These kind of BIOS modding shenannigans is why I got interesed in Coreboot, so that enhancing a Firmware capabilities becomes possible without touching propietary tools/code.
Other similar mods included adding NVMe Boot capabilities to older Firmwares that didn't supported them so that you could boot from modern NVMe drives if you used some kind of adapter (As none of those Motherboards have M.2 Slots, but you can use them with a PCIe-to-M.2 adapter card). Also adding required stuff to get certain CPUs booting in Motherboard that were never officially supported (Depending on generation, could be as simple as adding new CPU Microcode).
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.
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…
I patched my old Haswell system to add an NVMe DXE so I could boot from a modern SSD. I currently have a Lets Note laptop that definitely has UEFI, but pretty much pretends to be BIOS - apparently it's possible to modify to expose UEFI features but I have yet to try...
Since about 5 or 6 years ago, PCIe Video Cards advertise a capability known as Resizeable BAR. PCI Devices like GPUs require some memory to use for PCI MMIO, which is directly visible on the CPU Address Space. As a side note, this Address Space is shared with RAM, and anyone that was around when having 4 GiB RAM with a 32 Bit OS was common (Earlier than 2010 or so) knows that you only saw about 3.25 GiB RAM or so because of sharing the Address Space with PCI MMIO, and those fortunate enough to have used SLI usually saw even less than that, like 2.87 GiB RAM.
GPUs has been using a 256 MiB PCI MMIO window regardless of how much VRAM they actually have since... nearly forever? At least since PCIe is a thing, since I recall than AGP Aperture Size was seteable in era accurate BIOSes. PCIe 3.0 specification introduced a feature known as Resizeable BAR, where the PCI Device can tell a compatible Firmware how much MMIO it actually wants. GPUs uses that to tell a ReBAR capable UEFI Firmware that it wants more MMIO (Usually as big as the GPU VRAM), or uses legacy 256 MiB otherwise.
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.
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 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.
It's hard to explain without going into a few low-level details of PCI Express, but let me try.
Most PCI devices expose some memory and/or I/O ports to the CPU. That memory (or I/O ports) is mapped to somewhere in the address space visible to the CPU. Besides the memory and I/O ports, all PCI devices also expose a separate set of configuration registers; among these registers, there are the Base Address Registers (BARs), which configure where the memory or I/O ports is mapped.
Here's an example output from "lspci -vv" for a GPU:
Region 0: Memory at 7c00000000 (64-bit, prefetchable) [size=8G]
Region 2: Memory at 7e00000000 (64-bit, prefetchable) [size=256M]
Region 4: I/O ports at f000 [size=256]
Region 5: Memory at fca00000 (32-bit, non-prefetchable) [size=1M]
Expansion ROM at fcb00000 [disabled] [size=128K]
Note that regions 0 and 2 are above the 4GB addressable by old 32-bit CPUs. To be compatible with these old CPUs, this card and many others like it allow the firmware (and/or the operating system) to choose not only where the memory is mapped, but also its size. We can see this in the same "lspci -vv" output for this GPU:
Capabilities: [200 v1] Physical Resizable BAR
BAR 0: current size: 8GB, supported: 256MB 512MB 1GB 2GB 4GB 8GB
BAR 2: current size: 256MB, supported: 2MB 4MB 8MB 16MB 32MB 64MB 128MB 256MB
Older systems which do not understand this extended capability will still treat these regions as fixed size, probably with the first size in this list (256MB for region 0, 2MB for region 2). Newer systems can tell the device to "resize" the BAR to a bigger size, which obviously needs the first region to be placed above the 4GB barrier since it's too big.
Why is this useful? This particular GPU has 8GB of VRAM; it's quite obvious that region 0 is a direct view into that VRAM. When using the maximum BAR size, the CPU can directly read and write anywhere into the VRAM; when using a smaller BAR, the CPU can only see a small window into the VRAM, and has to use less direct methods to access it.
(As an aside: go right now and do a "sudo lspci -vv" on your computer, if you see a Resizable BAR capability which isn't using the maximum size, you can probably gain a bit more speed for free by going into the BIOS and enabling "Resizable BAR" and/or "Above 4G decoding". If you can't find these options, well, AFAIU that's what this project is all about..)
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.
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.
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.
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.
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.
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?
These kind of BIOS modding shenannigans is why I got interesed in Coreboot, so that enhancing a Firmware capabilities becomes possible without touching propietary tools/code.
Other similar mods included adding NVMe Boot capabilities to older Firmwares that didn't supported them so that you could boot from modern NVMe drives if you used some kind of adapter (As none of those Motherboards have M.2 Slots, but you can use them with a PCIe-to-M.2 adapter card). Also adding required stuff to get certain CPUs booting in Motherboard that were never officially supported (Depending on generation, could be as simple as adding new CPU Microcode).
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.
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…
I patched my old Haswell system to add an NVMe DXE so I could boot from a modern SSD. I currently have a Lets Note laptop that definitely has UEFI, but pretty much pretends to be BIOS - apparently it's possible to modify to expose UEFI features but I have yet to try...
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.)
Since about 5 or 6 years ago, PCIe Video Cards advertise a capability known as Resizeable BAR. PCI Devices like GPUs require some memory to use for PCI MMIO, which is directly visible on the CPU Address Space. As a side note, this Address Space is shared with RAM, and anyone that was around when having 4 GiB RAM with a 32 Bit OS was common (Earlier than 2010 or so) knows that you only saw about 3.25 GiB RAM or so because of sharing the Address Space with PCI MMIO, and those fortunate enough to have used SLI usually saw even less than that, like 2.87 GiB RAM.
GPUs has been using a 256 MiB PCI MMIO window regardless of how much VRAM they actually have since... nearly forever? At least since PCIe is a thing, since I recall than AGP Aperture Size was seteable in era accurate BIOSes. PCIe 3.0 specification introduced a feature known as Resizeable BAR, where the PCI Device can tell a compatible Firmware how much MMIO it actually wants. GPUs uses that to tell a ReBAR capable UEFI Firmware that it wants more MMIO (Usually as big as the GPU VRAM), or uses legacy 256 MiB otherwise.
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.
> sufficient tricks (that Intel GPUs lack)
Isn't it just being able to shift the window of GPU memory visible to the CPU?
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 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.
IOMMU branding at least makes a little bit of sense. AMDs and Intels virtualisation ISA are completely different, including their IOMMU.
It's hard to explain without going into a few low-level details of PCI Express, but let me try.
Most PCI devices expose some memory and/or I/O ports to the CPU. That memory (or I/O ports) is mapped to somewhere in the address space visible to the CPU. Besides the memory and I/O ports, all PCI devices also expose a separate set of configuration registers; among these registers, there are the Base Address Registers (BARs), which configure where the memory or I/O ports is mapped.
Here's an example output from "lspci -vv" for a GPU:
Note that regions 0 and 2 are above the 4GB addressable by old 32-bit CPUs. To be compatible with these old CPUs, this card and many others like it allow the firmware (and/or the operating system) to choose not only where the memory is mapped, but also its size. We can see this in the same "lspci -vv" output for this GPU: Older systems which do not understand this extended capability will still treat these regions as fixed size, probably with the first size in this list (256MB for region 0, 2MB for region 2). Newer systems can tell the device to "resize" the BAR to a bigger size, which obviously needs the first region to be placed above the 4GB barrier since it's too big.Why is this useful? This particular GPU has 8GB of VRAM; it's quite obvious that region 0 is a direct view into that VRAM. When using the maximum BAR size, the CPU can directly read and write anywhere into the VRAM; when using a smaller BAR, the CPU can only see a small window into the VRAM, and has to use less direct methods to access it.
(As an aside: go right now and do a "sudo lspci -vv" on your computer, if you see a Resizable BAR capability which isn't using the maximum size, you can probably gain a bit more speed for free by going into the BIOS and enabling "Resizable BAR" and/or "Above 4G decoding". If you can't find these options, well, AFAIU that's what this project is all about..)
It allows the CPU to access all GPU VRAM. This improves gaming performance, especially for AMD GPUs.
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.
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.
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.
> 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.
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.
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.
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.
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?