NvStrapsReBar

UEFI driver to enable and test Resizable BAR on Turing graphics cards (GTX 1600, RTX 2000).

This is a copy of the rather popular ReBarUEFI DXE driver. ReBarUEFI enables Resizable BAR for older motherboards and chipsets without ReBAR support from the manufacturer. NvStrapsReBar was created to test Resizable BAR support for GPUs from the RTX 2000 (and GTX 1600, Turing architecture) line. For the GTX 1000 cards (Pascal architecture) and older the tool can also enable a large BAR on the PCI bus, but it is fixed size and not resizable, so it is not the same as ReBAR. But then the NVIDIA driver for Windows shows a blue screen or resets the computer during boot if the BAR size has been changed. So GTX 1000 cards still can not enable ReBAR. The proprietary Linux driver does not crash, but does not pick up the new BAR size either (NVIDIA, could you please help fixing the Pascal driver ?)

Do I need to flash a new UEFI image on the motherboard, to enable ReBAR on the GPU ?

Yes, this is how it works for Turing GPUs (GTX 1600 / RTX 2000).

It's ususally the video BIOS (vBIOS) that should enable ReBAR, but the vBIOS is digitally signed (NVIDIA vBIOS is also encrypted) and can not be modified by modders and end-users (is locked-down). The motherboard UEFI image can also be signed or have integrity checks, but in general it is thankfully not as locked down, and users and UEFI modders often still have a way to modify it.

For older boards without ReBAR, adding ReBAR functionality depends on the Above 4G Decoding option in your UEFI setup, which must be turned on in advance, and CSM must be disabled.

Usage

Download latest release from the Releases page, or build the project using the build instructions. This should produce two files:

• NvStrapsReBar.ffs UEFI DXE driver
• NvStrapsReBar.exe Windows executable

After download or build you need to go through the following steps:

• update the motherbord UEFI image to add the new NvStrapsReBar.ffs driver (see below)
• enable ReBAR in UEFI Setup if the motherboard supports it. Otherwise enable "Above 4G Decoding" and disable CSM
• run NvStrapsReBar.exe as Administrator to enable the new BAR size, by following the text-mode menus. If you have a recent motherboard, you only need to input E to Enable ReBAR for Turing GPUs, then input S to save the new driver configuration to EFI variable. For older motherboards without ReBAR, you also need to input P and set BAR size on the PCI side (motherboard side).
• reboot after saving the menu options.
• if you make changes in UEFI Setup, NvStrapsReBar will be disabled automatically and you need to re-enable it. Same if you manually set back the current year in UEFI Setup (can be used to disable NvStrapsReBar without booting to Windows).
• if you make hardware changes like adding or changing a GPU: you have to disable ReBAR first. The reason is NvStrapsReBar depends on the GPU BAR0 address to enable ReBAR, and system firmware changes the allocated address for BAR0 when hardware is changed or settings in UEFI Setup are changed.

Warning

• Disable NvStrapsReBar before making hardware changes like adding a second GPU.
• NvStrapsReBar will be disabled automatically if you make changes in UEFI Setup. Re-enable it afterwards.

Credits go to the bellow github users, as I integrated and coded their findings and results:

• envytools project for the original effort on reverse-engineering the register interface for the GPUs, a very long time ago, for use by the nouveau open-source driver in Linux. Amazing how this old documentation could still help us today !
• @mupuf from envytools project for bringing up the idea and the exact (low level) registers from the documentation, that enable resizable BAR
• @Xelafic for the first code samples (written in assembly!) and the first test for using the GPU STRAPS bits, documented by envytools, to select the BAR size during PCIe bring-up in UEFI code.
• @xCuri0 for great support and for the ReBarUEFI DXE driver that enables ReBAR on the motherboard side, and allows intercepting and hooking into the PCIe enumeration phases in UEFI code on the motherboard.

Working GPUs

Check issue #1 for a list of known working GPUs (and motherboards).

Updating UEFI image

You can download the latest release of NvStrapsReBar from the Releases page, or build the UEFI DXE driver and the Windows executable using the instructions on the building page.

The resulting NvStrapsReBar.ffs file needs to be included in the motherboard UEFI image (downloaded from the montherboard manufacturer, usually under "BIOS update"), and the resulting image should be flashed onto the motherboard as if it were a new firmware version for that board. See the original project ReBarUEFI for the instructions to update motherboard UEFI. Replace "ReBarUEFI.ffs" with "NvStrapsReBar.ffs" where appropriate.

So you will still have to check the README page from the original project:

• https://github.com/xCuri0/ReBarUEFI

for all the details and instructions on working with the UEFI image, and patching it if necessary (for older motherboards and chipsets).

Enable ReBAR and choose BAR size

After flashing the motherboard with the new UEFI image, you need to enable ReBAR in UEFI Setup. For older motherboards without ReBAR, enable "Above 4G Decoding" and disable CSM. Then you need to run NvStrapsReBar.exe as Administrator.

NvStrapsReBar.exe prompts you with a small text-based menu. You can configure 2 values for the BAR size with this tool:

• GPU-side BAR size
• PCI BAR size (for older motherboards without ReBAR)

Newer boards with ReBAR support from the manufacturer can auto-configure PCI BAR size, so you only need to set the GPU-side value for the BAR size. If not, you should try and experiment with both of them, as needed.

Warning

• Disable NvStrapsReBar before making hardware changes like adding a second GPU.
• NvStrapsReBar will be disabled automatically if you make changes in UEFI Setup. Re-enable it afterwards.

Most people should choose the first menu option and press E to Enable auto-settings BAR size for Turing GPUs. Depending on your board, you may need to also input P at the menu prompt, to choose Target PCI BAR size, and select value 64 (for the option to configure PCI BAR for selected GPUs only). Before quitting the menu, input S to save the changes you made to the EFI variable store, for the UEFI DXE driver to read them.

If you choose a GPU BAR size of 8 GiB for example, and a Target PCI BAR size of 4 GiB, you will get a 4 GiB BAR.

For older boards without ReBAR support from the manufacturer, you can select other values for Target PCI BAR size, to also configure other GPUs for example. Or to limit the BAR size to smaller values even if the GPU supports higher values. Depending on the motherboard UEFI, for some boards you may need to use lower values, to limit BAR size to 4 GB or 2GB for example. Even a 2 GB BAR size still gives you the benefits of Resizable BAR in most titles, and NVIDIA tends to use 1.5 GB as the default size in the Profile Inspector. There are exceptions to this 'though (for some titles that can still see improvements with the higher BAR sizes).

If later you want to make further changes in UEFI Setup, or hardware changes like adding a new GPU, you have to disable NvStrapsReBar first. Because NvStrapsReBar depends on the GPU BAR0 address allocated by system firmware, and that changes with UEFI Setup changes or with hardware changes.

Using large BAR sizes

Remember you need to use the Profile Inspector because it enables ReBAR per-application, and that overrides the global value reported by the PCI bus. There appears to be a fake site for the Profile Inspector, so always downloaded it from github, or use the link above.