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

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Message-ID: <20190205085750.GM17528@hirez.programming.kicks-ass.net>
Date:   Tue, 5 Feb 2019 09:57:50 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Dave Hansen <dave.hansen@...el.com>
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 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_?  IOW, will my laptop in 5 years
> enumerate support for it?

I would bloody hope so. Just for giggles, create an little program that
does LOCK prefix across a line or page boundary in a while(1) loop and
'enjoy' your laptop experience.

> 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?

No, we very much do want to default enable this everywhere.

We might want to provide some chicken bits, like a (inheritable) PRCTL
or ELF flag to disable it for those broken apps.

But realistically, any app that triggers this is non-portable already,
most (if not all) the RISC architecture would already kill it with
SIGBUS for this.

We very much want the kernel _AND_ firmware to be #AC clean, always,
everywhere.

Heck, I'd love for #AC to be even stronger and not only trigger on
cross-line.