Thankfully, this doesn't seem to affect my Haswell machine. A microcode update was installed on 2021-06-09, but the microcode loader reports its build date as 2019-11-12. Does this vulnerability not affect older microarchitectures, or does Intel no longer bother updating them? I couldn't find any details in CVE.
It isn't clear if Haswell has this optimization. As an easy test you could run the zero-fill-bench linked in the post and check the fill0 vs fill1 numbers. If they are the same, Haswell probably never had this optimization in the first place.
Based on the Intel microcode release note [1] it doesn't seem like client HSW got any update this time, only HSX (Haswell Xeon).
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[1] https://github.com/intel/Intel-Linux-Processor-Microcode-Dat...