netdev - Re: [PATCH v7 18/21] x86/clearcpuid: Support feature flag string in kernel option clearcpuid

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Message-ID: <alpine.DEB.2.21.1904180815120.3174@nanos.tec.linutronix.de>
Date:   Thu, 18 Apr 2019 08:16:22 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Fenghua Yu <fenghua.yu@...el.com>
cc:     Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        H Peter Anvin <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        Xiaoyao Li <xiaoyao.li@...el.com>,
        Christopherson Sean J <sean.j.christopherson@...el.com>,
        Kalle Valo <kvalo@...eaurora.org>,
        Michael Chan <michael.chan@...adcom.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        x86 <x86@...nel.org>, kvm@...r.kernel.org,
        netdev@...r.kernel.org, linux-wireless@...r.kernel.org
Subject: Re: [PATCH v7 18/21] x86/clearcpuid: Support feature flag string in
 kernel option clearcpuid

On Wed, 17 Apr 2019, Fenghua Yu wrote:
> On Thu, Apr 18, 2019 at 01:19:41AM +0200, Thomas Gleixner wrote:
> > On Wed, 17 Apr 2019, Fenghua Yu wrote:
> > 
> > > The kernel option clearcpuid currently only takes feature bit which
> > > can be changed from kernel to kernel.
> > > 
> > > Extend clearcpuid to use cap flag string, which is defined in
> > > x86_cap_flags[] and won't be changed from kernel to kernel.
> > > And user can easily get the cap flag string from /proc/cpuinfo.
> > 
> > If your machine dies because init triggers #AC then please explain how that
> > easily can be read from /proc/cpuinfo and how the sysadmin can figure out
> > what the heck he needs to write on the kernel command line.
> > 
> > The whole 'clearcpuid' thing should have never been merged. It's a pure
> > testing/debugging thing. And no, we are not going to proliferate it and
> > extend it for dubious value. Quite the contrary, we should simply rip it
> > out.
> 
> So I can remove the four 'clearcpuid' related patches 0018-0021 in the next
> version, right?

Yes please. They are unrelated to this problem and 'noac' is way more admin
friendly than that.

Thanks,

	tglx