linux-kernel - Re: x86, microcode: BUG: microcode update that changes x86

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Message-ID: <CALCETrVoeqQUXH7g+reu=hUvCJoiy+edHKBqYKPWXPCS3_UtUw@mail.gmail.com>
Date:	Thu, 18 Sep 2014 17:23:40 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Henrique de Moraes Holschuh <hmh@....eng.br>
Cc:	Borislav Petkov <bp@...en8.de>,
	Chuck Ebbert <cebbert.lkml@...il.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: x86, microcode: BUG: microcode update that changes x86_capability

On Sep 18, 2014 5:13 PM, "Henrique de Moraes Holschuh" <hmh@....eng.br> wrote:
>
> On Thu, 18 Sep 2014, Henrique de Moraes Holschuh wrote:
> > On Thu, 18 Sep 2014, H. Peter Anvin wrote:
> > > We should, but this is also part of why we want the early ucode capability.
> >
> > Well, yes.  But that won't help the several stable and LTS distros with
> > kernels without early ucode update support.
>
> Here's a plan that might work, pending actually checking the libpthread TSX
> code to make sure it keys on /proc/cpuinfo flags:

Surely it checks cpuid directly, though.

Can we twiddle the cpuid bit?  I never noticed any way in the docs to
do it, but if BIOS has such an ability, maybe we do, too.  I wonder if
there's anything semi-documented in biosbits, or if we could just
reverse-engineer it.

--Andy

>
> Add a cpu quirk, triggered by the Haswell cpuids, to force-disable hle on
> the affected processors.
>
> This will work around the x86_capability capability issue (which should
> still be fixed, anyway), and it should also get userspace to stay away from
> TSX, therefore also working around the worst issue (processes getting
> SIGILL).
>
> This will disable the "user may ask the BIOS to keep TSX enabled"
> anti-feature, though.  This drawback can be avoided, but only if a future
> microcode update won't re-disable hle when the BIOS enabled it.  For now, I
> suggest that we decree that "hle is toast" for the current Haswells and add
> back ways to enable it for testing when we know more about it.
>
> --
>   "One disk to rule them all, One disk to find them. One disk to bring
>   them all and in the darkness grind them. In the Land of Redmond
>   where the shadows lie." -- The Silicon Valley Tarot
>   Henrique Holschuh
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