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

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Date:   Fri, 11 Aug 2023 11:38:51 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Borislav Petkov <bp@...en8.de>,
        Ashok Raj <ashok.raj@...el.com>,
        Arjan van de Ven <arjan@...ux.intel.com>
Subject: Re: [patch 29/30] x86/microcode: Prepare for minimal revision check

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@...utronix.de>
>> 
>> 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. :)