linux-kernel - Re: [PATCH v3 3/5] x86/microcode: Avoid any chance of MCE's during microcode update

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Date:   Wed, 17 Aug 2022 10:09:00 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Ashok Raj <ashok.raj@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tony Luck <tony.luck@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        LKML Mailing List <linux-kernel@...r.kernel.org>,
        X86-kernel <x86@...nel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Tom Lendacky <thomas.lendacky@....com>,
        Jacon Jun Pan <jacob.jun.pan@...el.com>
Subject: Re: [PATCH v3 3/5] x86/microcode: Avoid any chance of MCE's during
 microcode update

On Wed, Aug 17, 2022 at 09:58:03AM +0200, Ingo Molnar wrote:
> Also, Boris tells me that writing 0x0 to MSR_IA32_MCG_STATUS
> apparently shuts the platform down - which is not ideal...

Right, if you get an MCE raised while MCIP=0, the machine shuts down.

And frankly, I can't think of a good solution to this whole issue:

- with current hw, if you get an MCE and MCIP=0 -> shutdown

- in the future, even if you change the hardware to block MCEs from
being detected while the microcode update runs, what happens if a CPU
encounters a hw error during that update?

You raise it immediately after? What if there are multiple MCEs? Not
unheard of on a big machine...

Worse, what happens if there's a bitflip in the memory where the
to-be-updated microcode patch is?

You report the error afterwards?

Just thinking about this makes me real nervous.

-- 
Regards/Gruss,
    Boris.

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