linux-kernel - Re: [PATCH 2/2] x86/microcode/intel: Extend BDW late-loading with platform id and LLC check

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Message-ID: <20180115114851.npyeo4uztf5ww33o@khazad-dum.debian.net>
Date:   Mon, 15 Jan 2018 09:48:51 -0200
From:   Henrique de Moraes Holschuh <hmh@....eng.br>
To:     Jia Zhang <zhang.jia@...ux.alibaba.com>
Cc:     tony.luck@...el.com, bp@...en8.de, mingo@...hat.com, hpa@...or.com,
        tglx@...utronix.de, x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] x86/microcode/intel: Extend BDW late-loading with
 platform id and LLC check

On Mon, 15 Jan 2018, Jia Zhang wrote:
> For more details, see erratum BDF90 in document #334165 (Intel Xeon
> Processor E7-8800/4800 v4 Product Family Specification Update) from
> September 2017.

For the record, this erratum may well affect some E5v4 as well.
Anything with a LLC/core ratio >= 2.5 is potentially affected as far as
I could tell when I took a serious look at it months ago (based only on
crash reports and public information).

It would be safer to just blacklist by sig == 0x406f1, revision <
0x0b00021, and LLC/core ratio >= 2.5, ignoring platform IDs.

>  	/*
>  	 * Late loading on model 79 with microcode revision less than 0x0b000021
> -	 * may result in a system hang. This behavior is documented in item
> -	 * BDF90, #334165 (Intel Xeon Processor E7-8800/4800 v4 Product Family).
> +	 * and LLC size per core bigger than 2.5MB may result in a system hang.
> +	 * This behavior is documented in item BDF90, #334165 (Intel Xeon
> +	 * Processor E7-8800/4800 v4 Product Family).
>  	 */
>  	if (c->x86 == 6 &&
>  	    c->x86_model == INTEL_FAM6_BROADWELL_X &&
>  	    c->x86_mask == 0x01 &&
> +	    llc_size_per_core(c) > 2621440 &&
> +	    c->platform_id == 0xef &&
>  	    c->microcode < 0x0b000021) {
>  		pr_err_once("Erratum BDF90: late loading with revision < 0x0b000021 (0x%x) disabled.\n", c->microcode);
>  		pr_err_once("Please consider either early loading through initrd/built-in or a potential BIOS update.\n");

The c->platform_id test looks wrong.  The processor will only have a
single bit set, it is the microcode update that has more than a single
bit set.

And do you really want 0xef?  That is everyhing the public available
microcode updates can be applied to in the first place, so even a
corrected test would be useless (it would always match) unless you
actually expect to find never-seen-in-the-wild platform mask 0x10?

-- 
  Henrique Holschuh