What about P6 family (Ppro, I, II, III, IV) aka "ia32/64" microcode upload
in NetBSD ?

I saw codes in kernel supporting some hardwares, but nothing for processor ?

Not most motherboards have their bios and µP microcodes up to date, that's
why C:\Windows\System32\Drivers\Update.sys or Linux /dev/microcode may help
to filter some bugs... :]