RE: [PATCH v3] x86/microcode/intel: Reject problematic loading on Granite Rapids systems

[Van De Ven, Arjan]

> > Microcode updates can usually jump revisions. However, there is an
> > erratum on Granite Rapids systems. If they "jump over" revision
> > 0x1000405, they result in #MC. Avoid it.
> 
> A probably silly question.
> Is it valid to downgrade microcode?


Within SVN in theory it can be done

In practice, unless you have very special circumstances (and check with Intel if the exact downgrade you have in mind has been tested for downgrading) I would very strongly recommend against doing so. It may appear to work, but note that 

A -> B -> A 

Is NOT identical to just

A  (staying at A)

as part of the microcode load, some microcode runs as "setup" which may change settings in the CPU (versus runtime behavior changes) -- and that setup step is not undone as you go back to A.... which means you run A with a (partial or whole) setup of B.