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ResizeGpuBars
Type: plist integer
Failsafe: -1
Description: Configure GPU PCI BAR sizes.
This quirk sets GPU PCI BAR sizes as specified or chooses the largest available below the ResizeGpuBars value. The specified value follows PCI Resizable BAR spec. Use 0 for 1 MB, 1 for 2 MB, 2 for 4 MB, and so on up to 19 for 512 GB.
Resizable BAR technology allows to ease PCI device programming by mapping a configurable memory region, BAR, into CPU address space (e.g. VRAM to RAM) as opposed to a fixed memory region. This technology is necessary, because one cannot map the largest memory region by default, for the reasons of backwards compatibility with older hardware not supporting 64-bit BARs. Consequentially devices of the last decade use BARs up to 256 MB by default (4 remaining bits are used by other data) but generally allow resizing them to both smaller and larger powers of two (e.g. from 1 MB up to VRAM size).
Operating systems targeting x86 platforms generally do not control PCI address space, letting UEFI firmware decide on the BAR addresses and sizes. This illicit practice resulted in Resizable BAR technology being unused up until 2020 despite being standardised in 2008 and becoming widely available in the hardware soon after.
Modern UEFI firmware allow the use of Resizable BAR technology but generally restrict the configurable options to failsafe default (OFF) and maximum available (ON). This quirk allows to fine-tune this value for testing and development purposes.
Consider a GPU with 2 BARs:
• BAR0 supports sizes from 256 MB to 8 GB. Its value is 4 GB.
• BAR1 supports sizes from 2 MB to 256 MB. Its value is 256 MB.
Example 1: Setting ResizeGpuBars to 1 GB will change BAR0 to 1 GB and leave BAR1 unchanged. Example 2: Setting ResizeGpuBars to 1 MB will change BAR0 to 256 MB and BAR0 to 2 MB. Example 3: Setting ResizeGpuBars to 16 GB will change BAR0 to 8 GB and leave BAR1 unchanged.
Note 1 : This quirk shall not be used to workaround macOS limitation to address BARs over 1 GB. ResizeAppleGpuBars should be used instead.
Note 2: While this quirk can increase GPU PCI BAR sizes, this will not work on most firmware as is, because the quirk does not relocate BARs in memory, and they will likely overlap. Contributions to improve this feature are welcome.
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