Radeon Linux Drivers Now Only Officially Support Smart Access Memory On Zen 3 + RDNA2
Marek Olšák of AMD who led the original work on Smart Access Memory optimizations for RadeonSI led this change. This restriction isn't being done artificially for marketing purposes but in finding out those with older hardware combinations were sometimes hitting bugs and performance regressions with this recent S.A.M. support in Mesa. This basically puts the official Smart Access Memory support state the same under Windows and Linux.
With the newest Mesa 21.0 code as of today, the detection is in place for AMD Zen 3 (or newer) and GFX10.3 (RDNA 2) otherwise the functionality will now be disabled due to "many people experience performance degradation on some systems."
Via DriConf is now a "enable_sam" option for anyone regardless of Radeon GPU / CPU combination can try force-enabling Smart Access Memory if your system plays well with it enabled. Likewise, there is a "disable_sam" option if wanting to disable it for comparing the performance impact of this resizable BAR functionality. You must also (obviously) still have "Above 4G Decoding" enabled in your BIOS.
Also notable and tucked into this same merge request is getting Mesa's L3 cache pinning enabled for AMD Zen 3 systems. Previously the check was missing for Zen 3 but should now be there offering another performance boost for Ryzen 5000 series systems.
Fresh Mesa 21.0 benchmarks coming up soon on Phoronix.