Re: x86, microcode: BUG: microcode update that changes x86_capability

From: "H. Peter Anvin" <hpa@zytor.com>
To: Andy Lutomirski <luto@amacapital.net>,
	Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Cc: Borislav Petkov <bp@alien8.de>,
	Chuck Ebbert <cebbert.lkml@gmail.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: x86, microcode: BUG: microcode update that changes x86_capability
Date: Thu, 18 Sep 2014 17:28:08 -0700	[thread overview]
Message-ID: <713c7342-bdf4-4e38-8de2-37d840d7028b@email.android.com> (raw)
In-Reply-To: <CALCETrVoeqQUXH7g+reu=hUvCJoiy+edHKBqYKPWXPCS3_UtUw@mail.gmail.com>

The cpuid bit gets twiddled...

On September 18, 2014 5:23:40 PM PDT, Andy Lutomirski <luto@amacapital.net> wrote:
>On Sep 18, 2014 5:13 PM, "Henrique de Moraes Holschuh" <hmh@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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