My understanding is that currently Windows maps VRAM only in 256MB chunks. So if you need to access more memory, you need to do extra work (write stuff to registers) to map different part of memory. If you operate with a lot of memory, then you're spending time to adjust this base address. Mapping larger chunks, or even whole memory would eliminate this overhead. Which is called "resizable bar". AMD calls it smart access memory, just a marketing name. Using this requires support from GPU, BIOS, OS and GPU driver.
This overhead is global - to whole system, not one application. That probably allows to spend less time to remapping when your game needs to upload new uniforms for next frame, when in same time Windows switched it away for whatever it needs to do (update framebuffer/compositor/etc). If you don't stream a lot of new data, then you probably won't see much improvement - from what I've googled lot of benchmarks often show single digit % improvement, not 11% which is AMD marketing material number.
It seems that PCI spec have this since ~2008, so not a new technology, just needed to be enabled in GPU/OS/drivers: https://pcisig.com/specifications...ble%20bar&&&&order=title&sort=asc
Here's more info: https://docs.microsoft.com/en-us/...ers/display/resizable-bar-support
And from what it looks like Linux supported this feature already for a while, see: https://www.phoronix.com/forums/f...-rx-6000-series/page4#post1215694