From the FAQ of ETH Zurich's "Computer Security Group" website:
> Are only Linux systems affected?
> We’ve built the proof of concept code for Linux. But, because the fundamental issue is at the hardware level, Microsoft and Apple computers with the affected hardware have this issue too.
>“Note that Windows systems are not affected given that these systems use Indirect Branch Restricted Speculation (IBRS) by default which is also the mitigation being made available to Linux users. Intel is not aware of this issue being exploited outside of a controlled lab environment.”
It's referring to Intel processors with hardware IBRS support, e.g.: Coffee Lake Refresh (e.g.: 9700, 9900) and newer. On those CPUs, Windows also makes use of IBRS alongside retpoline compilation of the kernel. See the table[1] in the linked article.
> Are only Linux systems affected?
> We’ve built the proof of concept code for Linux. But, because the fundamental issue is at the hardware level, Microsoft and Apple computers with the affected hardware have this issue too.
--
[1] https://comsec.ethz.ch/research/microarch/retbleed/