Re: [patch 29/30] x86/microcode: Prepare for minimal revision check

[Peter Zijlstra]

On Thu, Aug 10, 2023 at 08:38:09PM +0200, Thomas Gleixner wrote:
> From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> 
> Applying microcode late can be fatal for the running kernel when the update
> changes functionality which is in use already in a non-compatible way,
> e.g. by removing a CPUID bit.

This includes all compatibility constraints? Because IIRC we've also had
trouble because a CPUID bit got set. Kernel didn't know about, didn't
manage it, but userspace saw the bit and happily tried to use it.

Ofc I can't remember the exact case :/ but anything that changes the
xsave size/state would obviously cause trouble.