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

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Date:   Tue, 5 Feb 2019 07:19:16 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Borislav Petkov <bp@...en8.de>,
        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 2/5/19 12:48 AM, Peter Zijlstra wrote:
> On Mon, Feb 04, 2019 at 12:46:30PM -0800, Dave Hansen wrote:
>> So, the compromise we reached in this case is that Intel will fully
>> document the future silicon architecture, and then write the kernel
>> implementation to _that_.  Then, for the weirdo deployments where this
>> feature is not enumerated, we have the setcpuid= to fake the enumeration
>> in software.
> 
> What user is _EVER_ going to use this? Nobody, I expect the answer to
> be.

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.

> Is this some transient state; where a few (early) models will not have
> the enumeration sorted but all later models will have it all neat and
> tidy?

>From my understanding, it's not just an early stepping.  It's a
generational thing.  The current generation lacks the enumeration and
the next generation will get it.  Both have the silicon to implement the
feature itself.

> If so, we can easily do the FMS solution for this.

Yeah, we can.  I honestly forget why we didn't do FMS. :)

Fenghua?

> But a cmdline features thing is not something I can see anybody but
> a limited set of developers ever using.

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.