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

[Thomas Gleixner]

On Thu, Aug 10 2023 at 22:54, Peter Zijlstra wrote:
> 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.

We don't know. If the microcoders screw that minrev constraint up, then
we are up the creek w/o a paddle as before.

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

Details. :)