Resizable BAR on systems which don't support it officially

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Resizable BAR on systems which don't support it officially

A UEFI DXE driver has been released, allowing users to enable Resizable BAR (Base Address Register) on systems that don't officially support it.

 

What is Resizable BAR (ReBAR)?

Resizable BAR is a PCIe capability that enables negotiation of the BAR size for PCIe devices like discrete graphics cards. This negotiation optimizes system resources and, when enabled, can lead to a notable performance improvement.

In games, GPUs rely on many small chunks of textures, 3D assets, and other graphics data from system memory. The GPU has to constantly request and wait for each piece it needs to render each frame. This results in lots of back-and-forth transfers between the CPU and a relatively small VRAM pool allocated to the GPU.

Resizable BAR enables something different - the GPU can simply request full access to graphics assets from the system memory as needed. This means the GPU isn't limited to only what will fit in its dedicated VRAM. The CPU can rapidly fulfill requests by providing complete packages of assets ready for immediate use.

ReBAR is also known as Re-Size BAR, Smart Access Memory, or Clever Access Memory, and users are encouraged to contact their system manufacturers for specific details.

 

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How Does ReBarDxe Work?

The UEFI DXE driver, called ReBarDxe, is integrated into the UEFI firmware's DXE volume, ensuring execution on every boot. This module replaces the original function PreprocessController of PciHostBridgeResourceAllocationProtocol. It checks for Resizable BAR capability and sets the size to the ReBarState NVRAM variable, ensuring proper allocation during PCI enumeration by the PciBus module.

Learn more about ReBarDxe: https://github.com/xCuri0/ReBarUEFI?tab=readme-ov-file

 

Performance Gains?

According to the FAQ on GitHub "On my system with an i5 3470 and Sapphire Nitro+ RX 580 8GB with Resizable BAR enabled in driver I get an upto 12% FPS increase with 2GB BAR size."

 

Use at Your Own Risk: As with any system modification, enabling unofficial modifications on your systems comes with potential risks, including damage or system failure. Users are strongly urged to read and understand all details before attempting this modification. Exercise caution and consider potential consequences before applying such changes to your system.

 

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