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

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Date:   Tue, 5 Feb 2019 07:18:01 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     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>,
        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 03:24:23PM -0800, Dave Hansen wrote:
> Actually, there's one part of all this that I forgot.  Will split lock
> detection be enumerated _widely_?

You never know what users will do. The moment it gets out, it better be
designed properly, along with the chicken bits.

> IOW, will my laptop in 5 years enumerate support for it?

Don't tell me this is going to be another MPX fiasco :-\

Or is this something along the lines of "let's see whether it takes off
and if yes, we'll commit to it or otherwise remove it and not even waste
a CPUID leaf"?

> If so, we surely don't want to enable this everyhwhere: it will break
> old apps. Doesn't that mean that we need both feature detection and
> another separate bit for folks to opt-in?

Well, if it breaks old apps, it probably needs to be opt-in anyway.

And for that you don't need setcpuid either - you simply boot with
"split_lock_ac" or whatever and the kernel pokes that MSR_TEST_CTL or
whatever else it needs to detect in hw for split lock and sets the
X86_FEATURE bits if detection is successful.

-- 
Regards/Gruss,
    Boris.

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