Even though it's supposedly not feasable to exploit on Linux, there are fixes in the upstream kernel now and the changelog entries have some technical details. If I'm understanding this correctly, AMD systems don't speculatively execute GS-based accesses after speculatively excuting SWAPGS like Intel ones do, but that isn't enough to fully fix the problem on Linux because there's a conditional branch over the SWAPGS instruction which can be speculatively executed. Since that execution path doesn't have a SWAPGS all processors will quite happily continue speculatively executing code that fetches data via GS.