Re: [RFC PATCH] tools/x86: add kcpuid tool to show raw CPU features

[Borislav Petkov]

On Wed, Sep 02, 2020 at 06:45:38PM +0200, peterz@xxxxxxxxxxxxx wrote:
> We really should clear the CPUID bits when the kernel explicitly
> disables things.

Actually, you want to *disable* the functionality behind it by clearing
a bit in CR4 - and yes, not all features have CR4 bits - so that
luserspace doesn't "probe" the existence of certain instructions.

Example: you can still try to run RDRAND and succeed even if the
corresponding CPUID bit is clear.

-- 
Regards/Gruss,
    Boris.

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