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

[peterz]

On Wed, Sep 02, 2020 at 06:55:01PM +0200, Borislav Petkov wrote:
> 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.

Well yes, but as you say, we don't have that :/ Clearing it in CPUID is
the best we can do.