linux-kernel - Re: [PATCH v4 6/7] x86/cpufeature: Blacklist SPEC

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Message-ID: <alpine.DEB.2.20.1801251146450.2020@nanos>
Date:   Thu, 25 Jan 2018 11:54:20 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     David Woodhouse <dwmw2@...radead.org>
cc:     arjan@...ux.intel.com, karahmed@...zon.de, x86@...nel.org,
        linux-kernel@...r.kernel.org, tim.c.chen@...ux.intel.com,
        bp@...en8.de, peterz@...radead.org, pbonzini@...hat.com,
        ak@...ux.intel.com, torvalds@...ux-foundation.org,
        gregkh@...ux-foundation.org, dave.hansen@...el.com,
        gnomes@...rguk.ukuu.org.uk, ashok.raj@...el.com, mingo@...nel.org
Subject: Re: [PATCH v4 6/7] x86/cpufeature: Blacklist SPEC_CTRL on early
 Spectre v2 microcodes

On Thu, 25 Jan 2018, David Woodhouse wrote:

> On Thu, 2018-01-25 at 09:23 +0000, David Woodhouse wrote:
> > 
> > +/*
> > + * Early microcode releases for the Spectre v2 mitigation were broken.
> > + * Information taken from;
> > + * • https://newsroom.intel.com/wp-content/uploads/sites/11/2018/01/microcode-update-guidance.pdf
> 
> Oh look, Intel have released a *third* version of that document
> already, and they've bumped the bad Kabylake versions to 0x84, although
> they're *still* missing out the KBL 906EA SKU which was updated to 0x80
> in the public 20180108 microcode release. I'll bump them all in my tree
> to 0x84.
> 
> Thomas, want another copy in email now, or were we waiting for another
> round of these before you merge them anyway?

Looking at this mess makes me even less convinced that a blacklist is a
good idea. We have already at least 3 different variants of blacklists.

So I rather see a whitelist approach which says:

   if (ucode_version < known_good(family, model))
       return;

I know it would require that Intel releases a set of known good ucodes at
some point in the future, but that's a reasonable request.

I rather take a few patches which add the cutoff for family/model (and NO,
I don't want to add stepping into the mix at all) than having this
blacklist mess which keeps changing every other day.

Thanks,

	tglx