PCIe device such as a GPU to map more VRAM to the system at once, which improves performance by reducing access overhead. This generated much fanfare in the early 2020s after AMD officially supported it in the newly released AMD Zen 3 CPUs with RX6800 series GPUs. It was marketed as a new technology to boost GPU/gaming performance. What AMD did was just rebranding an obscure feature in the PCIe specification [1]. As shown by this project, it was actually supported by the PCIe controller since Sandy Bridge, just disabled in the firmware. For a decade nobody bothered to use it. Presumably, AMD saw an opportunity and enabled it, presumably after validating the hardware and fixing any driver compatibility problems.

[1] This is nothing new in the tech industry. Intel rebrands DVFS as SpeedStep, IOMMU as VT-d, AMD rebrands the NX bit as Enhanced Virus Protection, etc.

Resizable Base Address Register. In short, rather than the 256MB of mapped memory available to the CPU for any PCI device, ReBAR capable devices can map larger memory to the CPU’s addressable memory space. Without ReBAR or sufficient tricks (that Intel GPUs lack) you have to shuffle 256MB chunks around between GPU and CPU.

It allows the CPU to access all GPU VRAM. This improves gaming performance, especially for AMD GPUs.

> what the purpose and use-case is for ReBAR

Literally the second sentence in the repo:

> This provides performance benefits and is even required for Intel Arc GPUs to function optimally.