[hansendc]

MSRs can do anything, really. They're also not universally "registers" in any meaningful way. Basically, instead of adding new instructions for everything, CPU designers just add a new MSR. Its kinda like how the kernel assigns a new major/minor for every new device driver instead of a new syscall.

MSRs can also be added by microcode update as we saw with MSR_IA32_SPEC_CTRL, which was used to mitigate Spectre (among other things).

Disclaimer: I work on Linux at Intel, and I was one of the ones posting in that LKML thread quoted in the article.