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Date:   Fri, 11 Aug 2023 07:20:04 -0700
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Subject: Re: [patch 29/30] x86/microcode: Prepare for minimal revision check
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To:     Peter Zijlstra <peterz@infradead.org>,
        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>
References: <20230810153317.850017756@linutronix.de>
 <20230810160806.617422055@linutronix.de>
 <20230810205427.GG212435@hirez.programming.kicks-ass.net>
From:   Arjan van de Ven <arjan@linux.intel.com>
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On 8/10/2023 1:54 PM, Peter Zijlstra wrote:
> 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

do you have the details on that -- I don't know of any of those outside
of enumerating the sidechannel status cpuid bits.

> manage it, but userspace saw the bit and happily tried to use it.

yes it contains all the compatibility constraints the OS folks (e.g. the intel kernel folks)
could convey to the microcode team. If you think the constraints are
not complete please help us improve them

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

of course that cant' happen at runtime at all correctly.
>