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

[Peter Zijlstra]

On Tue, Feb 05, 2019 at 09:57:50AM +0100, Peter Zijlstra wrote:
> 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.

Combined with a sysctl that disables the chickens; such that
administrators can configure their system to be #AC pure.

> 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.