linux-kernel - Re: [PATCH v3 08/10] x86/setcpuid: Add kernel option setcpuid

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Date:   Tue, 5 Feb 2019 16:43:09 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Ingo Molnar <mingo@...hat.com>, H Peter Anvin <hpa@...or.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Michael Chan <michael.chan@...adcom.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        Ricardo Neri <ricardo.neri@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>
Subject: Re: [PATCH v3 08/10] x86/setcpuid: Add kernel option setcpuid

On Tue, Feb 05, 2019 at 07:19:16AM -0800, Dave Hansen wrote:
> This is one of the few times that we're pretty confident that folks will
> use this.  The reason we're going to this trouble is that the split lock
> detection is wanted by actual customers, and they want it before it's
> implemented on a processor with real enumeration.
>
> This isn't something we want everybody and their grandma to turn on;
> it's a rather specialized feature.  It's really only for folks that care
> about the latency incurred across the entire system on split lock
> operations.

...

> It's not for developers.  This really is for (somewhat niche) end users
> that want split lock detection in production.  This is all really an
> effort to get them running mainline or real distro kernels.

This all sounds to me like it shouldn't even be mainline but in a
special, evaluation kernel. If anything, it should be default off and be
opted-in by a cmdline switch. None of that *cpuid=* toggling dance.

-- 
Regards/Gruss,
    Boris.

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