linux-kernel - Re: [v2 1/3] x86/microcode/intel: Check microcode revision before updating sibling threads

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Message-ID: <20180222134259.GD27489@pd.tnic>
Date:   Thu, 22 Feb 2018 14:44:58 +0100
From:   Borislav Petkov <bp@...e.de>
To:     "Raj, Ashok" <ashok.raj@...el.com>
Cc:     X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Tony Luck <tony.luck@...el.com>,
        Andi Kleen <andi.kleen@...el.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>
Subject: Re: [v2 1/3] x86/microcode/intel: Check microcode revision before
 updating sibling threads

On Thu, Feb 22, 2018 at 05:19:18AM -0800, Raj, Ashok wrote:
> This isn't a simple WRMSR like others. Microcode engine needs to do 
> a bunch of validation.

So this is slowly starting to resemble a real reason why not to. That
should be part of the code comment.

> We now have new guidance that the sibling must be spinning and not
> doing other things that can introduce instability around loading
> microcode.

Why isn't *this* in the code comment?

> I think its safer...

That's not good enough - it should be:

"It is not safe because... <insert reasons here>".

So that you can justify the overhead.

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
--