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

From: Borislav Petkov
Date: Thu Sep 25 2014 - 04:52:12 EST

On Wed, Sep 24, 2014 at 11:56:58AM -0300, Henrique de Moraes Holschuh wrote:
> I don't know about AMD, but on Intel, the time it takes to update the
> microcode on a core is anything but negligible[1], so the microcode
> version skew window still exists, and it is not small. It is much smaller
> than it once was, but it is still there.

I think that window is unsafe but yeah, we probably should take your
empirical observation as good enough for now.

> But IMHO we still need to detect and do something smart when
> x86_capability changes due to a microcode update.
> And I'd really prefer it to be "update x86_capability, warn the user and
> carry on" for anything that is not going to crash the kernel.

The problem is with hiding CPUID bits and userspace using HLE after
having detected it previously. I think we'll be on the safe side if we
reboot thus the suggestion to the user that rebooting should be done

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