Subject: Re: [RFC PATCH 07/13] powerpc/dexcr: Add sysctl entry for SBHE system override

From: Benjamin Gray <[email-masked]>
Date: Tue, 07 Mar 2023 16:58:22 +1100

On Tue, 2023-03-07 at 15:30 +1000, Nicholas Piggin wrote:
> On Mon Nov 28, 2022 at 12:44 PM AEST, Benjamin Gray wrote:
> > The DEXCR Speculative Branch Hint Enable (SBHE) aspect controls
> > whether
> > the hints provided by BO field of Branch instructions are obeyed
> > during
> > speculative execution.
> > 
> > SBHE behaviour per ISA 3.1B:
> > 
> > 0:      The hints provided by BO field of Branch instructions may
> > be
> >         ignored during speculative execution
> > 
> > 1:      The hints provided by BO field of Branch instructions are
> > obeyed
> >         during speculative execution
> > 
> > Add a sysctl entry to allow changing this aspect globally in the
> > system
> > at runtime:
> > 
> >         /proc/sys/kernel/speculative_branch_hint_enable
> > 
> > Three values are supported:
> > 
> > -1:     Disable DEXCR SBHE sysctl override
> >  0:     Override and set DEXCR[SBHE] aspect to 0
> >  1:     Override and set DEXCR[SBHE] aspect to 1
> > 
> > Internally, introduces a mechanism to apply arbitrary system wide
> > overrides on top of the prctl() config.
> 
> Why have an override for this, and not others?
> 

Should be in the commit message of course, but this aspect bleeds over
to other processes, so a user may wish to prevent all processes from
changing the value. The other aspects are probably only relevant to
their own process, though the implementation here should support
arbitrary system wide overrides.