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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Peter Zijlstra <peterz@infradead.org>
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
In-Reply-To: <20230810205427.GG212435@hirez.programming.kicks-ass.net>
References: <20230810153317.850017756@linutronix.de>
 <20230810160806.617422055@linutronix.de>
 <20230810205427.GG212435@hirez.programming.kicks-ass.net>
Date:   Fri, 11 Aug 2023 11:38:51 +0200
Message-ID: <871qg9j4us.ffs@tglx>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

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@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.

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. :)