The AMD Variant 2 fix is decided, It’s already hard as hell to make work on AMD as is. This kills any hope Variant 2 had on AMD.
On AMD CPUs, retpoline isn’t needed and usage of the LFENCE instruction is enough to protect against Variant 2 of Spectre.
Intel will need to be fixed via retpoline as is in Windows. This can carry a 5-10% performance impact.