linux-kernel - Re: [PATCH] x86/fpu: Parse comma separated list passed in clearcpuid

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Date:   Thu, 21 Feb 2019 14:37:45 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Prarit Bhargava <prarit@...hat.com>
Cc:     linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, Andi Kleen <ak@...ux.intel.com>,
        x86@...nel.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH] x86/fpu: Parse comma separated list passed in clearcpuid

On Thu, Feb 21, 2019 at 08:12:25AM -0500, Prarit Bhargava wrote:
> Users cannot disable multiple CPU features with the kernel parameter
> clearcpuid=.  For example, "clearcpuid=154 clearcpuid=227" only disables
> CPUID bit 154.
> 
> Previous to commit 0c2a3913d6f5 ("x86/fpu: Parse clearcpuid= as early XSAVE
> argument") it was possible to pass multiple clearcpuid options as kernel
> parameters using individual entries.  With the new code it isn't easy to
> replicate exactly that behaviour but a comma separated list can be easily
> implemented, eg) "clearcpuid=154,227"
> 
> Make the clearcpuid parse a comma-separated list of values instead of only
> a single value.

So I think the feature is broken as is; because it doesn't clear the
CPUID bits for userspace.

something along the lines of:

https://lkml.kernel.org/r/20190212164833.GK32494@hirez.programming.kicks-ass.net

would be required to make it so.