linux-kernel - Re: [RFC PATCH] Use feature bit names in clearcpuid=

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Date:   Tue, 22 Sep 2020 10:13:12 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Dave Hansen <dave.hansen@...el.com>
Cc:     x86-ml <x86@...nel.org>, lkml <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] Use feature bit names in clearcpuid=

On Tue, Sep 22, 2020 at 10:03:03AM +0200, Thomas Gleixner wrote:
> On Sun, Sep 20 2020 at 17:42, Borislav Petkov wrote:
> > Hi,
> >
> > so tglx hates this clearcpuid= interface where you have to give the
> > X86_FEATURE array indices in order to disable a feature bit for testing.
> > Below is a first attempt (lightly tested in a VM only) to accept the bit
> > names from /proc/cpuinfo too.
> >
> > I say "too" because not all feature bits have names and we would still
> > have to support the numbers. Yeah, yuck.
> >
> > An exemplary cmdline would then be something like:
> >
> > clearcpuid=de,440,smca,succory,bmi1,3dnow ("succory" is wrong on
> > purpose).
> >
> > and it says:
> >
> > [    0.000000] Clearing CPUID bits: de 13:24 smca bmi1 3dnow
> >
> > Also, I'm thinking we should taint the kernel when this option is used.
> >
> > Thoughts?
> 
> Yes, instead of making it differently horrible, can we finally remove that
> nonsense which should have never been there in the first place?

Fine with me - I don't need it.

I believe dhansen has a use-case or two, CCed.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette