You are now aware we live in a world where even CPUs have microcode patches.
Now, working TSX can have a security impact, yes. At least on TSX as implemented on later Broadwell/early Skylake, memory accesses done from inside an speculative region are allowed to bypass stuff they shouldn't. Is that what you meant?
"...CCROS [microcode] cards on the [IBM System/360] Model 30 could be replaced in the field. This made design modifications easier to accomplish, as well as providing for field diagnosis. ...
The later System/360 Model 25 and models of the later System/370 series were configured with at least one portion of the control store being read-write for loading microcode patches and microdiagnostics."
Was this not the same thing? Some microcode update to patch a CPU flaw essentially?
That was almost a decade ago.