AMD introduced the Smart Access Memory (or SAM) feature - which is a fancy term for PCIe Resizable BAR - back in October last year when it launched the Radeon RX 6000 series GPUs. Today, due to popular demand from previous generation Ryzen owners, the company has now added support for SAM on Ryzen 3000 series CPUs as well.
However, the feature is currently only available on select Ryzen 3000 CPUs, and you'll need a Radeon RX 6000 series GPU to take advantage of it as older Radeon cards do not have support for SAM, as of yet.
The PCIe Resizable BAR feature allows the CPU to have full access to the entire GPU memory, whereas typically a processor can only address 256MB for compatibility with 32-bit OSes. As a result, thanks to the entire VRAM now available, requests from the CPU can be queued parallelly instead of sequentially, which in turn improves the speed of their dispatch.
In terms of benefits, AMD has provided no separate numbers regarding performance improvement when enabling SAM on a Ryzen 3000 CPU. However, the company has stated up to a 16% uptick in performance when pairing its new Radeon RX 6700 XT with a Ryzen 9 5900X.
For Ryzen 5000 users, a 500-series chipset board with the AGESA firmware version 18.104.22.168 (or newer), is all that is needed to enable SAM. Ryzen 3000 series CPU owners, however, will probably have to wait for a compatible AGESA firmware.