ReBarUEFI: Resizable BAR for almost any UEFI system

marcus9999•about 1 hour ago

this fixes a real annoyance on older workstation boards where above 4g decoding is a requirement but the option isn't even in the bios menu. saw a build recently on an x79 board where it had to be exposed with a hand patched bios before anything like this or a modern gpu would work at all.

jedbrooke•37 minutes ago

thanks for the throwback, I remember having to hand patch the bios on my x79 system to use a gpu with 12GB vram. Fun times. My motherboard also mysteriously stopped working shortly after…

bigwheels•about 2 hours ago

What does this do? I perused the README but I'm still clueless on what the purpose and use-case is for ReBAR.

(Posting this under the assumption others will also appreciate a bit of quick context.)

Moto7451•about 1 hour ago

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.

segfaultbuserr•about 1 hour ago

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 it 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.

wmf•about 2 hours ago

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

willis936•about 1 hour ago

It should come with a warning label. If you're in a scenario where VRAM is being maxed out and thrashing main memory then ReBAR will worsen the frequency and magnitude of the stutters. I ran into this with a 3070 (8 GB) recently.

jmalicki•2 minutes ago

It's entirely possible a game manages paging VRAM badly. But allowing the game more flexibility isn't a problem with a larger BAR, it's that the game is stupid and gets dumber the more VRAM you give it.

If a program runs slower when you give it more RAM, the problem isn't giving more RAM.

Moto7451•about 1 hour ago

I think you’re experiencing paging to main memory rather than anything ReBAR related. ReBAR itself reduces memory copy operations. It’s of course possible you’re experiencing some sort of BIOS bug.

mathisfun123•about 2 hours ago

> 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.

qudat•about 1 hour ago

Your quote doesn’t help me understand this project at all.

What is a BAR let alone a resizable one? Readme just jumps in, which is fine, but I’m not sure why this is on HN or why I should care.

toast0•16 minutes ago

BARs are part of the PCI spec, they're the way for firmware and OS to determine the size of the memory or i/o window the card controls as well as the way for the system firmware (usually) to pick what address range the card should use.

In the old days, they were a fixed size. If your GPU has 16 GB of ram and you want to access all of it via memory addresses, you'd need a 16 GB BAR ... but lots of (older) systems wouldn't be able to map in a large BAR because of a lack of address lines (or wahtever), so GPUs had stayed with 256MB for VRAM access because it was compatible. With a smaller BAR than the VRAM, you have to use some sort of windowing / paging setup. Resizable BAR lets the BAR start small so older systems will work, but grow larger with capable systems.

Much better than having a jumper to set the BAR to big or small, and you can skip VRAM window management.

pizza234•about 1 hour ago

ReBAR stands for "Resizable BAR (Base Address Register)" - it makes the CPU's access window into GPU memory resizable; instead of accessing VRAM in small chunks, the CPU can access much more at once, which can improve performance on some modern GPUs.

mathisfun123•about 1 hour ago

i literally don't understand how people move through life - do you expect every single link on this site (or any site) to be ELI5 for you specifically? do you not understand that some things require effort/homework on your part?

ReBarUEFI: Resizable BAR for almost any UEFI system

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