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

From: Peter Zijlstra
Date: Tue Feb 05 2019 - 03:58:02 EST

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,

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