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ReBarUEFI: Resizable BAR for almost any UEFI system

243 points · 77 comments · nateb2022

  1. bigwheels · · focus · HN ↗
    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.)

    1. Moto7451 · · focus · HN ↗
      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.
      1. bpye · · focus · HN ↗
        > sufficient tricks (that Intel GPUs lack)

        Isn't it just being able to shift the window of GPU memory visible to the CPU?

        1. p_l · · focus · HN ↗
          Yes, intel lacks a remapping mechanism to support BAR smaller than the total VRAM size.
          1. wtallis · · focus · HN ↗
            Using an Intel discrete GPU on a system that lacks ReBAR support results in somewhat lower performance, not a complete inability to use more than 256MB of VRAM.
            1. p_l · · focus · HN ↗
              Then it changed, because early on I recall people reporting the drivers wouldn't work at all
              1. wtallis · · focus · HN ↗
                Source? I don't know what happened for the Linux drivers, but the Windows drivers definitely booted and ran benchmarks without ReBAR when the Arc A380 launched, because there were lots of published comparisons of the performance impact on workloads that definitely didn't fit in 256MB.
                1. p_l · · focus · HN ↗
                  Unfortunately I can't edit my earlier post, because I can't find it - might have been distorted rumour that reached me, then :/

                  Wasn't helped by my experience reacting very strongly to story of early Arc drivers being problematic because the devs were too used to GPU being just a ring-bus away ;-)

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