ReBarUEFI: Resizable BAR for almost any UEFI system
summary
ReBarUEFI provides a UEFI DXE driver that allows users to enable Resizable BAR on hardware that lacks official manufacturer support. By modifying the UEFI firmware's DXE volume, the tool intercepts PCI resource allocation to force the GPU BAR size based on user-defined settings. This modification unlocks performance gains for various GPUs and is essential for the optimal operation of Intel Arc GPUs on older platforms. The process requires 4G decoding and often necessitates additional UEFI patches to handle 64-bit BARs correctly.
key points
- The driver works by replacing the PreprocessController function in the PciHostBridgeResourceAllocationProtocol to override default BAR sizes during PCI enumeration.
- Users must ensure 4G decoding is enabled and CSM is disabled to allow the system to allocate the necessary memory address space for large BARs.
- Additional UEFI patches are available to remove size limits, prevent BAR downgrades, and increase MMIO space for specific CPU architectures like Sandy Bridge through Coffee Lake.
who should read it
Advanced PC enthusiasts and gamers who want to improve GPU performance on older motherboards and those using Intel Arc GPUs on unsupported hardware.
discussion positive
consensus
Commenters agree that Resizable BAR (ReBAR) is a PCIe feature that removes the traditional 256MB memory limit, allowing the CPU to access a GPU's entire VRAM directly. This reduces overhead and can improve gaming performance by 5% to 15%. There is a general consensus that while modern hardware supports it, older systems often require firmware modifications or tools to enable the feature.
top insight
The discussion highlights the utility of Coreboot as a way to enhance firmware capabilities, such as adding NVMe boot or ReBAR support, without relying on proprietary tools. A key insight is that Coreboot's usability for average users depends heavily on the choice of payload (like EDK II or SeaBIOS) to provide a configuration UI, rather than the core firmware itself.
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