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

From: Andy Lutomirski
Date: Thu Sep 18 2014 - 20:24:08 EST

On Sep 18, 2014 5:13 PM, "Henrique de Moraes Holschuh" <hmh@xxxxxxxxxx> 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.


> 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
> 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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