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

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Date:   Mon, 4 Feb 2019 13:09:12 -0800
From:   Fenghua Yu <fenghua.yu@...el.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, H Peter Anvin <hpa@...or.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        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 Mon, Feb 04, 2019 at 11:05:52AM -0800, Dave Hansen wrote:
> On 2/4/19 9:49 AM, Thomas Gleixner wrote:
> > On Fri, 1 Feb 2019, Fenghua Yu wrote:
> >> This option behaves like existing kernel option clearcpuid.
> > 
> > No it does NOT. clearcpuid allows to disable things.
> > 
> > This allows to enable random CPUID bits without any sanity checking. Not
> > going to happen. We made it clear in the past that functionality needs to
> > be detectable by enumeration. We do quirks for broken crap, but this is
> > just not how it works.
> 
> Hi Thomas,
> 
> I think we are trying persuade you like mentioned here:
> 
> > http://lkml.kernel.org/r/alpine.DEB.2.21.1807122153170.1597@nanos.tec.linutronix.de
> 
> But, we're not being very persuasive because we kinda forgot about the
> "if and only if" condition that you mentioned.

Intel SDM published TODAY does have IA32_CORE_CAPABILITY MSR enumerateion
bit CPUID.0x7.0:EDX[30] now. Please check today's SDM for the bit:
https://software.intel.com/en-us/download/intel-64-and-ia-32-architectures-sdm-combined-volumes-1-2a-2b-2c-2d-3a-3b-3c-3d-and-4

I will add the two patches that enumerate the MSR and #AC for split lock
in the next version.

Thanks.

-Fenghua