alt Hacker NewsSince 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.
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Just to make sure I understand: Is the resizable BAR/MMIO a RAM buffer for PCie packets? Or does it have some deeper integration with DMA or something? I assume it's not like memory mapped peripherals on an AXI bus which is why you need the buffer?
GPUs have been using a 256MB window since they started coming with 256MB of VRAM, as it would otherwise be a pointless waste of address space to have a window larger than the actual amount of VRAM. Previously, they would have a window exactly equal to the size of VRAM (likely rounded up to the next power of 2.)