The firmware of intel 8-series motherboards from the era of Haswell was initially provided with skipping the memory mapping beyond 4GB by default. Modern GPUs couldn't use all their video memory as a result. This negatively affects the performance in the games on the old systems. roschank (a community developer) has created reBART.
New open-source UEFI driver restores modern GPU memory features on old Haswell boards
With it you can correct this shortcoming of the initial firmware provided by the manufacturers of the motherboards. This allows to use resizable bar without any new hardware for the old ones.
- Compatible Motherboards: Intel 8-series (Z87, H87, Q87, B85, Q85, H81)
- Compatible GPUs: Intel Arc (A750, B580)
- Technical Requirement: DSDT Patching
reBART – driver in the form of the open-source UEFI tool, it is intended to move the gpu memory window above 4gb right before os loading. It supports intel 8-series (z87, h87, q87, b85, q85, h81 and so on). Also experimental support for intel arc graphics cards is added (at least the a750 and b580). People confirmed that these cards are working with unlimited bar size on ubuntu.
To turn resizable bar on you need to patch the dsdt tables with reBART. Module patching as in ReBarUEFI is not necessary (as stated by the developer), but the dsdt alteration is a must. This could be a stumbling block for the common user who's not accustomed to change something at the low level. Although this way you managed to bypass the initial limitation of the firmware you now have to make a manual setup.
No benchmarks were published by the developer to show a performance gain from this functionality. At least one user stated that his asus z87-plus motherboard didn't boot after he applied a patch to the dsdt. Keep in mind that when you change anything in your bios/dsdt you risk not to boot anymore. As much as this feature works fine, the developer didn't confirm the actual stability and performance boost.
Roschank provides us this driver in order to correct the firmware shortcoming shared by all the big manufacturers of this era. Thanks to this tool it's possible to get modern gpu memory functionality working on the old intel 8-series boards. Resizable bar can be turned on for the compatible gpu of intel arc (a750, b580 and so on).
You need to patch your dsdt. Take into consideration the possible boot issues due to lack of testing.
DSDT patching is a nightmare for the average user. The kernel cannot fix a broken ACPI table once the OS has booted.
Does this actually give a performance boost or is it just a placebo for Haswell boards?