Re: x86, microcode: BUG: microcode update that changes x86_capability
From: Andy Lutomirski
Date: Thu Sep 18 2014 - 21:00:39 EST
On Sep 18, 2014 5:28 PM, "H. Peter Anvin" <hpa@xxxxxxxxx> wrote:
> The cpuid bit gets twiddled...
Yes, but how? I assume that BIOS isn't switching between two
different ucode blobs, and I don't know about any wrcpuid instruction.
So there must be *some* way, at least on new ucode (and maybe on old
ucode) to change that bit. If we could do that in the kernel, we
might be able to come up with a more intelligent way to handle this
ucode gotcha (such as, ideally, clearing the bit ourselves on old
> On September 18, 2014 5:23:40 PM PDT, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> >On Sep 18, 2014 5:13 PM, "Henrique de Moraes Holschuh" <hmh@xxxxxxxxxx>
> >> 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
> >> >
> >> > Well, yes. But that won't help the several stable and LTS distros
> >> > 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
> >> 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
> >> 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
> >> them all and in the darkness grind them. In the Land of Redmond
> >> where the shadows lie." -- The Silicon Valley Tarot
> >> Henrique Holschuh
> Sent from my mobile phone. Please pardon brevity and lack of formatting.
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