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Date:   Thu, 10 Aug 2023 22:54:27 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     LKML <linux-kernel@vger.kernel.org>, x86@kernel.org,
        Borislav Petkov <bp@alien8.de>,
        Ashok Raj <ashok.raj@intel.com>,
        Arjan van de Ven <arjan@linux.intel.com>
Subject: Re: [patch 29/30] x86/microcode: Prepare for minimal revision check
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 <20230810160806.617422055@linutronix.de>
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On Thu, Aug 10, 2023 at 08:38:09PM +0200, Thomas Gleixner wrote:
> From: Thomas Gleixner <tglx@linutronix.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.

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