Resizable BAR

Resizable BAR, or ReBAR, is a PCI Express capability that lets a device's base address register (BAR) be resized past its default, so the CPU can address all of a GPU's memory. A BAR is the window of address space through which the CPU reaches a device. AMD markets the feature on Ryzen and Radeon as Smart Access Memory. Server BIOS menus in AMD's Instinct guide list it as Resizable BAR Support. AMD's Instinct guide lists Above 4G Decoding as GPU large BAR support.

NVIDIA's L40 brief shows how much the setting varies. BAR1 is 64 GiB in the default Display Off mode, 8 GiB in Display On 8GB mode and 256 MiB in Display On 256MB mode. For passthrough, Hyper-V gives each VM 128 MB of low and 512 MB of high memory-mapped I/O space by default, and Microsoft notes a GPU can need more. AMD's Instinct guide enables Resizable BAR Support for virtualization, and the option may not show on every BIOS. AMD notes that before Smart Access Memory, Radeon drivers exposed only a 256 MiB aperture to the CPU.

Enable Above 4G Decoding and Resizable BAR Support before installing a compute GPU. Size each passthrough VM's MMIO space to its GPUs' BARs.

Sources

Last verified October 30, 2026.