I think there's some misunderstanding in this discussion thread on what Ingo is working on.
This isn't a general fix or "pure-software alternative to IBRS" - it is specific to handling the 16 stack depth ret being vulnerable on skylake+ processors, as it falls back to an indirect branch prediction. The zero overhead for non-skylake+ chips isn't that it is a zero-overhead general IBRS solution, it's that it does not incur additional overhead on non-skylake+ chips as it is not doing any work to mitigate the issue on non-skylake chips.
This is because all of the major distros already compile their kernels with the CONFIG_FUNCTION_TRACER=y option set, so no additional work is being done unless the conditions appear and it is on a skylake+ chip.
This isn't a general fix or "pure-software alternative to IBRS" - it is specific to handling the 16 stack depth ret being vulnerable on skylake+ processors, as it falls back to an indirect branch prediction. The zero overhead for non-skylake+ chips isn't that it is a zero-overhead general IBRS solution, it's that it does not incur additional overhead on non-skylake+ chips as it is not doing any work to mitigate the issue on non-skylake chips.
This is because all of the major distros already compile their kernels with the CONFIG_FUNCTION_TRACER=y option set, so no additional work is being done unless the conditions appear and it is on a skylake+ chip.